Workshop "Optical GEMS in the Quickly Variable Sky: New Insights on Neutron Star Fundamental Physics from Multi-Messenger Observations"

13 May 2026, 12:00 → 15 May 2026, 14:00 Europe/Rome

Aula Gratton (INAF Astronomical Observatory of Rome)

Alessandro Papitto (Istituto Nazionale di Astrofisica (INAF)), Arianna Miraval Zanon (Agenzia Spaziale Italiana)

Millisecond pulsars (MSPs) are unique laboratories for probing the Neutron Star (NS) Equation of State (EoS) at supra-nuclear densities. The GEMS project - funded by the Italian MUR under the PRIN 2020 program (PI: Astone) - strategically connected innovative observational strategies across the gravitational wave (CW) and electromagnetic (EM) domains to maximise CW search sensitivity in LIGO-Virgo data.

GEMS leveraged dedicated hardware, like the high-time resolution SiFAP2 optical photometer operated from the Telescopio Nazionale Galileo, to conduct multi-wavelength surveys (radio, optical, X-ray, gamma-ray) of candidate CW sources like bright accreting NSs in Low-Mass X-ray Binaries and MSPs. Major astrophysical results include bringing the total number of detected optical millisecond pulsars to three and identifying promising candidates, advancing the understanding of particle acceleration and optical signal amplification in these systems. The project also pioneered X-ray polarimetry of compact objects in X-ray binaries, contributing to the first X-ray polarisation measurement from an accreting MSP. This critical contribution provides geometric constraints on magnetic fields and emission regions, which are vital for breaking parameter degeneracies in pulse profile modelling to accurately constrain the NS mass and radius, directly informing the EoS. Additionally, fast optical photometry played a role in testing models for Fast Radio Bursts (FRBs) in terms of fast-spinning magnetars.

This workshop will gather experts from multi-wavelength and multi-messenger astrophysics to present and synthesise the GEMS project's observational and theoretical astrophysical outcomes. The goals are to share validated search algorithms and improved hardware, and collaboratively plan future observing strategies that will push the boundaries of NS fundamental physics with next-generation detectors.

The workshop will take place from Wednesday, 13 May to Friday, 15 May 2026 at the INAF-Observatory of Rome. 

 

 

Optical GEMS  è un workshop dedicato alla divulgazione dei risultati astrofisici del progetto PRIN 2020 GEMS "Cutting-edge strategies to identify new GEMS (Gravitational- and ElectroMagnetic-wave Sources) in the Universe with current and next-generation detectors" - 2020BRP57Z - PI: Pia Astone; Resp. Locale: L. Stella, A. Papitto, elencati in questa pagina NASA/ADS.

Wednesday 13 May

1 Registration

2 Welcome - A. Papitto

Papitto

3 More and more FRBs, more and more questions - Andrea Possenti

Fast Radio Bursts (FRBs) are short (from millisecond to tens of millisecond) radio flashes originating from sources located across the whole Universe up (so far) to a redshift around 2. Although about 5000 sources have been now catalogued, the engine of the FRB emission is still unconstrained, and the nature of the underlying source(s) debated. This contribution will focus on some recent developments in this blooming astrophysical field, while presenting a new experiment, which INAF is significantly involved in, and which promises to play a major role for both addressing the many still unsolved questions and for fully exploiting the FRBs also in order to perform fundamental physics and cosmology research.

Possenti

4 Fast Radio Bursts outside the radio band - Maura Pilia

Emission outside the radio band is predicted for most models trying to explain the FRB phenomenon. Its nature, however, is still to be determined, with the exception of the possible example provided by the Galactic FRB20200428A, where X-ray emission was observed simultaneously with the radio emission, with an energy ratio E_X/E_radio = 10^5.
I will review the targeted attempts to search for the multi-wavelength counterpart of FRBs and I will highlight the results of six years of multi-wavelength campaigns targeting FRB20180916B in optical, X- and gamma-rays, simultaneously with high sensitivity radio observations. Through this campaign the deepest prompt upper limits were obtained in optical and X-rays in terms of E_MWL/E_radio.
I will discuss the challenges of such large scale campaigns and the physical implications of our findings.

Pilia

5 Gravitational self-lensing of Fast Radio Bursts in neutron star magnetospheres - Simone Dall'Osso

Nearly two decades after their discovery, the origin of fast radio bursts (FRBs) remains a mystery.
To date their bright, coherent radio emission has no detected counterparts beyond the radio band,
with the only exception of FRB-like flares emitted by the galactic magnetar SGR 1935+215. The
latter lend support to the proposed association of FRB sources with a cosmic population of magnetars.
I will summarize the main properties of FRB emission, and introduce our novel interpretation
in terms of strong gravitational lensing of neutron star (NS) flares due to the NS gravitational field,
i.e. the gravitational self-lensing (GSL) model. I show how this idea explains in a unified picture
the large FRB luminosities, the co-existence of rare repeaters and more numerous one-off sources,
and their overall energy budget, allowing at the same time to bridge the gap between the apparent
paucity of FRBs in the local universe and their comparatively large all-sky rate. Moreover, the
GSL model accounts for (a) the double-peaked energy distribution of individual FRBs observed in
the most active repeaters, in terms of a simple, pulsar-like emission geometry, and (b) the redshift
(and fluence) distribution of FRBs in the first CHIME catalog, in terms of a cosmic population –
consistent with magnetars - following the star formation history.

