arXiv subjects

Sandro Mereghetti

Publications and source records attributed to Sandro Mereghetti.

At least 37 records · Page 2Linked to original sources

arXiv2024

INTEGRAL observations of magnetars

The INTEGRAL satellite has collected a large amount of data on magnetars in our Galaxy, spanning more than 20 years starting from 2003. The large data set obtained with the IBIS/ISGRI instrument at energies above 20 keV allows us to study both the properties and long-term evolution of their persistent hard X-ray emission and the population characteristics of the short bursts emitted during active periods. We are carrying out a comprehensive analysis of the observed magnetars, exploiting the most recent calibrations and analysis software. Here we report on the long term evolution of the hard X-ray flux of the magnetars detected with ISGRI and the results of a sensitive search for short bursts in SGR J1935+2154.

astro-ph.HE↗

arXiv2024

The Proper Motion of the High Galactic Latitude Pulsar Calvera

Calvera (1RXS J141256.0+792204) is a pulsar of characteristic age 285 kyr at a high Galactic latitude of b=+37°, detected only in soft thermal X-rays. We measure a new and precise proper motion for Calvera using Chandra HRC-I observations obtained 10 years apart. We also derive a new phase-connected ephemeris using 6 years of NICER data, including the astrometric position and proper motion as fixed parameters in the timing solution. Calvera is located near the center of a faint, circular radio ring that was recently discovered by LOFAR and confirmed as a supernova remnant (SNR) by the detection of gamma-ray emission with Fermi/LAT. The proper motion of $78.5 \pm 2.9$ mas/yr at position angle $241°.3 \pm 2°.2$ (in Galactic coordinates) points away from the center of the ring, a result which differs markedly from a previous low-significance measurement, and greatly simplifies the interpretation of the SNR/pulsar association. It argues that the supernova indeed birthed Calvera <10 kyr ago, with an initial spin period close to its present value of 59 ms. The tangential velocity of the pulsar depends on its uncertain distance, $v_t=(372 \pm 14) d_{1 kpc}$ km/s, but is probably dominated by the supernova kick, while its progenitor could have been a runaway O or B star from the Galactic disk.

astro-ph.HE↗

arXiv2024

Long-term study of the 2020 magnetar-like outburst of the young pulsar PSRJ1846-0258 in Kes 75

Magnetar-like activity has been observed in a large variety of neutron stars. PSR J1846-0258 is a young 327 ms radio-quiet pulsar with a large rotational power ($\sim 8 \times 10^{36}$ erg s$^{-1}$), and resides at the center of the supernova remnant Kes 75. It is one of the rare examples of a high magnetic field pulsar showing characteristics both of magnetars and radio pulsars, and can thus provide important clues on the differences in the emission mechanisms between these two classes. In 2006, PSR J1846-0258 was detected to undergo an outburst for the first time, accompanied by a large flux increase, millisecond X-ray bursts, significant spectral changes and a large timing glitch. In the period between May-June 2020, after fourteen years of quiescent stable emission, the source underwent a second magnetar-like outburst, which was followed up with several observations by NICER, XMM-Newton, NuSTAR and Swift. In this work, we report on the long-term timing and X-ray spectral properties of the source following the 2020 outburst, and place upper limits on any source activity at radio wavelengths. We demonstrate that the pulsed flux increased by a factor $> 6$ during the outburst, followed by non-trivial variability in the spin-down rate. Our timing analysis shows that the spin frequency and its derivative are clearly affected by magnetospheric activity due to the outburst. We find hints for an oscillation in the frequency derivative with a timescale of 50-60 days, recovering later on to stable quiescence.

astro-ph.HE↗

arXiv2024

XMM-Newton observations of the peculiar Be X-ray binary A0538-66

A0538-66 is a neutron star/Be X-ray binary located in the Large Magellanic Cloud and, since its discovery in the seventies, it showed a peculiar behavior which makes it a unique object in the high-mass X-ray binaries scene: the extremely eccentric orbit (e=0.72), the short spin period of the neutron star (P=69 ms), the episodes of super-Eddington accretion. These characteristics contribute to a remarkable bursting activity that lasts from minutes to hours and increases the flux by a factor 10^3-10^4. In 2018, A0538-66 was observed by XMM-Newton in a particularly active state, characterized by a forest of short bursts lasting 0.7-50 seconds each. In this contribution we present a reanalysis of these observations. The timing analysis allowed us to distinguish between the epochs of direct accretion and propeller state, that do not correlate with the orbital position of the neutron star. The spectral analysis revealed that during the accretion regime three components (a soft one, a hard one, and a ~6.4-keV emission line) equally contribute to the overall emission, while the propeller regime is characterized by a single soft component. We discuss these findings in the context of spherical and disk accretion regimes, highlighting the similarities and the differences with other X-ray binary systems.

