arXiv subjects

Luigi Stella

Publications and source records attributed to Luigi Stella.

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 Nançay, 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 $3σ$ 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

Peering through the dip: IXPE unveils the extended scattering environment of GX 13+1

Neutron star low-mass X-ray binaries feature complex accretion geometries, often including an accretion disk corona or disk winds. Here, a study of the highly inclined dipping source GX 13+1, using coordinated observations from the IXPE, NuSTAR, and Swift-XRT, is presented; this is the first time that such a campaign was conducted to monitor its periodic dip. Our analysis reveals significant variations in polarimetric properties tracking the dip. At the center of the dip, the polarization degree reaches a maximum of ${\sim}$9% (at more than $8σ$ confidence level). This is accompanied by a highly significant polarization angle rotation of roughly 60$^\circ$ when passing from the dip to the subsequent off-dip state. Furthermore, the dip state exhibits energy-dependent polarization. By modeling the polarization during the periodic dip, we constrain the geometry of the scattering medium. In the scenario of an oblate extended accretion disk corona (ADC), an equatorial radius at least 1.5 times its polar radius is needed with $τ{\sim}0.3$. Alternatively, the results can be modeled as scattering in a disk wind with a most probable electron density of $1.3\times10^{14}$ cm$^{-3}$ and an opening angle near 40$^\circ$, corresponding to an optical depth of ${\sim}0.2$. The obtained results highlight the crucial role of X-ray polarimetric data to unveil the geometry of the extended scattering environment surrounding the accretion disks, advancing our understanding of neutron star low-mass X-ray binaries.

astro-ph.HE↗

arXiv2026

Gravitational self-lensing of Fast Radio Bursts in neutron star magnetospheres: II. Applications to strong repeaters and the CHIME population

Paper I in this series introduced a model in which seed radio bursts produced by a hotspot anchored in the magnetosphere of a highly-magnetic neutron star (NS) are greatly amplified by strong gravitational self-lensing and thus give rise to Fast Radio Bursts (FRBs). Key features of the FRB population such as the observed dichotomy between repeating and non-repeating sources, their large luminosities and the high-energy power-law distribution of their bursts naturally arise in the model from the amplification dependence on the relative orientation of the rotation axis with respect to the hotspot and the line of sight. Here we compare the model predictions with Five-hundred-meter Aperture Spherical radio Telescope (FAST) data from repeaters and with the general population of FRBs. We find that the burst energy distribution from FRB 20121102A can be explained by assuming two antipodal hotspots in the NS magnetosphere, both producing seed bursts with the same log-normal energy distribution. This scenario implies a well-aligned system geometry, with the rotation axis, line of sight, and hotspot sites separated by $\lesssim 2$°. Similar constraints are found for FRB 20201124A and FRB 20220912A, and weaker ones for FRB 20190520B, owing to its smaller burst sample. We also show that precession of the NS rotation axis can explain the time evolution of the burst energy distribution from FRB 20121102A as well as its temporary disappearance. In application to a cosmological population of randomly-oriented sources the model predicts distance and fluence distributions of FRBs in good agreement with those from a completeness-selected subsample of the first CHIME/FRB catalogue, provided the energy distribution of seed bursts spans a range of ${\sim10^{35}-10^{38}}$ erg.

astro-ph.HE↗

arXiv2026

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.

astro-ph.HE↗

arXiv2025

X-ray Dips and Polarization Angle Swings in GX 13+1

We present the result from the April 2024 observation of the low-mass X-ray binary GX 13+1 with the Imaging X-ray Polarimetry Explorer (IXPE), together with NICER and Swift-XRT coordinated observations. Two light curve dips were observed; during them, the harder Comptonized spectral component was dominant and the polarization degree higher than in the softer, off-dip intervals. Through a joint analysis of the three IXPE observations, which also included the dip from the first observation, we demonstrate that the polarization properties varied in response to the intensity and spectral hardness changes associated with the dips. The polarization degree attained values up to ~4%. The polarization angle showed a swing of ~70° across the dip and off-dip states, comparable to the continuous rotation seen during the first IXPE observation. We discuss these results in the context of models for polarized emission from the accretion disk and the boundary/spreading layer on the neutron star surface. We also draw attention to the role that an extended accretion disk corona or disk wind can play in generating high polarization degrees and, possibly, swings of the polarization angle.