Dall'Osso

6 Optical monitoring of FRB 20220912A and characterization of fast optical foreground events - Alessia Spolon

Repeating fast radio bursts (FRBs) are prime targets for high–time-resolution optical searches
aimed at identifying fast optical counterparts. Between October 2022 and December 2023, we
monitored the highly active repeater FRB 20220912A at the Asiago Observatory using the fast
photon-counting instruments Aqueye+ and IFI+Iqueye. During prolonged optical observations,
we identified short (∼1 ms) statistically significant optical pulses, here referred to as fast optical
bursts (FOBs).
To assess the nature of these events, we performed a detailed foreground characterization using
archival Aqueye+ observations of a comparison field, centered on the faint millisecond pulsar
PSR J1023+0038. Exploiting the availability of both on-source and on-sky detectors, we find that
about 60% of significant events are detected simultaneously in both regions, consistent with diffuse
atmospheric contamination, likely associated with meteor activity. A comparable fraction is observed
in the FRB 20220912A field, suggesting that the detected FOBs are dominated by foreground
events. Further optical monitoring and coordinated multi-wavelength campaigns, including Iqueye
at Gemini South, are required to improve the statistics and to search for any excess of isolated
on-source events potentially attributable to the FRB.

Spolon

7 A tricky statistical search for Fast Optical Bursts - Carlo Campa

The long unexplored fast optical sky is now accessible thanks to new large ground-based telescopes
and new generation instruments. Studying it may lead to new discoveries, but it is also challenging
due to optical pollution.
In this talk, I will present our preliminary results obtained from an automated search and characterization
of mysterious Fast (millisecond) Optical Bursts (FOBs) in about 400 hours of observation
with the fast photometer SiFAP2 at the Telescopio Nazionale Galileo.
Using a Bayesian Blocks–based algorithm we developed, we found a surprisingly high rate of 2
FOBs per hour. Our analysis suggests that many of these bursts likely have a non-astrophysical
origin. However, maybe some bursts show marginally significant evidence to the contrary.

Campa

8 Multifrequency observations of Crab giant pulses - Luca Beduzzi

The Crab pulsar is the youngest known neutron star and it has been monitored for over five decades.
Its giant pulses (GPs) provide a unique laboratory for probing neutron-star emission physics. Fast
radio bursts (FRBs), often linked to neutron stars, offer a complementary framework for studying
extreme energy-release processes. Crab GPs have typical fluences larger than a few tens of Jy ms.
They may therefore represent a so far missing link between pulsar physics and repeating FRBs
emission. We conducted observations at 408 MHz with the Northern Cross radio telescope, 820
MHz with the Green Bank Telescope, and 1500 MHz with the Green Bank and Noto telescopes. Our
observations, carried out simultaneously with optical ones, are expected to provide the optical-toradio
energy ratio of Crab giant pulses. I will present the analysis of GPs energy and waiting-time
distributions. I will compare them with those of the most hyperactive repeating FRBs, such as FRB
121101, FRB 201124A, FRB 220114A, and FRB 220912A, including a search for periodicities.

Beduzzi

16:15 Coffee break

9 The optical beat of magnetars - Alice Borghese

Magnetars are the most magnetised isolated neutron stars known in the Universe. Their signature
is the unpredictable and and variable flaring activity in the X/gamma-ray bands, with bursts lasting
from a few milliseconds to tens of seconds and reaching peak luminosities of 1037-1043 erg/s. These
flaring events are often accompanied by outbursts in which the persistent X-ray flux increases by
a factor of ~10-1000 and then decays over months to years. Magnetars have also been proposed
as possible progenitors of some of the most extreme transient events in the Universe, such as fast
radio bursts and gamma-ray bursts.
Investigating magnetar emission outside the X-ray band is challenging. In this talk, I will focus
on the treasure hunt for optical magnetar counterparts, reviewing the few known detections and
the ongoing effort with the high-speed optical photometer SiFAP2. I will also discuss possible
mechanisms at the origin of magnetar optical emission - a still open problem for which detailed
theoretical models are currently lacking.

Borghese

10 CCOs with fast optical photometry - James Turner

Central compact objects (CCOs) are a rather unusual manifestation of neutron stars (NSs). They
show all the characteristics of being old compared to the bulk NS population; they have slower spin
periods, weaker surface magnetic fields and their pulsed emission is almost exclusively comprised
of thermal X-rays. We nevertheless know they must be young because they are found in the
centre of supernova remnants, hence their namesake. Their formation and evolutionary history is
contested, with theories of an intrinsically weak magnetic field at birth (anti-magnetar scenario)
being opposed by ideas of rapid spin-down events (fallback disk and B-field crustal emergence)
in order to explain their observed properties. A recent detection of radio pulsations from one
CCO has suggested that, whilst relatively weakly magnetised, their magnetospheres may possess
temporarily favourable configurations for non-thermal emission. Now that the link between CCOs
and canonical pulsars has been established, we can probe further at optical and nIR wavelengths.
Studying the nIR polarisation and spectra across the rotational phase will be particularly pertinent
for constraining atmosphere models, and explaining the true origin of the weak magnetic fields.

Turner

11 High-time-resolution optical photometry of supernovae with SiFAP2: probing the late-time emission of SN 2023ixf and the compact remnant connection - Fabio Ragosta

Core-collapse supernovae are the birth sites of neutron stars and black holes, yet the properties of
the newly formed compact object remain elusive at optical wavelengths. Fast optical photometry
—with sub-second sampling —opens a new window on the rapidly evolving phases of supernovae
and on the late-time emission that may betray the presence of an active compact remnant.
We report on the first application of the SiFAP2 high-time-resolution photon counter at the TNG
(La Palma) to supernova photometry, using the nearby Type II-L SN 2023ixf in M101 as a test case.
Observations were carried out at 1 s sampling across six epochs spanning from ~2 days to ~772
days post-explosion, covering the rise, the plateau, the radioactive tail, and the late nebular phase.
We describe the methodological challenges of adapting an instrument designed for stellar oscillations
and pulsars to the calibration of a spectrally evolving transient, including the development
of an empirical time-dependent colour correction.
Our late-time photometry reveals a significant excess above the expected radioactive decay at
epochs beyond 700 days, consistent with an additional energy source such as ejecta–CSM interaction
or emission powered by the nascent compact remnant. We discuss the implications of these
findings in the context of neutron star formation and early pulsar activity, and we highlight the potential
of high-cadence optical facilities for the multi-messenger follow-up of nearby core-collapse
events in the era of gravitational wave astronomy.

