arXiv subjects

Christian Malacaria

Publications and source records attributed to Christian Malacaria.

At least 19 recordsLinked to original sources

arXiv2026

Searching for optical pulsations from spider millisecond pulsars with the fast photometer SiFAP2

Optical pulsations from millisecond pulsars (MSPs) have recently been discovered from a transitional MSP and an accreting MSP. Evidence for optical pulsations has also been found for a rotation-powered redback MSP. Observations suggest that optical pulsed emission is produced by different mechanisms depending on whether an accretion disk is present in the system or not. We explore a sample of rotation-powered MSPs in close binaries (P$_{orb}<1\,$day, redbacks and black widows), searching for optical periodic signals with the $SiFAP2$ photometer mounted on the $Telescopio\ Nazionale\ Galileo$ (TNG). We perform pulsation searches using the epoch folding method with the most updated (radio or gamma-ray) orbital solution for each system, employing already published solutions or recent observations with Nan\c{c}ay, the Giant Metrewave Radio Telescope, the Parkes Observatory, and $Fermi$-LAT. We update or confirm radio and gamma-ray ephemerides for five of the presented systems. No optical pulsations were detected with a significance exceeding a $3\sigma$ confidence level in any of our searches, despite often achieving sensitivities on the verge of the instrument's limit. This places correspondingly stringent upper limits on the systems' optical pulsed magnitude and conversion efficiency. Our upper limits are consistent with the pulsed optical luminosity of optical pulsations from the redback PSR J2339-0533. Compared to accreting and transitional MSPs, redback and black widow systems show much lower efficiency in converting their spin down power into pulsed optical emission, supporting the recently advanced idea that the accretion disk around the neutron star plays a role in accelerating the particles responsible for producing bright optical pulsed emission.

astro-ph.HE↗

arXiv2026

17 Yr of Magnetar Bursts Observed with the Fermi Gamma-ray Burst Monitor

The Fermi Gamma-ray Burst Monitor (GBM) has been in operation for over 17 years, during which it has observed more than a thousand bursts from soft gamma repeaters (SGRs), also known as magnetars. Serving as a laboratory for extreme physics, magnetars are a sub-family of neutron stars characterized by extreme magnetic field strength, observed through a combination of persistent and short transient emission across the electromagnetic spectrum. We present the comprehensive GBM catalog of SGR short bursts which supersedes the 5-year catalog of Collazzi et al. 2015. The new catalog contains 1254 SGR short bursts observed over 17 years, providing the longest uninterrupted, high-sensitivity all-sky monitoring of magnetar bursts with unprecedented spectral and temporal resolution. Our catalog contains bursts from 17 unique Galactic sources, with major contributions by bursts from SGR J1935+2154 and SGR J1550-5418. We present overall characteristics of these bursts, such as the durations, spectral parameters for various photon models, fluxes, as well as their comparison with recently published catalogs of other missions and the previous GBM magnetar catalog. The machine readable catalog, as well as burst spectra and response files are made publicly available for the community.

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arXiv2026

Pulse profiles of accreting neutron stars - A review of analysis methods, observations, and theoretical models

X-ray pulsars are highly magnetized ($B\sim 10^{12}$ G) neutron stars accreting from a donor star. Their characteristic X-ray emission arises from accreted material decelerated from relativistic velocities near the magnetic poles of the neutron star. As our line of sight onto the magnetic poles changes with the rotation of the neutron star, the X-rays are periodically modulated, resulting in X-ray pulsations. The shape of the pulse profiles depends on the physics of the interaction between the bright X-rays from the magnetic poles with the infalling matter, the location of the magnetic poles on the neutron star with respect to its spin axis, and on the properties of the space-time around the neutron star. In this review we give a pedagogical introduction to the accretion mechanisms operating in the various types of accreting neutron star systems and the observational techniques used to characterize the pulse profiles. We summarize how the pulse profiles depend on X-ray luminosity and energy and discuss the attempts to connect theoretically these observables with the physical accretion mechanisms. We conclude with an outline of future observational needs and further developments for theoretical models of magnetic accretion.