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↗

arXiv2024

Periodicity from X-ray sources within the inner Galactic disk

For many years, it has been claimed that the Galactic ridge X-ray emission at the Galactic Center (GC) is truly diffuse in nature. However, with the advancement of modern X-ray satellites, it has been found that most of the diffuse emission is actually comprised of thousands of previously unresolved X-ray point sources. Further, many studies suggest that a vast majority of these X-ray point sources are magnetic cataclysmic variables (mCVs) and active binaries. One unambiguous way to identify these mCVs and other sources is by detecting their X-ray periodicity. Therefore, we systematically searched for periodic X-ray sources in the inner Galactic disk, including the GC region. We have used data from our ongoing XMM-Newton Heritage survey of the inner Galactic disk ($350^{\circ}\lesssim l\lesssim+7^{\circ}$ and $-1^{\circ}\lesssim b\lesssim +1^{\circ}$) plus the XMM-Newton archival observations of the GC. We computed the Lomb-Scargle periodogram of the light curves for the periodicity search. We fitted the energy spectra of the sources using a simple power-law model plus three Gaussians at 6.4, 6.7, and 6.9 keV for the iron $K$ emission complex. We detected periodicity in 26 sources. For 14 of them, this is the first discovery of periodicity. For the other 12 sources, we found periods similar to those already known, indicating no significant period evolution. We also searched for the Gaia counterparts of the periodic sources to estimate their distances using the Gaia parallax. We found a likely Gaia counterpart for seven sources. We have classified the sources into four categories based on the periodicity, hardness ratio, and the equivalent width of Fe $K$ line emission. Of the 14 sources where we detect the periodicity for the first time, four are likely to be intermediate polars, five are likely to be polars, two are neutron star X-ray binaries, and three are of unknown nature.

astro-ph.HE↗

arXiv2024

A magnetar giant flare in the nearby starburst galaxy M82

Giant flares, short explosive events releasing up to 10$^{47}$ erg of energy in the gamma-ray band in less than one second, are the most spectacular manifestation of magnetars, young neutron stars powered by a very strong magnetic field, 10$^{14-15}$ G in the magnetosphere and possibly higher in the star interior. The rate of occurrence of these rare flares is poorly constrained, as only three have been seen from three different magnetars in the Milky Way and in the Large Magellanic Cloud in about 50 years since the beginning of gamma-ray astronomy. This sample can be enlarged by the discovery of extragalactic events, since for a fraction of a second giant flares reach peak luminosities above 10$^{46}$ erg/s, which makes them visible by current instruments up to a few tens of Mpc. However, at these distances they appear similar to, and difficult to distinguish from, regular short gamma-ray bursts (GRBs). The latter are much more energetic events, 10$^{50-53}$ erg, produced by compact binary mergers and originating at much larger distances. Indeed, only a few short GRBs have been proposed, with different levels of confidence, as magnetar giant flare candidates in nearby galaxies. Here we report the discovery of a short GRB positionally coincident with the central region of the starburst galaxy M82. Its spectral and timing properties, together with the limits on its X-ray and optical counterparts obtained a few hours after the event and the lack of an associated gravitational wave signal, qualify with high confidence this event as a giant flare from a magnetar in M82.

astro-ph.HE↗

arXiv2023

XMM-Newton and INTEGRAL observations of the bright GRB 230307A : vanishing of the local absorption and limits on the dust in the Magellanic Bridge

230307A is the second brightest gamma ray burst detected in more than 50 years of observations and is located in the direction of the Magellanic Bridge. Despite its long duration, it is most likely the result of the compact merger of a binary ejected from a galaxy in the local universe (redshift z=0.065). Our XMM-Newton observation of its afterglow at 4.5 days shows a power-law spectrum with photon index $Γ=1.73 \pm0.10$, unabsorbed flux $F_{0.3-10\,\rm keV}=(8.8\pm0.5)\times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$ and no absorption in excess of that produced in our Galaxy and in the Magellanic Bridge. We derive a limit of $N_{\rm H}^{\rm HOST} < 5\times 10^{20}$ cm$^{-2}$ on the absorption at the GRB redshift, which is a factor $\sim\,$5 below the value measured during the prompt phase. We searched for the presence of dust scattering rings with negative results and set an upper limit of the order of $A_V<0.05$ on the absorption from dust in the Magellanic Bridge.