Ragosta

12 Catching Gamma-Ray Bursts in the Act: Prospects for Prompt Optical Emission Detection with Fast Photometry - Riccardo Brivio

Gamma-ray bursts (GRBs) are the most energetic explosions in the Universe, yet the physics of
their prompt emission phase remains poorly constrained. Simultaneous optical observations during
the prompt γ-ray phase provide a unique and direct diagnostic of the jet structure, emission
mechanism, and outflow Lorentz factor. To date, only a handful of events have been captured in
optical light during the prompt phase; the most remarkable case, GRB 080319B, reached nakedeye
brightness (V ~ 5.3) and was serendipitously observed by the wide-field TORTORA camera,
revealing a two-component jet geometry and an extremely relativistic outflow. We discuss the observational
requirements and instrument strategies needed to systematically detect GRB prompt
optical emission, focusing on two complementary approaches: (i) wide-field, high-cadence imagers
capable of monitoring large sky areas without the need for rapid slewing, and (ii) SIFAP-like
fast-photometry instruments mounted on telescopes with fast-response capability, which are wellsuited
for long-duration events (T90 > 90 s) and ultra-long GRBs. By rescaling from GRB 080319B,
we show that O(1) detectable events per year are expected with existing or near-future Italian
facilities. The scientific return of each such detection —from sub-second variability to real-time
optical–γ correlation —would be transformative for our understanding of the central engine.

Brivio-Ferro

13 Fast Optical Variability as a Probe of GRB Central Engine Physics: Opportunities with Italian Facilities - Matteo Ferro

The physical mechanism powering the prompt emission of gamma-ray bursts (GRBs) —internal
shocks, magnetic reconnection, or photospheric dissipation —is one of the longest-standing open
questions in high-energy astrophysics. While gamma-ray observations alone cannot break the
model degeneracy, simultaneous optical monitoring at sub-second cadence offers a powerful and
largely unexploited discriminant: the temporal correlation (or lack thereof) between optical and
gamma-ray pulses directly constrains the emission radius, the magnetization of the outflow, and
the presence of a reverse shock. We argue that the Italian astronomical community is uniquely
positioned to contribute to this field. On one hand, wide-field high-cadence imagers (analogous
to TORTORA or the proposed Mezzocielo concept) can serendipitously catch the prompt phase
without any slew delay, at the cost of shallow limiting magnitude. On the other hand, SIFAPclass
photon-counting instruments on medium-to-large aperture telescopes, if paired with a fastresponse
alert system, can reach the field of long-duration GRBs (T90 > 90 s) and ultra-long GRBs
(T90 > 1000 s) while the prompt phase is still ongoing, providing photon-by-photon light curves
with millisecond resolution. We quantify the expected detection rates under conservative assumptions,
discuss the minimum resolvable timescales as a function of telescope aperture and GRB
brightness, and outline a realistic observational strategy to maximize the scientific yield of each
event. A single well-observed prompt optical light curve, obtained simultaneously with Swift/BAT
or Fermi/GBM data, would provide diagnostics unavailable by any other means.

Brivio-Ferro

Thursday 14 May

14 The high-speed eye of the TNG: Reviewing SiFAP2 successes starting from the first optical millisecond pulsar - Giulia Illiano

High-time resolution astronomy in the visible band has historically been limited by the readout
scales of standard detectors. SiFAP2, a fast optical photometer based on Silicon Photo-Multipliers
mounted at the 3.58m Telescopio Nazionale Galileo, overcomes these constraints by recording individual
photon arrival times with a sub-nanosecond resolution. This instrument has been pivotal
in investigating the rapidly variable optical sky, particularly in the study of millisecond pulsars
(MSPs). This talk reviews the key milestones of SiFAP2, starting from the discovery of the first
optical millisecond pulsations from the transitional MSP PSR J1023+0038, which overturned our
understanding of the emission mechanisms in these systems. The enigma has only deepened with
the subsequent detection of optical pulsations from two other sources: the accreting MSP SAX
J1808.4-3658 and the rotation-powered MSP PSR J2339-0533. We now face a scenario where three
distinct systems exhibit optical pulsations that appear to stem from different physical processes,
prompting a deeper exploration of the pulsar-disk interaction and the search for new candidates
to test current theoretical models.

Illiano

15 Investigating the long-term timing properties of the transitional millisecond pulsar PSR J1023+0038 with Aqueye+, NICER, and Iqueye@Gemini South - Silvia Conforti

PSR J1023+0038 is a transitional millisecond pulsar (tMSP), the only one where coherent pulsations
were discovered in both the Optical and X-ray energy bands to date. We present a detailed
timing analysis of new Aqueye+ optical data taken from 2021 through 2023, obtaining an updated
timing solution for the pulsar. We perform a new measurement of the phase lag between optical
and X-ray pulsations and investigate the recent evolution of the binary system, monitoring the
variation of the time of passage at the ascending node. We measured a phase lag between the optical
and X-ray pulses of 112.3 ± 30.7 μs, confirming a common origin. After 2017, the evolution of
the passage at the ascending node follows a well-defined parabolic increasing trend, suggesting a
corresponding increment in the orbital period and orbital separation of the system. This evolution
is consistent with a scenario in which the binary system is evolving through non-conservative
Roche lobe overflow and the donor is interacting with the pulsar wind, losing mass at a rate much
higher than the inferred accretion rate. Finally, we offer a glimpse of the results from observations
of PSR J1023+0038 obtained with Iqueye
recently installed at Gemini South, which achieved unprecedented photon statistics and time resolution.