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arXiv2026

Pulse profile modelling of the 2024 outburst of the accreting millisecond pulsar SRGA J144459.2-604207

Pulse profile modelling via relativistic ray-tracing can constrain the system parameters of neutron stars, notably their mass and radius. Among these objects, accreting millisecond pulsars (AMPs) are promising targets, because they are bright in X-rays and their potentially polarized radiation can lead to complementary constraints on the emission geometry. We perform combined analysis of NICER and IXPE observations of the recently discovered the 448-Hz pulsar SRGA J144459.2-604207, with IXPE providing X-ray polarization information. NICER and IXPE jointly favour a large mass and radius for our best-fitting model, for which the neutron star has two independent hotspots. The primary hotspot is centered near the northern rotational pole, the secondary in the southern hemisphere, and the observer inclination is in the range 50-75 degrees. The primary hotspot is large (up to half the surface area) and contributes the majority of the non-pulsed X-rays, while the secondary is hotter and the major contributor to the overall pulse profile shape. However, many parameters are inferred to be near the prior bounds, which could indicate that the model does not adequately account for important physics. Furthermore, we tested several different methodologies for joint analysis of the two data sets: the results are sensitive to the method used, something that merits further study with synthetic data. In the future, we expect simultaneously recorded data will lead to improved parameter constraints, especially when multi-band and polarized data are combined.

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arXiv2026

Energy-resolved pulse profiles of Vela X-1: cross-calibrating XMM-Newton and NuSTAR to trace spectral features

Pulse profiles probe the emission geometry of accreting X-ray pulsars, but their observed shapes may depend on instrumental response and observational setup. The pulsed fraction spectrum provides a compact spectro-timing observable that can both trace localized spectral features and serve as a quantitative cross-calibration diagnostic. We assess the consistency of energy-resolved pulse profiles obtained with simultaneous XMM-Newton/EPIC-pn and NuSTAR/FPM observations of Vela X-1, and investigate the broadband pulsed fraction spectrum as a diagnostic of spectral features from 1 to 70 keV. We construct energy-phase matrices for both instruments and derive pulsed fraction spectra after carefully accounting for instrumental and observational effects. We quantify the residual systematics in the overlapping 3-10 keV band. We then model the broadband pulsed fraction spectra phenomenologically and search for timing signatures of spectral features. After correcting for instrumental effects, the pulsed fraction spectra derived strictly over the common exposure intervals of the two instruments agree within 5% in their overlapping 3-10 keV range. Remaining discrepancies larger than 5% are confined to the iron-line region and can be attributed to the different energy resolutions of the two instruments. The broadband pulsed fraction spectrum reveals significant localized features corresponding to known emission lines in the soft band and to cyclotron resonant scattering features. Orbital-phase-resolved modeling of the EPIC-pn pulsed fraction spectrum shows that the soft-band features depend strongly on the equivalent absorption column, with emission-line signatures becoming progressively suppressed during highly absorbed intervals. The pulsed fraction spectrum serves both as a quantitative cross-calibration diagnostic and as a powerful spectro-timing diagnostic.

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arXiv2026

A simple relation: Neutron star magnetic field strength and spectral shape at low mass accretion rates