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

Investigating the interplay between the coronal properties and the hard X-ray variability of active galactic nuclei with NuSTAR

Active galactic nuclei (AGN) are extremely variable in the X-ray band down to very short timescales. However, the driver behind the X-ray variability is still poorly understood. Previous results suggest that the hot corona responsible for the primary Comptonized emission observed in AGN is expected to play an important role in driving the X-ray variability. In this work, we investigate the connection between the X-ray amplitude variability and the coronal physical parameters; namely, the temperature ($kT$) and optical depth ($τ$). We present the spectral and timing analysis of 46 {\it NuSTAR} observations corresponding to a sample of 20 AGN. For each source, we derived the coronal temperature and optical depth through X-ray spectroscopy and computed the normalized excess variance for different energy bands on a timescale of $10$ ks. We find a strong inverse correlation between $kT$ and $τ$, with correlation coefficient of $r<-0.9$ and negligible null probability. No clear dependence was found among the temperature and physical properties, such as the black hole mass or the Eddington ratio. We also see that the observed X-ray variability is not correlated with either the coronal temperature or optical depth under the thermal equilibrium assumption, whereas it is anticorrelated with the black hole mass. These results can be interpreted through a scenario where the observed X-ray variability could primarily be driven by variations in the coronal physical properties on a timescale of less than $10$~ks; whereas we assume thermal equilibrium on such timescales in this work, given the capability of the currently available hard X-ray telescopes. Alternatively, it is also possible that the X-ray variability is mostly driven by the absolute size of the corona, which depends on the supermassive black hole mass, rather than resulting from any of its physical properties.

astro-ph.HE↗

arXiv2024

Skipping a beat: discovery of persistent quasi-periodic oscillations associated with pulsed fraction drop of the spin signal in M51 ULX-7

The discovery of pulsations in (at least) six ultraluminous X-ray sources (ULXs) has shown that neutron stars can accrete at (highly) super-Eddington rates, challenging the standard accretion theories. M51 ULX-7, with a spin signal of $P\simeq2.8$ s, is the pulsating ULX (PULX) with the shortest known orbital period ($P_\mathrm{orb}\simeq2$ d) and has been observed multiple times by XMM-Newton, Chandra, and NuSTAR. We report on the timing and spectral analyses of three XMM-Newton observations of M51 ULX-7 performed between the end of 2021 and the beginning of 2022, together with a timing re-analysis of XMM-Newton, Chandra, and NuSTAR archival observations. We investigated the spin signal by applying accelerated search techniques and studied the power spectrum through the fast Fourier transform, looking for (a)periodic variability in the source flux. We analysed the energy spectra of the 2021-2022 observations and compared them to the older ones. We report the discovery of a recurrent, significant ($>$3$σ$) broad complex at mHz frequencies in the power spectra of M51 ULX-7. We did not detect the spin signal, setting a 3$σ$ upper limit on the pulsed fraction of $\lesssim10\%$ for the single observation. The complex is significantly detected also in five Chandra observations performed in 2012. M51 ULX-7 represents the second PULX for which we have a significant detection of mHz-QPOs at super-Eddington luminosities. These findings suggest that one should avoid using the observed QPO frequency to infer the mass of the accretor in a ULX. The absence of spin pulsations when the broad complex is detected suggests that the mechanism responsible for the aperiodic modulation also dampens the spin signal's pulsed fraction. If true, this represents an additional obstacle in the detection of new PULXs, suggesting an even larger occurrence of PULXs among ULXs.

astro-ph.HE↗

arXiv2024

Gravitational self-lensing of Fast Radio Bursts in neutron star magnetospheres: I. The model