Conforti

16 Optical spectroscopy of the transitional millisecond pulsars - Marco Messa

Transitional millisecond pulsars (tMSPs) represent a dynamic class of systems linking low-mass
X-ray binaries to rotation-powered millisecond radio pulsars, providing key laboratories to study
accretion–ejection physics and neutron star environments. These sources undergo transitions between
accretion- and rotation-powered states, and in their active phase, they display rapid switches
between distinct high and low luminosity modes, occasionally accompanied by flares.
In this talk, I will present high time-resolution optical spectroscopic observations of the prototypical
tMSP J1023+0038 in its sub-luminous disc state. The dataset probes variability on minute
timescales, revealing significant changes in colour as well as in the strength and profile of the main
emission lines. These variations offer insight into the structure and dynamics of the accretion flow
and its interaction with the neutron star during its disc state.
I will emphasise how combining high-time resolution optical spectroscopy with fast optical photometry,
as enabled by instruments such as SiFAP2, can provide a more complete view of these
systems. In particular, the synergy between fast photometry and spectroscopy, combined with
X-ray observations, can be used to trace correlations between high/low mode transitions across
different energy bands while simultaneously probing the spectral response of the accretion disc.
This approach opens the possibility of linking rapid flux variability to changes in emission line
properties and disc structure.

Messa

17 What powers millisecond optical pulsations in SAX J1808 and PSR J1023? - Tiziana Di Salvo

The discovery of millisecond optical pulsations in SAX J1808.4−3658 and PSR J1023+0038 has
opened a new window on the interplay between accretion, magnetic fields, and particle acceleration
in neutron-star binaries. In this talk I will review the main physical ideas that have been
proposed to explain these signals, from accretion-powered hotspot scenarios to synchrotron models
associated with compact magnetospheric interaction regions or striped/pulsar-wind-like dissipation.
I will then compare the observational constraints provided by the two prototype systems
and discuss how they may point to different radiative regimes: a more clearly two-component
picture in SAX J1808, with X-ray pulsations linked to polar accretion and optical/UV pulsations to
a non-thermal synchrotron component, versus a more nearly optically thin and spectrally unified
pulsed component in PSR J1023. The goal is to identify which elements of the phenomenology
may be source-specific and which may reflect a common underlying engine.

Di Salvo

18 3D particle-in-cell simulations of pulsar wind – disk interaction: application to the transitional millisecond pulsar PSR J1023+0038 - Valentina Richard Romei - REMOTE

Transitional millisecond pulsars constitute a peculiar subclass of neutron stars in which the pulsar
alternates between accretion-powered and rotation-powered states. A third intermediate state,
referred to as ‘sub-luminous disk state’, has been identified. During this state, the system exhibits
intriguing features, such as broad optical and X-ray pulsations characterized by an unexpectedly
high luminosity. To date, no ab initio model of a pulsar wind interacting with an accretion disk has
been developed to address these observables. We aim to investigate the origin of the enhanced electromagnetic
emission and to reproduce the observable properties of such systems. To model the
interaction between the pulsar wind and the disk, we perform 3D particle-in-cell simulations of a
pulsar magnetosphere surrounded by a perfectly conducting torus. We find that the presence of the
disk induces a significant reconfiguration of the magnetosphere. As a result, the synchrotron radiation
is substantially enhanced, and characterized by a strong continuous component and either
one or two-peaked light curves, depending on the pulsar’s magnetic obliquity. The polarization
properties are also explored. The model successfully reproduces the main features of the optical
and X-ray pulsed emission originating from PSR J1023+0038, thereby confirming the scenario in
which these pulsations originate from synchrotron radiation generated as the pulsar wind interacts
with the inner edge of the disk.

Richard Romei

10:35 Coffee break

19 Searching for pulsars associated with Fermi sources with MeerKAT - Marta Burgay

Pulsars, thanks to their extreme characteristics and their clock-like nature, are unique laboratories
to probe many aspects of astrophysics and fundamental physics: from stellar and binary evolution
to the study of the interstellar medium and of the Galactic magnetic field, from nuclear physics to
relativistic gravity and gravitational waves.
The extraordinary sensitivity of the MeerkAT telescope has, in only a few years, led to much
progress and many discoveries in this field. In this talk I will describe the pulsar search experiments
carried out in the last five years in the framework of the TRAPUM (TRAnsients and Pulsars
with Meerkat) Large Survey Projects. I will report on some of the most interesting results, both
from pulsar searches and follow-up studies, with a particular focus on searches targeting Fermi
unidentified sources, real treasure maps to find exotic binary pulsars, often exhibiting a rich phenomenology
across the electromagnetic spectrum.