astro-ph.HE↗

arXiv2023

Timing the X-ray pulsating companion of the hot-subdwarf HD 49798 with NICER

HD 49798 is a hot subdwarf of O spectral type in a 1.55 day orbit with the X-ray source RX J0648.0-4418, a compact object with spin period of 13.2 s. We use recent data from the NICER instrument, joined with archival data from XMM-Newton and ROSAT, to obtain a phase-connected timing solution spanning ~30 years. Contrary to previous works, that relied on parameters determined through optical observations, the new timing solution could be derived using only X-ray data. We confirm that the compact object is steadily spinning up with Pdot = -2.28(2)x10^-15 s/s and obtain a refined measure of the projected semi-major axis of the compact object aX sini = 9.60(5) lightsec. This allows us to determine the inclination and masses of the system as i = 84.5(7) deg, MX = 1.220(8) Msun and Mopt = 1.41(2) Msun. We also study possible long term (~year) and orbital variations of the soft X-ray pulsed flux, without finding evidence for variability. In the light of the new findings, we discuss the nature of the compact object, concluding that the possibility of a neutron star in the subsonic propeller regime is unlikely, while accretion of the subdwarf wind onto a massive white dwarf can explain the observed luminosity and spin-up rate for a wind velocity of ~800 km/s.

astro-ph.HE↗

arXiv2023

The power of the rings: the GRB 221009A soft X-ray emission from its dust-scattering halo

GRB 221009A is the brightest gamma-ray burst (GRB) ever detected and occurred at low Galactic latitude. Owing to this exceptional combination, its prompt X-ray emission could be detected for weeks in the form of expanding X-ray rings produced by scattering in Galactic dust clouds. We report on the analysis of 20 rings, generated by dust at distances ranging from 0.3 to 18.6 kpc, detected during two XMM-Newton observations performed about 2 and 5 days after the GRB. By fitting the spectra of the rings with different models for the dust composition and grain size distribution, we reconstructed the spectrum of the GRB prompt emission in the 0.7-4 keV energy range as an absorbed power law with photon index 1-1.4 and absorption in the host galaxy nHz=(4.1-5.3)E21 cm-2. Taking into account the systematic uncertainties on the column density of dust contained in the clouds producing the rings, the 0.5-5 keV fluence of GRB 221009A can be constrained between 1E-3 and 7E-3 erg cm-2.

astro-ph.HE↗

arXiv2023

The IXPE view of GRB 221009A

We present the IXPE observation of GRB 221009A which includes upper limits on the linear polarization degree of both prompt and afterglow emission in the soft X-ray energy band. GRB 221009A is an exceptionally bright gamma-ray burst (GRB) that reached Earth on 2022 October 9 after travelling through the dust of the Milky Way. The Imaging X-ray Polarimetry Explorer (IXPE) pointed at GRB 221009A on October 11 to observe, for the first time, the 2-8 keV X-ray polarization of a GRB afterglow. We set an upper limit to the polarization degree of the afterglow emission of 13.8% at a 99% confidence level. This result provides constraints on the jet opening angle and the viewing angle of the GRB, or alternatively, other properties of the emission region. Additionally, IXPE captured halo-rings of dust-scattered photons which are echoes of the GRB prompt emission. The 99% confidence level upper limit to the prompt polarization degree depends on the background model assumption and it ranges between ~55% to ~82%. This single IXPE pointing provides both the first assessment of X-ray polarization of a GRB afterglow and the first GRB study with polarization observations of both the prompt and afterglow phases.

astro-ph.HE↗

arXiv2022

Limiting the accretion disk light in two mass transferring hot subdwarf binaries

We report the results from follow-up observations of two Roche-lobe filling hot subdwarf binaries with white dwarf companions predicted to have accretion disks. ZTF J213056.71+442046.5 (ZTF J2130) with a 39-minute period and ZTF J205515.98+465106.5 (ZTF J2055) with a 56-minute period were both discovered as subdwarf binaries with light curves that could only be explained well by including an accretion disk in their models. We performed a detailed high-resolution spectral analysis using Keck/ESI to search for possible accretion features for both objects. We also employed polarimetric analysis using the Nordic Optical Telescope (NOT) for ZTF J2130. We did not find any signatures of an accretion disk in either object, and placed upper limits on the flux contribution and variation in degree of polarisation due to the disk. Owing to the short 39-minute period and availability of photometric data over six years for ZTF J2130, we conducted an extensive $O - C$ timing analysis in an attempt to look for orbital decay due to gravitational wave radiation. No such decay was detected conclusively, and a few more years of data paired with precise and consistent timing measurements were deemed necessary to constrain $\dot P$ observationally.