The X-ray spectra of neutron stars with moderate magnetic fields ($B\sim 10^{12}$ G) in high-mass X-ray binaries (HMXBs) at low X-ray luminosities ($L_\mathrm{X}\lesssim 10^{35}$ erg/s) are characterized by a double humped shape. This shape has been explained either as the radiation from a two-temperature magnetized atmosphere, where thermal radiation dominates at soft X-rays below about 10 keV, and cyclotron radiation with an imprinted cyclotron line dominates at high energies, or by the complex redistribution of primary X-rays in a structured atmosphere. The theoretical explanations of the double humped structure predict the spectra to depend on the magnetic field. We aim to connect the model predictions with observations. We analyzed archival NuSTAR observations of four HMXBs consisting of a neutron star and a Be star (BeXRBs), with known magnetic fields at luminosities low enough to show the characteristic double-hump spectrum. We modeled these spectra empirically and derived a relation between the energy of the intersection of the two humps and the magnetic field strength. In a second step, we tested whether this correlation is supported by fitting synthetic spectra simulated with the physically self-consistent polcap model. We find a linear correlation between the magnetic field strength and the intersection energy for the real BeXRB NuSTAR spectra and polcap-based simulated NuSTAR spectra alike. The effect of the magnetic field on spectral formation results in an observable correlation between the field strength and spectral shape. This derived positive correlation between intersection energy and magnetic field strength also allowed us to roughly estimate the magnetic field strength. Additional observations of XRBs and dedicated modeling efforts will be necessary to determine whether this approach is valid beyond the B-field range that was tested in this work.

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arXiv2026

The elusive cyclotron line in 4U 1901+03: hidden, yet present

Context. Cyclotron resonant scattering features (CRSFs) in accreting X-ray pulsars are often difficult to detect, especially when shallow or variable. Recent studies have shown that combining spectral and timing analyses enhances their detectability. Aims. We investigated the evolution of energy-resolved pulse profiles of the X-ray pulsar 4U 1901+03 during its 2019 giant outburst, focusing on the 30-40 keV range where there have been disputed claims of a cyclotron line detection. Methods. We analysed four NuSTAR observations of 4U 1901+03 at different luminosities. We studied energy-resolved pulse profiles using harmonic decomposition, cross-correlation analysis, energy-phase maps, and pulsed-fraction spectra. We also used Bayesian spectral modelling to assess the presence and properties of a cyclotron line. Results. We detected significant spectral-timing variability in the 30-40 keV range, which becomes stronger at lower luminosities. We found a pronounced drop in the pulsed fraction near 35 keV only in the lowest accretion state and in the first harmonic of one intermediate-luminosity observation. Adopting a Bayesian informative approach, we find evidence for a cyclotron line in all examined energy spectra, with an average centroid energy of E_cyc approx 32 keV (varying by only 1.6%), and an anti-correlation between line depth and luminosity. Conclusions. We show that a combined spectral-timing approach is more sensitive than phase-averaged spectroscopy to shallow cyclotron features. The luminosity-dependent evolution of pulse profiles and cyclotron line depth point to a drastic change in the emission geometry and accretion flow structure.

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arXiv2026

XMM-Newton Observations of Flares and a Possible Pulse Dropout in the Supergiant X-ray binary 4U 1909+07

We report on a pair of X-ray Multi-Mirror Mission (XMM-Newton) observations of the Supergiant X-ray binary 4U 1909+07, which were performed on 2021 October 3 and 2021 October 8, respectively. We measure the neutron star rotation period in each observation to be $\sim$602.62 s. This continues a long spin-up trend that has persisted since 2001 where the neutron star spin period was found to be $\sim$604.66 s. In our timing analysis, we observe strong variations in the amplitude of the 1--10 keV pulse profile as a function of time, and for the first time we find a low flux interval extending for a single pulse period in which pulsations are no longer detected. We interpret this low flux interval as a pulse dropout similar to those observed in Vela X-1 and GX 301-2, which were each explained by a low-density cavity in the wind driving the propeller effect. In our time-resolved spectral analysis, we observed the spectral continuum, which can be described as an absorbed power law modified by a high-energy cutoff, to significantly soften during the pulse-dropout phase. No evidence of an increasing absorption column density was found. The observed softening in 4U 1909+07 also supports an interpretation that the observed pulse dropout may be driven by the propeller effect, but the quasi-spherical settling accretion regime cannot be ruled out.