Burgay

20 The many faces of millisecond pulsars in globular clusters - Alessandro Ridolfi - REMOTE

About half of all currently known millisecond pulsars (MSPs) reside in globular clusters. These are
extremely ancient, self-gravitating, spherical stellar systems that orbit our Galaxy. In their cores,
the stellar density can exceed a million solar masses per cubic parsec, providing a formidable
breeding ground for the formation of binary systems in which a neutron star can be spun up by a
companion and eventually become an MSP. Indeed, over 80% of all the pulsars hosted by GCs are
MSPs (with periods smaller than 10 ms).
The number of MSPs in GCs has also increased dramatically over the past few years, thanks to
major surveys conducted with the most sensitive cm-wavelength radio facilities, most notably
MeerKAT and FAST. As of May 2026, more than 300 MSPs are known across 46 different GCs.
In this talk, I will review the rich and diverse population of MSPs in globular clusters. I will
highlight their general characteristics, relating them to the structural parameters of their host
clusters, and briefly discuss some peculiar objects that deserve special attention for their broader
scientific implications.

Ridolfi

21 Spider millisecond pulsars with SiFAP2 - Christian Malacaria

High-time-resolution detectors are playing a crucial role in advancing our understanding of neutron
stars and transient phenomena, enabling precise measurements of their properties. SiFAP2,
the high-speed optical photometer permanently mounted at the 3.6m INAF Telescopio Nazionale
Galileo, obtained significant breakthroughs discovering optical pulsations of three millisecond pulsars
in different luminosity states. SiFAP2 is also involved in multiwavelength campaigns on FRBs
to search for optical counterparts. In this talk, I will present the improvements applied to the instrument
from the point of view of detectors and timing devices within the “GEMS” project. I will
also present the enhanced e-SiFAP instrument, a 6-band Vis/SWIR fast photometer with added
polarimetric capabilities as well as the new SiFAP-SoFT photometer mounted at the 1.5m Loiano
Telescope especially conceived for simultaneous optical measurements targeted on FRBs and their
possible link with magnetars.

Malacaria

22 Unravelling the web: discovery of nine new spider pulsar candidates - Marco Turchetta - REMOTE

Since its launch in 2008, Fermi-LAT has been uncovering a new nearby population of compact
binary millisecond pulsars, also known as “spiders”. Expanding the currently known spider population
holds the key to finding the fastest-rotating and most massive neutron stars, which have
crucial implications for nuclear physics and gravitational wave astronomy. Although many of
these systems are found via radio follow-up of Fermi sources, searching for the variable optical
emission of their companions has also proven successful, and holds potential for revealing a hidden
population of radio-obscured spiders.
We present the discovery of nine variable optical counterparts to pulsar-like Fermi unidentified
sources, which we identify as new spider candidates. Their optical light curves show flux modulation
larger than 0.2 mag in the sub-day period range and effective temperatures of 4000−6000 K, all
properties consistent with the known redback subtype of binary millisecond pulsars. Among our
newly discovered candidates, 4FGL J0736.9-3231, if confirmed as millisecond pulsar, would be the
closest known spider to Earth (D=659 pc, from Gaia-DR3 parallax). Unlike the mildly irradiated
companion in the previous system, 4FGL J1639.3-5146—one of our most promising candidates—
shows strong irradiation, with temperature variations up to 1500 K. We also find an X-ray source
at the location of another redback candidate, 4FGL J1748.8-3915, which shows direct evidence of
the intrabinary shock between the pulsar and its companion’s winds. These results facilitate radio
observations of these candidates currently being acquired with the GBT, but also enable neutron
star mass estimates via optical light curve modelling, supported by upcoming GTC and VLT spectroscopic
follow-ups.

Turchetta

23 The power of optical and ultraviolet observations of neutron stars - Roberto Mignani - REMOTE

Neutron stars are among the faintest sources of optical radiation. Nonetheless, their study has
been pursued since right after the pulsar discovery and has continued through the years with
renewed effort. In this review, I will summarise the state of the art of optical and UV observations
of neutron stars, with particular emphasis on the scientific implications of such observations and
their impact in astrophysics and fundamental physics.

Mignani

12:55 Lunch

24 Accreting Millisecond X-ray Pulsars: Spin Evolution and Accretion Physics - Andrea Sanna

Accreting millisecond X-ray pulsars represent a key phase in the formation of rotation-powered
millisecond pulsars, providing a direct observational window onto the interaction between accretion
flows and magnetised neutron stars. Their X-ray pulsations offer a unique probe of how
angular momentum is transferred to the neutron star and how spin evolution proceeds under the
action of accretion torques.
In this talk, I will present a review of the observational properties of accreting millisecond X-ray
pulsars, focusing on their spin distributions, transient behaviour, and accretion regimes. Recent
studies indicate that the observed diversity of these systems cannot be interpreted solely in terms
of instantaneous accretion rates, but likely reflects the imprint of their long-term evolutionary history.
Within this framework, the observed spin properties can be understood as the result of the
interplay among accretion torques, magnetic field strength, and the system’s secular evolution.
I will discuss how X-ray timing observations constrain the physics of the disc–magnetosphere interaction
and the efficiency of spin-up processes, and how these constraints can be connected to
broader evolutionary scenarios. In this context, multi-wavelength observations provide an important
complementary perspective. In particular, the detection of optical pulsations at the neutron
star spin period in a handful of systems indicates that part of the optical emission can be directly
linked to magnetospheric processes operating on millisecond timescales, offering an additional
diagnostic of the accretion–ejection interplay.
This overview highlights how accreting millisecond X-ray pulsars serve as a laboratory for connecting
short-term accretion physics with long-term neutron star spin evolution, offering clear synergies
with ongoing efforts to characterise millisecond pulsars across the electromagnetic spectrum,
including the optical domain explored within the GEMS project.