astro-ph.SR↗

arXiv2022

A puzzling 2-hour X-ray periodicity in the 1.5-hour orbital period black widow PSR J1311-3430

Time-domain analysis of an archival XMM-Newton observation unveiled a very unusual variability pattern in the soft X-ray emission of PSR J1311-3430, a black widow millisecond pulsar in a tight binary (P_B=93.8 min) with a very low-mass (M~0.01 Msun) He companion star, known to show flaring emission in the optical and in the X-rays. A series of six pulses with a regular recurrence time of ~124 min is apparent in the 0.2-10 keV light curve of the system, also featuring an initial, bright flare and a quiescent phase lasting several hours. The X-ray spectrum does not change when the pulses are seen and is consistent with a power law with photon index Gamma~1.6, also describing the quiescent emission. The peak luminosity of the pulses is of several 10^32 erg/s. Simultaneous observations in the U band with the Optical Monitor onboard XMM and in the g' band from the Las Cumbres Observatory do not show any apparent counterpart of the pulses and only display the well-known orbital modulation of the system. We consider different hypotheses to explain the recurrent pulses: we investigate their possible analogy with other phenomena already observed in this pulsar and in similar systems and we also study possible explanations related to the interaction of the energetic pulsar wind with intrabinary material, but we found none of these pictures to be convincing. We identify simultaneous X-ray observations and optical spectroscopy as a possible way to constrain the nature of the phenomenon.

astro-ph.HE↗

arXiv2022

Two decades of X-ray observations of the isolated neutron star RX J1856.5-3754: detection of thermal and non-thermal hard X-rays and refined spin-down measurement

The soft X-ray pulsar RX J1856.5-3754 is the brightest member of a small class of thermally-emitting, radio-silent, isolated neutron stars. Its X-ray spectrum is almost indistinguishable from a blackbody with $kT^\infty\approx 60$ eV, but evidence of harder emission above $\sim 1$ keV has been recently found. We report on a spectral and timing analysis of RX J1856.5-3754 based on the large amount of data collected by XMM-Newton in 2002--2022, complemented by a dense monitoring campaign carried out by NICER in 2019. Through a phase-coherent timing analysis we obtained an improved value of the spin-down rate $\dotν=-6.042(4)\times10^{-16}$ Hz s$^{-1}$, reducing by more than one order magnitude the uncertainty of the previous measurement, and yielding a characteristic spin-down field of $1.47\times10^{13}$ G. We also detect two spectral components above $\sim1$ keV: a blackbody-like one with $kT^\infty=138\pm13$ eV and emitting radius $31_{-16}^{+8}$ m, and a power law with photon index $Γ=1.4_{-0.4}^{+0.5}$. The power-law 2--8\,keV flux, $(2.5_{-0.6}^{+0.7})\times10{-15}$ erg cm$^{-2}$ s$^{-1}$, corresponds to an efficiency of $10^{-3}$, in line with that seen in other pulsars. We also reveal a small difference between the $0.1$--$0.3$ keV and $0.3$--$1.2$ keV pulse profiles, as well as some evidence for a modulation above $1.2$ keV. These results show that, notwithstanding its simple spectrum, \eighteen still has a non-trivial thermal surface distribution and features non-thermal emission as seen in other pulsars with higher spin-down power.

astro-ph.HE↗

arXiv2022

X-ray observations of the isolated black hole candidate OGLE-2011-BLG-0462 , and other collapsed objects discovered through gravitational microlensing

Isolated black holes and neutron stars can be revealed through the observation of long duration gravitational microlensing events. A few candidates have been found in surveys of stars in the direction of the Galactic bulge. Recently, thanks to the addition of astrometric information at milliarcsecond level, it has been possible to reduce the uncertainties in the masses and distances for some of these "dark" gravitational lenses and select the most promising candidates. These isolated compact objects might emit X-rays powered by accretion from the interstellar medium. Using data of the Chandra, XMM-Newton, and INTEGRAL satellites, we searched for X-ray emission in the isolated black hole candidate OGLE-2011-BLG-0462, and in several other putative collapsed objects found with gravitational microlensing. OGLE-2011-BLG-0462 has been recently interpreted as a 7.1 M_sun black hole at a distance of 1.6 kpc, although a different group obtained a mass range (1.6-4.4 M_sun) that cannot exclude a massive neutron star. We have derived upper limits on the flux from OGLE-2011-BLG-0462 of 9$\times10^{-15}$ erg cm$^{-2}$ s$^{-1}$ in the 0.5-7 keV range and $\sim2\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 17-60 keV range. The implied X-ray luminosity is consistent with the small radiative efficiency expected for a black hole and disfavours a neutron star interpretation. Limits down to a factor about five lower are obtained for the soft X-ray flux of other candidates, but their interpretation is affected by larger uncertainties in the masses, distances and spatial velocities.