Fast Radio Bursts (FRBs) are cosmological sub-second bursts of coherent radio emission, whose source is still unknown. To date, the galactic magnetar SGR 1935+2154 is the only astrophysical object known to emit radio bursts akin to FRBs, albeit less powerful, supporting suggestions that FRBs originate from magnetars. Many remarkable properties of FRBs, e.g. the dichotomy between repeaters and one-off sources, and their power-law energy distributions (with typical index $\sim 2-3$), are not well understood yet. Moreover, the huge radio power released by the most active repeaters is challenging even for the magnetic energy reservoir of magnetars. Here we assume that FRBs originate from co-rotating hot-spots anchored in neutron star magnetospheres and get occasionally amplified by large factors via gravitational self-lensing in the strong NS field. We evaluate the probability of amplification and show that (i) a power-law energy distribution of events $\propto E^{-(2- 3)}$ is generally expected, (ii) all FRB sources may be regarded as repeating, their appearance as one-off sources or repeaters being determined by the critical dependence of the amplification probability on the emission geometry and source orientation relative to Earth and (iii) the most active repeaters, in particular, correspond to extremely rare and finely-tuned orientations ($\sim$ one in $10^6$), leading to large probabilities of amplification which make their bursts frequently detectable. At the same time, their power release appears enhanced, typically by factors $\gtrsim 10$, easing their energy budget problem.

astro-ph.HE↗

arXiv2024

The Orbit of NGC 5907 ULX-1

We report on the orbit of the binary system powering the most extreme ultraluminous X-ray pulsar known to date: NGC 5907 ULX-1 (hereafter ULX1). ULX1 has been the target of a substantial multi-instrument campaign, mainly in the X-ray band, but no clear counterparts are known in other bands. Although ULX1 is highly variable and pulsations can be transient (regardless of the source flux), the timing data collected so far allow us to investigate the orbit of this system. We find an orbital period $P_{orb}=5.7^{+0.1}_{-0.6}\text{ d}$ and a projected semi-axis $A_1 =3.1^{+0.8}_{-0.9}\text{ lts}$. The most likely ephemeris is: $P_{orb}=5.6585(6)\text{ d}$, $A_1 = 3.1(4)\text{ lts}$, and the epoch of ascending nodes passage is: $T_{asc} = 57751.37(5)\text{ MJD}$. However, there are 6 similar solutions, acceptable within $3\,σ$. We find further indications that ULX1 is a high-mass X-ray binary. This implies that we are observing its orbit face-on, with an inclination $<5\text{ deg}$.

astro-ph.HE↗

arXiv2023

First detection of X-ray polarization from the accreting neutron star 4U 1820-303

This paper reports the first detection of polarization in the X-rays for atoll-source 4U 1820-303, obtained with the Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed in the radio with the Australia Telescope Compact Array (ATCA). The IXPE observations of 4U 1820-303 were coordinated with Swift-XRT, NICER, and NuSTAR aiming to obtain an accurate X-ray spectral model covering a broad energy interval. The source shows a significant polarization above 4 keV, with a polarization degree of 2.0(0.5)% and a polarization angle of -55(7) deg in the 4-7 keV energy range, and a polarization degree of 10(2)% and a polarization angle of -67(7) deg in the 7-8 keV energy bin. This polarization also shows a clear energy trend with polarization degree increasing with energy and a hint for a position-angle change of about 90 deg at 96% CL around 4 keV. The spectro-polarimetric fit indicates that the accretion disk is polarized orthogonally to the hard spectral component, which is presumably produced in the boundary/spreading layer. We do not detect linear polarization from the radio counterpart, with a 99.97% upper limit of 50% at 7.25 GHz.

astro-ph.HE↗

arXiv2023

Magnetar central engines in gamma-ray busts follow the universal relation of accreting magnetic stars