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arXiv2025

GRB 250702B: Discovery of a Gamma-Ray Burst from a Black Hole Falling into a Star

Gamma-ray bursts are the most luminous electromagnetic events in the universe. Their prompt gamma-ray emission has typical durations between a fraction of a second and several minutes. A rare subset of these events have durations in excess of a thousand seconds, referred to as ultra-long gamma-ray bursts. Here, we report the discovery of the longest gamma-ray burst ever seen with a ~25,000 s gamma-ray duration, GRB 250702B, and characterize this event using data from four instruments in the InterPlanetary Network and the Monitor of All-sky X-ray Image. We find a hard spectrum, subsecond variability, and high total energy, which are only known to arise from ultrarelativistic jets powered by a rapidly-spinning stellar-mass central engine. These properties and the extreme duration are together incompatible with all confirmed gamma-ray burst progenitors and nearly all models in the literature. This burst is naturally explained with the helium merger model, where a field binary ends when a black hole falls into a stripped star and proceeds to consume and explode it from within. Under this paradigm, GRB 250702B adds to the growing evidence that helium stars expand and that some ultra-long GRBs have similar evolutionary pathways as collapsars, stellar-mass gravitational wave sources, and potentially rare types of supernovae.

astro-ph.HE↗

arXiv2025

A semi-coherent search for optical pulsations from Scorpius X-1

The emission of continuous gravitational waves (CWs) possibly explains why pulsars spinning with a period shorter than a millisecond have not been observed so far. Neutron stars accreting mass at the highest rates are the most promising targets for a search for CWs, because a strong emission of gravitational waves is required to balance the torque exerted by mass accretion onto the neutron star. Detecting coherent pulsations in the electromagnetic emission maximizes the search sensitivity, but has so far not been successful for most of the brightest accreting neutron stars. Here, we present the first search for pulsations in the optical band from the brightest accreting neutron star known, Sco X-1. To this end, we tailored semi-coherent search strategies to data obtained over four years, for a total of $\sim$$56$ ks, by the SiFAP2 fast photometer mounted at the Telescopio Nazionale Galileo (TNG). These searches are especially suited to analysing long observations of systems for which only limited knowledge on the orbital parameters is available, and involve joining coherent analyses on shorter segments without connecting the spin phase between them. The large count rates afforded by an optical telescope and the efficiency of the search strategy employed allowed us to set an upper limit of $9 \times 10^{-5}$ to the pulsed amplitude, which is lower by a factor of four with respect to previous searches in the X-ray band. We also show that the application of semi-coherent searches to SiFAP2 observations of the first detected optical millisecond pulsar, PSR J1023+0038, could have preceded its detection in the radio band. These results highlight the role played by high-time-resolution optical observations in performing deep searches of quickly rotating pulsars.

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arXiv2025

Detection of low-luminosity X-ray pulsations from the accreting millisecond pulsar IGR J17511-3057: An ever-thinning thread between bright accretion and sub-luminous states

After nearly a decade in quiescence, the accreting millisecond pulsar IGR J17511$-$3057 displayed a new outburst on 2025 February 11, its third since discovery, following previous activity in 2009 and 2015. We report on an XMM-Newton Target of Opportunity observation performed on 2025 March 4, more than twenty days after the outburst onset. From the X-ray spectrum - well described by an absorbed Comptonization model - we estimated an unabsorbed 0.5$-$10 keV luminosity of $L_X \sim 7 \times 10^{33} \, \mathrm{erg \, s^{-1}}$ (assuming a source distance equal to the upper limit of $6.9$ kpc). To place this in context, we analyzed an archival Chandra observation performed in 2019, which yielded a quiescent luminosity of $L_\mathrm{X,q} \sim 2 \times 10^{32} \, \mathrm{erg \, s^{-1}}$ in the same energy band. Although this comparison indicates that the source was still well above its quiescent level during the XMM-Newton observation, the estimated low luminosity during the late stage of the 2025 outburst would typically place the source in the propeller regime. Nevertheless, we unexpectedly detected coherent X-ray pulsations with an amplitude peaking at $\sim$42% in the 0.3$-$3 keV band. We also observed a spectral softening compared to the early stages of the outburst. Finally, we report a 3$\sigma$ upper limit of 60 $\mu$Jy beam$^{-1}$ on the source flux density at 5.5 GHz from ATCA observations acquired on 2025 April 12, following a decline of the accretion activity, as indicated by our analysis of NICER data from 2025 March 15, which revealed no significant X-ray pulsations at a luminosity level of $L_X \sim 1 \times 10^{34} \, \mathrm{erg \, s^{-1}}$. We discuss our findings in the context of other accreting millisecond pulsars and draw comparisons with transitional systems in the sub-luminous disk state.