Sanna

25 Probing the origin of optical and UV pulsations in the accreting millisecond pulsar SAX J1808.4-3658 - Caterina Ballocco

SAX J1808.4-3658 is the prototype of accreting millisecond X-ray pulsars and the first of this class
to show optical and UV pulsations during an accretion outburst. These pulsations challenge standard
rotation- or accretion-powered scenarios and provide a unique window into the emission
mechanisms of pulsating neutron stars. I will present a multi-band timing study based on XMMNewton,
HST/STIS, and TNG/SiFAP2 observations obtained during the 2022 outburst. Optical and
UV pulsations observed in 2022 are compared with those first detected in 2019, allowing us to
study their evolution across different outbursts. HST observations confirmed the presence of significant
UV pulsations at an X-ray luminosity approximately a factor of two lower than during
the 2019 outburst, extending the range of mass accretion rates at which UV pulsations have been
detected. Analyzing optical observations performed with the fast optical photometer SiFAP2, we
find an anti-correlation between the X-ray flux and the amplitude of the optical pulsations, with
the latter increasing as the X-ray flux decreases. I will also discuss optical observations obtained
during the quiescent state of the source, probing the pulse emission mechanism over a large range
of mass accretion rates.

Ballocco

26 Millisecond pulsars phenomenology under the light of graph theory - Carlos Rodriguez - REMOTE

Millisecond pulsars (MSPs) in binary systems display a rich phenomenology, including spider pulsars,
commonly classified as black widows or redbacks, as well as transitional millisecond pulsars.
Understanding how these systems relate to one another within the broader MSP population remains
an open question.
In this talk, I present a set of algorithms in the context of graph theory, specifically those based
on the Minimum Spanning Tree (MST), to analyze and classify the binary MSP population in a
multidimensional parameter space.
The MST provides a graph-based representation in which distinct pulsar classes naturally emerge
as separate branches. In particular, we show that black widows, redbacks, and transitional systems
occupy well-defined regions of the tree, enabling their separation without relying on predefined
boundaries. We also define a similarity-based ranking to identify candidate systems associated
with specific classes. This approach enables the promotion of new candidates.
Building on this structure, we apply an algorithm to partition the MST into statistically meaningful
subgroups and systematically explore their phenomenological classification.
Finally, we introduce a method for locating pulsars with uncertain measurements within the MST.
By analyzing their positions relative to well-characterized sources, we constrain the plausible
ranges of unknown parameters and guide targeted observational searches.
These results demonstrate that MST-based methods offer an interpretable and flexible framework
for classifying and exploring MSP populations, with direct applicability to current and future pulsar
surveys.

27 Optical and X-ray Polarised Emission from Transitional Millisecond Pulsars - Cristina Baglio

Transitional millisecond pulsars (tMSPs) switch between a rotation-powered radio pulsar state and
an X-ray active state in which an accretion disc is present. What powers the emission in this active
state (pulsar rotation, mass accretion, or some combination of both) has been debated for over a
decade. Multiwavelength polarimetry offers a powerful tool to break this degeneracy.
In this talk, I will present results from the first multiwavelength polarimetric campaign of the prototypical
tMSP PSR J1023+0038, combining simultaneous observations with IXPE, the VLT, and
the VLA. I will show how these results provide direct evidence that both the polarised and pulsed
emission arise from synchrotron radiation at the boundary region where the pulsar particle wind
collides with the inner accretion disk.
Moreover, the same multiwavelength polarimetric campaign also allowed us to shed new light on
the mechanisms powering the flaring mode.
Looking ahead, fast-time resolved polarimetry with photon-counting optical instruments, combined
with the expected capabilities of eXTP in the X-rays, promises to take this field to a new level,
enabling us to probe the geometry and emission mechanisms of these systems on the timescales
of the pulsations.

Baglio

28 Time variability of X-ray polarization as a tool to investigate the geometry of the accretion flow in NS-LMXBs - Alessandro Di Marco

Millisecond pulsars (MSPs) are the quickest-spinning neutron stars (NSs) known. The X-ray pulsations
produced by hot spots on the NS surface encode information about the mass and size of
these compact objects; in this context, accreting MSPs (AMSPs) are among the best targets because
they become relatively bright X-ray sources during X-ray outbursts. Yet, breaking the degeneracy
among the many parameters that shape their pulse profiles was not possible due to the limited
effective area of X-ray instruments flown to date. X-ray polarimetry is a potential game-changer,
providing an independent measure of the geometry of the hot spots. In this talk, the IXPE results
obtained by observations of the two AMSPs, SRGA J144459.2-604207 and SAX J1808.4-3658, are
presented. Although the spin-phase variability of the normalized Stokes parameters is not highly
significant, applying the relativistic rotating vector model, the geometry of the hot spots was determined
for SRGA J144459.2-604207. On the other hand, unexpected time variability on longer
time scales has been observed in non-pulsating NS-LMXBs, particularly in response to intensity
drop during dips. These variations could be explained by a possible contribution from an extended
accretion-disk corona or a disk-wind component. In this talk, results for GX 13+1 and XTE J1701-
462 are presented, along with possible constraints on the geometry of the extended accretion-disk
corona or the disk wind.

Di Marco

29 New insights from X-ray polarimetry into emission geometry of Z sources - Fabio La Monaca

X-ray timing and broadband spectroscopy have been the main methods for studying the rich and
complex phenomenology of neutron stars with weak magnetic fields for a long time. Nowadays,
X-ray polarimetry offers the missing, independent set of observables that are directly relevant to
comprehending the geometry and nature of the emission regions: polarization degree and angle.
This allows breaking degeneracies because different geometries and physical conditions might
yield equivalent spectral and timing signatures. Some of the main results obtained in this frame
by using the Imaging X-ray Polarimetry Explorer (IXPE) will be reported in this talk. In particular,
IXPE observations of Z sources have shown that the polarization properties are not constant but
evolve significantly with spectral state. A common trend has emerged in all the Z sources studied
so far: the polarization decreases markedly during the transition from the horizontal branch (HB)
to the normal branch (NB), indicating substantial changes as the accretion flow evolves.