Gamma-ray bursts (GRBs), both long and short, are explosive events whose inner engine is generally expected to be a black hole or a highly magnetic neutron star (magnetar) accreting high density matter. Recognizing the nature of GRB central engines, and in particular the formation of neutron stars (NSs), is of high astrophysical significance. A possible signature of NSs in GRBs is the presence of a plateau in the early X-ray afterglow. Here we carefully select a subset of long and short GRBs with a clear plateau, and look for an additional NS signature in their prompt emission, namely a transition between accretion and propeller in analogy with accreting, magnetic compact objects in other astrophysical sources. We estimate from the prompt emission the minimum accretion luminosity below which the propeller mechanism sets in, and the NS magnetic field and spin period from the plateau. We demonstrate that these three quantities obey the same universal relation in GRBs as in other accreting compact objects switching from accretion to propeller. This relation provides also an estimate of the radiative efficiency of GRBs, which we find to be several times lower than radiatively efficient accretion in X-ray binaries and in agreement with theoretical expectations. These results provide additional support to the idea that at least some GRBs are powered by magnetars surrounded by an accretion disc.

astro-ph.HE↗

arXiv2022

Timing analysis of the 2022 outburst of the accreting millisecond X-ray pulsar SAX J1808.4$-$3658: hints of an orbital shrinking

We present a pulse timing analysis of NICER observations of the accreting millisecond X-ray pulsar SAX J1808.4$-$3658 during the outburst that started on 2022 August 19. Similar to previous outbursts, after decaying from a peak luminosity of $\simeq 1\times10^{36} \, \mathrm{erg \, s^{-1}}$ in about a week, the pulsar entered in a $\sim 1$ month-long reflaring stage. Comparison of the average pulsar spin frequency during the outburst with those previously measured confirmed the long-term spin derivative of $\dotν_{\textrm{SD}}=-(1.15\pm0.06)\times 10^{-15} \, \mathrm{Hz\,s^{-1}}$, compatible with the spin-down torque of a $\approx 10^{26} \, \mathrm{G \, cm^3}$ rotating magnetic dipole. For the first time in the last twenty years, the orbital phase evolution shows evidence for a decrease of the orbital period. The long-term behaviour of the orbit is dominated by a $\sim 11 \, \mathrm{s}$ modulation of the orbital phase epoch consistent with a $\sim 21 \, \mathrm{yr}$ period. We discuss the observed evolution in terms of a coupling between the orbit and variations in the mass quadrupole of the companion star.

astro-ph.HE↗

arXiv2021

Millisecond Magnetars

Two classes of X-ray/$γ$-ray sources, the Soft Gamma Repeaters and the Anomalous X-ray Pulsars have been identified with isolated, slowly spinning magnetars, neutron stars whose emission draws energy from their extremely strong magnetic field ($\sim 10^{15}-10^{16}$ G). Magnetars are believed to form with millisecond spin period and to represent an important fraction of the whole population of young neutron stars. Newborn magnetars can convert very quickly their rotational energy into electromagnetic and/or gravitational waves, by virtue of their strong magnetic fields and fast spins. This chapter provides a brief summary of astrophysical problems and scenarios in which millisecond magnetars are believed to play a key role: these include Gamma Ray Bursts, Supernovae, Gravitational Wave events and Fast Radio Bursts.

astro-ph.HE↗

arXiv2020

A new method to constrain neutron star structure from quasi-periodic oscillations

We develop a new method to measure neutron star parameters and derive constraints on the equation of state of dense matter by fitting the frequencies of simultaneous Quasi Periodic Oscillation modes observed in the X-ray flux of accreting neutron stars in low mass X-ray binaries. To this aim we calculate the fundamental frequencies of geodesic motion around rotating neutron stars based on an accurate general-relativistic approximation for their external spacetime. Once the fundamental frequencies are related to the observed frequencies through a QPO model, they can be fit to the data to obtain estimates of the three parameters describing the spacetime, namely the neutron star mass, angular momentum and quadrupole moment. From these parameters we derive information on the neutron star structure and equation of state. We present a proof of principle of our method applied to pairs of kHz QPO frequencies observed from three systems (4U1608-52, 4U0614+09 and 4U1728-34). We identify the kHz QPOs with the azimuthal and the periastron precession frequencies of matter orbiting the neutron star, and via our Bayesian inference technique we derive constraints on the neutrons stars' masses and radii. This method is applicable to other geodesic-frequency-based QPO models.

