Christian Malacaria

We present an analysis of a long NuSTAR observation of the intermediate supergiant fast X-ray transient (SFXT) IGR J00370+6122, obtained around the peak of its orbital modulation. The observation has a net exposure ∼93 ks. During the observation, the source displayed a bright flaring event that was resolved into numerous short flares with durations of 10 − 10 ³ s. The 3 − 70 keV luminosity spanned a dynamic range of ∼200, from ∼10 ³⁴ to ∼1.8 × 10 ³⁶ erg s ⁻¹ ; the maximum was reached for only ∼40 s in total. The spectrum is well described by an absorbed double-Comptonisation model, with parameters consistent with those found in other X-ray pulsars accreting at similarly low luminosities. This is the first detection of a double-hump continuum in IGR J00370+6122, which extends this spectral state to the class of intermediate SFXTs. We searched for the previously reported ∼346 s and ∼674 s periodicities and find no significant signal at either period, which makes their interpretation as the neutron star spin period less likely. Using ∼21 yr of Swift /BAT monitoring, we refine the orbital period of the system to P _orb = 15.6654 ± 0.0008 d. Finally, we discuss the observed flaring activity in the framework of quasi-spherical settling accretion, which is expected at the observed luminosities.

Context. Optical pulsations from millisecond pulsars (MSPs) have recently been detected in 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. Aims. We explored 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). Methods. We performed pulsation searches using the epoch-folding method with the most up-to-date (radio or gamma-ray) orbital solution for each system. These solutions were based either on previously published works or recent observations from Nançay, the Giant Metrewave Radio Telescope, the Parkes Observatory, and Fermi -LAT. Results. 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 σ confidence level in any of our searches, despite achieving sensitivities close to 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. This supports the recent suggestion that the accretion disk around the neutron star plays a role in accelerating the particles responsible for producing bright optical pulsed emission.

The Fermi Gamma-ray Burst Monitor (GBM) has been in operation for over 17 yr, during which it has observed more than 1000 bursts from soft gamma repeaters (SGRs), also known as magnetars. Serving as a laboratory for extreme physics, magnetars are a subfamily 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 yr catalog of A. C. Collazzi et al. The new catalog contains 1254 SGR short bursts observed over 17 yr, 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, and 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.

Context. 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. Aims. 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. Methods. We constructed energy–phase matrices for both instruments and derived pulsed fraction spectra after carefully accounting for instrumental and observational effects. We quantified the residual systematics in the overlapping 3–10 keV band. We then modeled the broadband pulsed fraction spectra phenomenologically and searched for timing signatures of spectral features. Results. 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 the fundamental and harmonic cyclotron resonant scattering features at ∼25 and ∼55 keV. An orbital-phase-resolved modeling of the pulsed fraction spectrum of EPIC-pn shows that the soft band features strongly depend on the value of the equivalent absorption column, with emission line signatures becoming progressively suppressed during highly absorbed intervals. Conclusions. The pulsed fraction spectrum serves both as a quantitative cross-calibration diagnostic and as a powerful spectro-timing diagnostic. Its modeling provides independent constraints on spectral features, complementing traditional phase-averaged spectroscopy.

We present a comprehensive analysis of the X-ray spectrum of the highly obscured high-mass X-ray binary IGR J16318−4848 based on ∼250 ks Chandra/HETGS observations covering different orbital phases. We detected a strong narrow Fe K α emission line and interpret our finding as the iron line originating in the stellar wind close to the compact object. We tentatively identify a Compton shoulder associated with the Fe K α line. The ratio of the Compton-shoulder flux to that of the Fe K α core exhibits orbital-phase variability, reaching a maximum of ∼10% around orbital phase ϕ _orb ≃ 0.33. In addition, we do not detect ionized lines, implying that the plasma in the system is predominantly cold. We preliminarily report that the Fe K α line centroid exhibits an orbital modulation with an amplitude of 167 ± 32 km s ⁻¹ , adopting the best-known circular orbit in the source. This study presents the first detailed view of the high-resolution X-ray spectra of IGR J16318−4848 across different orbital phases, providing timely context for the newly taken XRISM observations of the source.

Context. The X-ray spectra of neutron stars with moderate magnetic fields ( B ∼ 10 ¹² G) in high-mass X-ray binaries (HMXBs) at low X-ray luminosities ( L _X ≲ 10 ³⁵ erg s ⁻¹ ) 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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 via our proposed 2-B-12 rule. Additional observations of XRBs and dedicated modeling efforts will be necessary to determine whether this approach is valid beyond the B -field range of a few 10 ¹² –10 ¹³ G that was tested in this work.

Superorbital modulations have been detected in the supergiant high-mass X-ray binary 4U 1538-52 using long-term monitoring with the Neil Gehrels Swift Observatory Burst Alert Telescope (BAT). The source also exhibits a long-term pulse-period evolution as seen with Rossi X-ray Timing Explorer, INTEGRAL, and Fermi Gamma-Ray Burst Monitor that appears uncorrelated with changes in its X-ray flux. To investigate the mechanisms causing these superorbital modulations and its possible dependence on pulse-period changes, we analyzed long-term monitoring with Swift-BAT and Monitor of All Sky X-ray Image Gas Slit Camera (MAXI-GSC) to construct dynamic power spectra and superorbital intensity profiles. In addition, we used pointed X-ray observations from Nuclear Spectroscopic Telescope Array and Neutron Star Interior Composition Explorer mission to investigate the pulsation and spectral properties across different superorbital and orbital phase intervals. We find the presence of superorbital modulations in the MAXI-GSC 2–20 keV lightcurves, consistent with the periodicity observed with the Swift-BAT lightcurves. However, no significant changes are detected in the pulse profiles or spectral parameters across different superorbital, orbital, or pulse-change intervals. This lack of spectral or timing variations with orbital and superorbital phases suggests that the mechanisms driving the observed superorbital modulation and pulse-period changes are likely associated with large-scale stellar wind structures, such as corotating interaction regions, within the stellar wind of the supergiant companion.

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 8, respectively. We measure the neutron star rotation period in each observation to be ∼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 ∼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.

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 ~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 × 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.

We present a detailed X-ray and UV high-time-resolution monitoring of the final reflaring phase of the 2022 outburst of the accreting millisecond pulsar SAX J1808.4−3658, based on simultaneous XMM-Newton and Hubble Space Telescope ( 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 one 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} ≃ 6.21 0.20 _−0.15 d ² _3.5 erg s ⁻¹ ; this is among the lowest ever observed in this source during the outburst state. 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 ∼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 ∝ Ṁ ^Λ , with Λ = −0.17(9), which is 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) × 10 ³² erg s ⁻¹ , was consistent with that observed during the 2019 outburst. Yet, such a UV luminosity exceeds the predictions of standard emission models, as further confirmed by the shape of the pulsed spectral energy distribution.