astro-ph.HE↗

arXiv2022

The first 7 months of the 2020 X-ray outburst of the magnetar SGR J1935+2154

The magnetar SGR J1935+2154 underwent a new active episode on 2020 April 27-28, when a forest of hundreds of X-ray bursts and a large enhancement of the persistent flux were detected. For the first time, a radio burst with properties similar to those of fast radio bursts and with a X-ray counterpart was observed from this source, showing that magnetars can power at least a group of fast radio bursts. In this paper, we report on the X-ray spectral and timing properties of SGR J1935+2154 based on a long-term monitoring campaign with Chandra, XMM-Newton, NuSTAR, Swift and NICER covering a time span of ~7 months since the outburst onset. The broadband spectrum exhibited a non-thermal power-law component (photon index~1.2) extending up to ~20-25 keV throughout the campaign and a blackbody component with temperature decreasing from ~1.5 keV at the outburst peak to ~0.45 keV in the following months. We found that the luminosity decay is well described by the sum of two exponential functions, reflecting the fast decay (~1 d) at the early stage of the outburst followed by a slower decrease (~30 d). The source reached quiescence about ~80 days after the outburst onset, releasing an energy of ~6e40 erg during the outburst. We detected X-ray pulsations in the XMM-Newton data sets and derived an average spin-down rate of ~3.5e-11 s/s using the spin period measurements derived in this work and three values reported previously during the same active period. Moreover, we report on simultaneous radio observations performed with the Sardinia Radio Telescope. No evidence for periodic or single-pulse radio emission was found.

astro-ph.HE↗

arXiv2022

Thermal and non-thermal X-ray emission from the rotation-powered radio/$γ$-ray pulsar PSR J1740+1000

We report the results of new XMM-Newton observations of the middle-aged ($\sim$10$^5$ yr) radio pulsar PSR J1740+1000 carried out in 2017-2018. These long pointings ($\sim$530 ks) show that the non-thermal emission, well described by a power-law spectrum with photon index $Γ=1.80\pm0.17$, is pulsed with a $\sim$30% pulsed fraction above 2 keV. The thermal emission can be well fit with the sum of two blackbodies of temperatures $kT_1=70\pm4$ eV, $kT_2=137\pm7$ eV, $R_1=5.4_{-0.9}^{+1.3}$ km and $R_2=0.70_{-0.13}^{+0.15}$ km (for a distance of 1.2 kpc). We found no evidence for absorption lines as those observed in the shorter XMM-Newton observations ($\sim$67 ks) of this pulsar carried out in 2006. The X-ray thermal and non-thermal components peak in anti-phase and none of them is seen to coincide in phase with the radio pulse. This, coupled with the small difference in the emission radii of the two thermal components, disfavors an interpretation in which the dipolar polar cap is heated by magnetospheric backward-accelerated particles. Comparison with the other thermally-emitting isolated neutron stars with spectra well described by the sum of two components shows that the ratios $T_2$/$T_1$ and $R_2$/$R_1$ are similar for objects of different classes. The observed values cannot be reproduced with simple temperature distributions, such as those caused by a dipolar field, indicating the presence of more complicated thermal maps.

astro-ph.HE↗

arXiv2022

INTEGRAL contributions to magnetars and multimessenger astrophysics

The INTEGRAL satellite, in orbit since October 2002, has significantly contributed to the study of magnetars and, thanks to its unique capabilities for the study of transient gamma-ray phenomena, it is now playing an important role in multimessenger astrophysics. The most recent results include the discovery of a peculiar burst from SGR J1935+2154, which gave the first observational evidence for the connection between magnetars and fast radio bursts, and extensive searches for bursting activity in peculiar sources, such as the repeating FRB 20200120E in M81 and the ultra-long period magnetar candidate GLEAMX J162759.5--523504.3.

astro-ph.HE↗