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arXiv2025

Probing multi-band variability and mode switching in the candidate transitional millisecond pulsar 3FGL J1544.6-1125

We present the most extensive high-time resolution multi-band campaign to date on the candidate transitional millisecond pulsar (tMSP) 3FGL J1544.6-1125 in the sub-luminous disk state, with coordinated observations from the radio to the X-ray band. While XMM-Newton and NuSTAR X-ray light curves exhibit the characteristic high- and low-mode bimodality, the source faintness prevents firm evidence for similar bimodality in the ultraviolet and near-infrared light curves, presented here for the first time. A re-analysis of archival XMM-Newton/OM data reveals an optical flare without an X-ray counterpart, likely originating from the outer accretion disk or the companion star. During our observations, no radio emission was detected, with a 3$σ$ flux density upper limit of 8 $μ$Jy at 6 GHz. While past works have already reported radio variability in the source, this limit is a factor of 3.5 below the average value measured in 2019 in similar conditions, underscoring significant radio variability despite the relatively stable X-ray flux. Simultaneous optical light curves in five filters with GTC/HiPERCAM revealed flickering and dipping activities that resemble the observed X-ray variability, along with a reddening trend at lower fluxes. The latter is consistent with discrete mass ejections that disrupt the inner flow and reduce both X-ray and optical fluxes, thereby driving the high-to-low-mode switches. This suggests a common origin for most optical and X-ray emission at the boundary region between the pulsar wind and the inner disk, as also supported by our modelling of the spectral energy distribution in the high mode. Overall, our findings reinforce the mini-pulsar nebula picture for tMSPs in the sub-luminous state and demonstrate how coordinated, high-time resolution, multi-wavelength campaigns are essential to probe the processes governing rapid mode switches in these systems.

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arXiv2025

A deep X-ray and UV look into the reflaring stage of the accreting millisecond pulsar SAX J1808.4-3658

We present an X-ray and UV high-time-resolution monitoring of the final flaring phase of the 2022 outburst of the AMSP SAX J1808.4-3658, based on simultaneous XMM-Newton and HST observations. The uninterrupted coverage provided by XMM-Newton enabled a detailed characterization of the spectral and temporal evolution of the source X-ray emission, as the flux varied by approximately 1 order of magnitude. We detected coherent X-ray pulsations during the whole X-ray observation, down to a 0.5-10 keV luminosity of $L_{X(low)0.5-10} \simeq 6.21^{+0.20}_{-0.15}\times 10^{34} d^2_{3.5}erg/s$, among the lowest ever observed in this source. At the lowest flux levels, we observed significant variations in pulse amplitude and phase. These variations were anticorrelated with the X-ray source flux. We found a sharp phase jump of $\sim 0.4$ cycles, accompanied by a doubling of the pulse amplitude and a softening of the X-ray emission. We interpreted changes in the X-ray pulse profiles as drifts of emission regions on the neutron-star surface, driven by an increase in the inner-disk radius when the mass-accretion rate decreased. The dependence of the pulse phase on the X-ray flux was consistent with a magnetospheric radius scaling as $R_{m} \propto \dot{M}^{\Lambda}$, with $\Lambda = -0.17(9)$, in broad agreement with theoretical predictions. Simultaneous 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. The measured pulsed UV luminosity, $L_{pulsed}^{UV}=1.1(3) \times 10^{32}erg/s$, was consistent with that observed during the 2019 outburst. Such a UV luminosity exceeds the predictions of standard emission models, as further confirmed by the shape of the pulsed spectral energy distribution.