La Monaca

30 X-ray Polarimetry as a Geometrical Probe of Neutron-Star Low-Mass X-ray Binaries - Alessio Anitra

Fast X-ray timing of transient sources has historically been the domain of large, monolithic space
observatories. Here we show that, for the first time, sub-millisecond timing can be achieved with
a wide-field nanosatellite instrument operating without collimation. Using data from the HERMES
X/γ-ray spectrometer on board the SpIRIT CubeSat, we demonstrate sub-millisecond timing
capabilities in the keV range.
This performance is enabled by an algorithmic framework based on cross-correlation techniques
applied to photon Time-of-Arrival (ToA) data, combined with adaptive binning and Monte Carlo
methods to robustly recover fast variability in the low-count regime typical of CubeSat observations.
These results establish CubeSats as a viable platform for fast X-ray photometry, opening
new opportunities for timing studies of both transient and persistent high-energy sources.
This methodology naturally enables cross-band lag measurements and delay studies between optical
and high-energy emission, as targeted by the Optical GEMS project.

Anitra

16:45 Coffee break

31 Unveiling the accretion-ejection interplay in black hole transients with multi-band variability - Alessio Marino

Black Hole Transients (BHTs) are binary systems composed of a stellar-mass BH and a companion
star typically less massive than the Sun. These sources emit over a wide range of electromagnetic
frequencies, from radio to gamma-rays. Their multi-band spectral energy distribution is dominated
by two main components: an accretion disk, emitting from the optical to the X-rays, and a jet,
emitting from radio to NIR. Despite being apparently opposite phenomena, matter inflows (in the
disk) and matter outflows (in the jet) are profoundly intertwined, to the extent that we can call
them two faces of the same medal. Unfortunately, the details of such a coupling are far to be fully
understood, including the physical engine that launches jets and the role that the mass and/or
spin of the BH (if any) plays in this scenario. In this context, multi-band observations, especially
in the optical-NIR band, as it lies at the interface between jet and disk emission, are fundamental
to solve these questions. Fast optical photometry has in particular emerged as a powerful tool
to disentangle these components in the time domain, providing unique insights into the causal
connection between inflow and outflow. IIn this talk, I will review our current understanding of
the disk–jet connection in black hole transients and highlight how fast optical photometry has
advanced, and will continue to advance, the field, providing access to physical source scales, jet
speeds, the origin of quasi-periodic oscillations, and the coupling between accretion and ejection.

Marino

32 Sub-second multi-wavelength emission from X-ray binaries - Federico Vincentelli

Accretion onto compact objects is a fundamental process in astrophysics, powering some of the
brightest sources in the sky. It also drives powerful outflows that can shape the surrounding
environment, making accretion central to the broader inflow–outflow cycle. In recent years, subsecond
optical and infrared observations of accreting X-ray binaries have opened a new window
onto this connection, uncovering a rich variety of previously inaccessible phenomena. For example,
the discovery of a 0.1 s lag between the X-ray and optical/infrared variable emission has
provided a powerful probe of the inner jet physics, while also placing constraints on the jet speed
and inclination. In this talk, I will review the current observational picture and discuss the main
physical components responsible for rapid optical/infrared variability. I will then show how new
data are placing stringent constraints on the properties of jets and accretion discs, highlighting the
need for more realistic physical models.

Vincentelli

33 Quickly variable optical polarization and e-SiFAP - Francesco Leone

e-SIFAP has been designed to study sources characterized by quickly-variable polarized emission.
After a brief overview of polarimetric measurement methods, we provide a detailed description of
e-SIFAP. The discussion focuses on its unique capabilities, particularly its ultra-high temporal resolution
and its broad spectral coverage. e-SIFAP operates across an extensive range that includes
the full SLOAN filter set and extends into the near-infrared with the J and H bands. e-SIFAP is a
full-Stokes polarimeter designed for high-fidelity measurements. The instrument is capable of measuring
the Stokes Q and U parameters simultaneously, while the Stokes V parameter is acquired
independently. Both measurement modes benefit from the implementation of the dual-beam swapping
technique. This method is essential to cancel out instrumental polarization and systematic
errors, allowing us to accurately measure even the most subtle linear and circular polarization
signals.
While e-SIFAP is crucial for probing rotating magnetospheres, mapping the underlying magnetic
field geometry requires solving the radiative transfer equation in polarized light within the magnetized
medium. This step is necessary to correctly interpret the observed polarization signatures.
In this context, the knowledge gained from modeling the magnetic Ap stars is essential, as their
magnetospheres have been extensively studied through phase-resolved Stokes parameters in both
the visible and radio bands.

Leone

Friday 15 May

34 On searches for optical pulsations from NSs in binaries - Riccardo La Placa

Timing neutron star systems is crucial to follow the evolution of binary and the interplay between
the two objects, test strong-field general relativity, and study the neutron star itself. Millisecond
pulsars in particular are ideal targets to probe the strong interaction at supranuclear densities
and search for continuous gravitational wave (CW) sources. I will review the main techniques
behind the efforts to find these pulsations across the EM spectrum, comparing their main use cases.
Moreover, I will describe our implementation of a semi-coherent algorithm paricularly suited for
binary systems for which standard blind searches would prove computationally unfeasible. I will
explain the main workflow of the technique and show our first results in its application to optical
data of one of the strongest candidates for CW emission, Scorpius X-1, using the fast photometer
SiFAP2 mounted at the TNG.