astro-ph.HE↗

arXiv2020

Neutron Star Radius-to-mass Ratio from Partial Accretion Disc Occultation as Measured through Fe K$α$ Line Profiles

We present a new method to measure the radius-to-mass ratio (R/M) of weakly magnetic, disc-accreting neutron stars by exploiting the occultation of parts of the inner disc by the star itself. This occultation imprints characteristic features on the X-ray line profile that are unique and are expected to be present in low mass X-ray binary systems seen under inclinations higher than ~65 degrees. We analyse a NuSTAR observation of a good candidate system, 4U 1636-53, and find that X-ray spectra from current instrumentation are unlikely to single out the occultation features owing to insufficient signal-to-noise. Based on an extensive set of simulations we show that large-area X-ray detectors of the future generation could measure R/M to ~2÷3% precision over a range of inclinations. Such is the precision in radius determination required to derive tight constraints on the equation of state of ultradense matter and it represents the goal that other methods too aim to achieve in the future.

astro-ph.HE↗

arXiv2019

Three-dimensional general relativistic Poynting-Robertson effect II: Radiation field from a rigidly rotating spherical source

We investigate the three-dimensional, general relativistic Poynting-Robertson effect in the case of rigidly rotating spherical source which emits radiation radially in the local comoving frame. Such radiation field is meant to approximate the field produced by the surface of a rotating neutron star, or by the central radiating hot corona of accreting black holes; it extends the purely radial radiation field that we considered in a previous study. Its angular momentum is expressed in terms of the rotation frequency and radius of the emitting source. For the background we adopt a Kerr spacetime geometry. We derive the equations of motion for test particles influenced by such radiation field, recovering the classical and weak-field approximation for slow rotation. We concentrate on solutions consisting of particles orbiting along circular orbits off and parallel to the equatorial plane, which are stabilized by the balance between gravitational attraction, radiation force and PR drag. Such solutions are found to lie on a critical hypersurface, whose shape may morph from prolate to oblate depending on the Kerr spin parameter and the luminosity, rotation and radius of the radiating sphere. For selected parameter ranges, the critical hypersurface intersects the radiating sphere giving rise to a bulging equatorial region or, alternatively, two lobes above the poles. We calculate the trajectories of test particles in the close vicinity of the critical hypersurface for a selected set of initial parameters and analyze the spatial and angular velocity of test particles captured on the critical hypersurface.

gr-qc↗

arXiv2019

Diffuse X-ray emission around an ultraluminous X-ray pulsar

Ultraluminous X-ray sources (ULXs) are extragalactic X-ray emitters located off-center of their host galaxy and with a luminosity in excess of a few ${10^{39}\text{ erg s}^{-1}}$, if emitted isotropically. The discovery of periodic modulation revealed that in some ULXs the accreting compact object is a neutron star, indicating luminosities substantially above their Eddington limit. The most extreme object in this respect is ${NGC 5907~ULX-1}$ (ULX1), with a peak luminosity that is 500 times its Eddington limit. During a Chandra observation to probe a low state of ULX1, we detected diffuse X-ray emission at the position of ULX1. Its diameter is $2.7 \pm 1.0$ arcsec and contains 25 photons, none below 0.8 keV. We interpret this extended structure as an expanding nebula powered by the wind of ULX1. Its diameter of about ${200\text{ pc}}$, characteristic energy of ${\sim 1.9\text{ keV}}$, and luminosity of ${\sim 2\times10^{38}\text{ erg s}^{-1}}$ imply a mechanical power of ${1.3\times10^{41}\text{ erg s}^{-1}}$ and an age ${\sim 7 \times 10^{4}\text{ yr}}$. This interpretation suggests that a genuinely super-Eddington regime can be sustained for time scales much longer than the spin-up time of the neutron star powering the system. As the mechanical power from a single ULX nebula can rival the injection rate of cosmic rays of an entire galaxy, ULX nebulae could be important cosmic ray accelerators.

astro-ph.HE↗