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arXiv2025

How the spin-phase variability of cyclotron lines shapes the pulsed fraction spectra: insights from 4U 1538-52

We study the energy-dependent pulse profile of 4U 1538-52 and its phase-dependent spectral variability, with emphasis on the behavior around the cyclotron resonant scattering feature at around 21 keV. We analyze all available NuSTAR observations of 4U 1538-52. We decompose energy-resolved pulse profiles into Fourier harmonics to study their energy dependence. Specifically, we compute pulsed fraction spectra, cross-correlation and lag spectra, identifying discontinuities and linking them to features in the phase-averaged spectra. We perform phase-averaged and phase-resolved spectral analyses to probe spectral variability and its relation to pulse profile changes. Finally, we interpret our findings via physical modeling of energy- and angle-dependent pulse profile emission, performing radiative transfer in a homogeneous slab-like atmosphere under conditions relevant to 4U 1538-52. The emission is projected onto the observer's sky plane to derive expected observables. In contrast to the dips in pulsed fraction spectra observed in other sources (e.g., Her X-1), we find a broad bump near the cyclotron resonance energy in 4U 1538-52. This increase is driven primarily by phase-dependent spectral variability, especially by strong variations in cyclotron line depth across different phase intervals. We interpret the observed contrast between dips and bumps in various sources as arising from phase-dependent variations of cyclotron line depth relative to the phase-modulated flux. We model the X-ray emission from an accreting neutron star and find that our simulations indicate high values of both the observer's inclination and the magnetic obliquity, along with a 10-15 degrees asymmetry between the locations of the magnetic poles. Assuming this geometry, we satisfactorily reproduce the observed pulse profiles and introduce general trends in the observables resulting from the system's geometry.

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arXiv2025

Disk reflection and energetics from the accreting millisecond pulsar SRGA J144459.2-604207

Accreting millisecond pulsars (AMSPs) are excellent laboratories to study reflection spectra and their features from an accretion disk truncated by a rapidly rotating magnetosphere near the neutron star surface. These systems also exhibit thermonuclear (type-I) bursts that can provide insights on the accretion physics and fuel composition. We explore spectral properties of the AMSP SRGA J144459 observed during the outburst that recently led to its discovery in February 2024. We aim to characterize the spectral shape of the persistent emission and to analyze type-I bursts properties employing XMM + NuSTAR overlapping observations taken during the most recent outburst. We perform spectral analysis of the time-averaged persistent (i.e., non-bursting) emission. For this, we first employ a semi-phenomenological continuum model made of a dominant thermal Comptonization plus two thermal contributions. A separate fit has also been performed employing a physical reflection model. We also perform time-resolved spectral analysis of a type-I burst employing a blackbody model. We observe a broadened iron emission line, thus suggesting relativistic effects, supported by the physical model accounting for relativistically blurred reflection. The resulting accretion disk extends down to 6 gravitational radii, inclined at ~$53^{\circ}$, and only moderately ionized (log$ξ\simeq2.3$). We observe an absorption edge at ~9.7 keV that can be interpreted as an Fe XXVI edge blueshifted by an ultrafast ($\simeq0.04$c) outflow. Our broadband observations of type-I bursts do not find evidence of photospheric radius expansion. The burst recurrence time shows a dependence on the count rate with the steepest slope ever observed in these systems. We also observe a discrepancy of ~3 between the observed and expected burst recurrence time, which we discuss in the framework of fuel composition and high NS mass scenarios.