La Placa

35 Genetic and evolutionary algorithms for finding pulsations - Nicolò Pinciroli

Detecting periodic signals from accreting pulsars, including millisecond pulsars, is challenging due
to distortions introduced by orbital motion in binary systems. This talk presents the use of evolutionary
algorithms, specifically genetic algorithms, to efficiently explore the orbital parameter
space and reconstruct a corrected light curve, reducing computational cost compared to exhaustive
grid searches. This approach is particularly useful for millisecond pulsars, whose search is both
resource- and time-consuming when using traditional grid-search methods.

Pinciroli

36 Fast X-ray Timing with CubeSats: An Adaptive Approach to the Low-Count Regime - Wladimiro Leone

Fast X-ray timing of transient sources has historically been the domain of large, monolithic space
observatories. Here we show that, for the first time, sub-millisecond timing can be achieved with
a wide-field nanosatellite instrument operating without collimation. Using data from the HERMES
X/γ-ray spectrometer on board the SpIRIT CubeSat, we demonstrate sub-millisecond timing
capabilities in the keV range.
This performance is enabled by an algorithmic framework based on cross-correlation techniques
applied to photon Time-of-Arrival (ToA) data, combined with adaptive binning and Monte Carlo
methods to robustly recover fast variability in the low-count regime typical of CubeSat observations.
These results establish CubeSats as a viable platform for fast X-ray photometry, opening
new opportunities for timing studies of both transient and persistent high-energy sources.
This methodology naturally enables cross-band lag measurements and delay studies between optical
and high-energy emission, as targeted by the Optical GEMS project.

Leone

10:25 Coffee break

37 Fast optical photometry with TNG@SiFAP2, e-SiFAP@TNG and SiFAP-SoFT@Loiano - Filippo Ambrosino

High-time-resolution detectors are playing a crucial role in advancing our understanding of neutron
stars and transient phenomena, enabling precise measurements of their properties. SiFAP2,
the high-speed optical photometer permanently mounted at the 3.6m INAF Telescopio Nazionale
Galileo, obtained significant breakthroughs discovering optical pulsations of three millisecond pulsars
in different luminosity states. SiFAP2 is also involved in multiwavelength campaigns on FRBs
to search for optical counterparts. In this talk, I will present the improvements applied to the instrument
from the point of view of detectors and timing devices within the “GEMS” project. I will
also present the enhanced e-SiFAP instrument, a 6-band Vis/SWIR fast photometer with added
polarimetric capabilities as well as the new SiFAP-SoFT photometer mounted at the 1.5m Loiano
Telescope especially conceived for simultaneous optical measurements targeted on FRBs and their
possible link with magnetars.

Ambrosino

38 Future of Aqueye+, Iqueye and synergies with SiFAP2 - Michele Fiori - REMOTE

Aqueye+ and Iqueye are two astronomical instruments designed to exploit the highest time resolution
achievable using modern electronics and detectors in the optical band. They are used
to explore the quantum properties of light (e.g., through intensity interferometry measurements)
and to enable the study of rapid optical variability down to the sub-millisecond level. This extreme
timing capability is crucial for characterising certain important classes of astrophysical sources,
in particular (millisecond) pulsars and X-ray binaries. Along with SiFAP2, Aqueye+ and Iqueye
are among the most powerful optical timing facilities currently in operation. In this presentation,
I will highlight some of the most recent results that we have obtained, emphasising the multiinstrument
and multi-wavelength synergies. I will also present future instrumental developments
of this project and the scientific prospects for the coming years, including the possibility of offering
Iqueye at Gemini South.

Fiori

39 Einstein Probe and Synergies with Fast Optical Photometry - Francesco Coti Zelati

Einstein Probe, launched in January 2024, is opening a new window on the transient X-ray Universe
through the wide-field capabilities of its Wide-field X-ray Telescope and the rapid, sensitive
follow-up provided by the Follow-up X-ray Telescope. In this talk, I will present the main characteristics
of the mission and its role in the discovery and characterization of X-ray transients, with
a focus on accreting compact objects. I will then discuss the synergy between Einstein Probe and
fast optical photometry, highlighting simultaneous Einstein Probe and TNG/SiFAP2 observations
of the accreting millisecond pulsar SAX J1808.4-3658 during its 2025 outburst. These data include
a thermonuclear burst observed simultaneously in X-rays and optical wavelengths, as well as coordinated
observations probing the connection between X-ray and optical pulsations across different
stages of the outburst.

Coti Zelati

40 The new SOXS instrument for the ESO NTT: overview and synergies with fast optical photometry - Paolo D'Avanzo

SOXS (Son Of X-Shooter) will be a unique spectroscopic facility built by an international consortium
for the ESO-NTT 3.6-m telescope in La Silla (Chile). The design foresees a single-object,
high-efficiency spectrograph with a resolution-slit product of ~ 4,500, capable of simultaneously
observing the complete spectral range 350 - 2000 nm with a good sensitivity and with imaging
capabilities in the visible band (ugrizY). It is designed to observe all kind of transients and variable
sources over a wide range of time scales with a highly flexible schedule maintained by the
consortium, based on the Target of Opportunity concept. SOXS is going to be a fundamental spectroscopic
partner for any kind of imaging survey, becoming one of the premier transient follow-up
instruments in the Southern hemisphere.

D'Avanzo

41 TDT and synergies with fast optical photometry - Simone Scaringi - REMOTE

Scaringi

42 Quantum Gravity With Pulsars - Luciano Burderi

Burderi