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arXiv2025

Energy-resolved pulse profile changes in V 0332+53: Indications of wings in the cyclotron absorption line profile

We aim to investigate the energy-resolved pulse profile changes of the accreting X-ray pulsar V 0332+53, focusing in the cyclotron line energy range, using the full set of available NuSTAR observations. We applied a tailored pipeline to study the energy dependence of the pulse profiles and to build the pulsed fraction spectra (PFS) for the different observations. We studied the profile changes also using cross-correlation and lag spectra. We re-analysed the energy spectra to search for links between the local features observed in the PFS and spectral emission components associated with the shape of the fundamental cyclotron line. In the PFS data, with sufficiently high statistics, we observe a consistent behaviour around the cyclotron line energy. Specifically, two Gaussian-shaped features appear symmetrically on either side of the putative cyclotron line. These features exhibit minimal variation with source luminosity, and their peak positions consistently remain on the left and right of the cyclotron line energy. Associated with the cyclotron line-forming region, we interpret them as evidence for the resonant cyclotron absorption line wings, as predicted by theoretical models of how the cyclotron line profile should appear along the observer's line of sight. A phase-resolved analysis of the pulse in the energy bands surrounding these features enables us to determine both the spectral shape and the intensity of the photons responsible for these peaks in the PFS. Assuming these features correspond to a spectral component, we used their shapes as priors for the corresponding emission components, finding a statistically satisfactory description of the spectra. To explain these results, we propose that our line of sight is close to the direction of the spin axis, while the magnetic axis is likely orthogonal to it.

astro-ph.HE↗

arXiv2024

Discovery of Polarized X-Ray Emission from the Accreting Millisecond Pulsar SRGA J144459.2-604207

We report on the discovery of polarized X-ray emission from an accreting millisecond pulsar. During a 10-day-long coverage of the February 2024 outburst of SRGA J144459.2-604207, the Imaging X-ray Polarimetry Explorer (IXPE) detected an average polarization degree of the 2-8 keV emission of 2.3% +/- 0.4% at an angle of 59° +/- 6° (East of North; uncertainties quoted at the 1$σ$ confidence level). The polarized signal shows a significant energy dependence with a degree of 4.0% +/- 0.5% between 3 and 6 keV and < 1.5% (90% c.l.) in the 2-3 keV range. We used NICER, XMM-Newton, and NuSTAR observations to obtain an accurate pulse timing solution and perform a phase-resolved polarimetric analysis of IXPE data. We did not detect any significant variability of the Stokes parameters Q and U with the spin and the orbital phases. We used the relativistic rotating vector model to show that a moderately fan-beam emission from two point-like spots at a small magnetic obliquity ($\simeq$ 10°) is compatible with the observed pulse profile and polarization properties. IXPE also detected 52 type-I X-ray bursts, with a recurrence time $Δt_{rec}$ increasing from 2 to 8 h as a function of the observed count rate $C$ as as $Δt_{rec} \simeq C^{-0.8}$ We stacked the emission observed during all the bursts and obtained an upper limit on the polarization degree of 8.5% (90% c.l.).

astro-ph.HE↗

arXiv2024

Tentative Blazar Candidate EP240709A Associated with 4FGL J0031.5-5648: NICER and Archival Multiwavelength Observations

We report on follow-up observations of the recently discovered transient by the Einstein Probe, EP240709A, with the Neutron star Interior Composition Explorer (NICER). We also incorporated archival multiwavelength survey data from the Neil Gehrels Swift Observatory (X-ray), Gaia (optical), the Fermi Gamma-ray Space Telescope (gamma-ray), and the Wide-field Infrared Survey Explorer (infrared) to distinguish between blazars and stellar systems. We suggest that EP240709A is likely an active blazar.

astro-ph.HE↗