Search

The accretion flow/jet correlation in neutron star (NS) low-mass X-ray binaries (LMXBs) is far less understood when compared to black hole (BH) LMXBs. In this paper we will present the results of a dense multiwavelength observational campaign on the NS LMXB 4U 1820-30, including X-ray (NICER, NuSTAR , and AstroSat) and quasi-simultaneous radio (ATCA) observations in 2022. 4U 1820-30 shows a peculiar 170 d super-orbital accretion modulation, during which the system evolves between 'modes' of high and low X-ray flux. During our monitoring, the source did not show any transition to a full hard state. X-ray spectra were well described using a disc blackbody, a Comptonization spectrum along with a Fe K emission line at ∼6.6 keV. Our results show that the observed X-ray flux modulation is almost entirely produced by changes in the size of the region providing seed photons for the Comptonization spectrum. This region is large (∼15 km) in the high mode and likely coincides with the whole boundary layer, while it shrinks significantly (≲10 km) in low mode. The electron temperature of the corona and the observed rms variability in the hard X-rays also exhibit a slight increase in low mode. As the source moves from high to low mode, the radio emission due to the jet becomes ∼5 fainter. These radio changes appear not to be strongly connected to the hard-to-soft transitions as in BH systems, while they seem to be connected mostly to variations observed in the boundary layer. [ABSTRACT FROM AUTHOR]

The transient X-ray source MAXI J1810−222 was discovered in 2018 and has been active ever since. A long combined radio and X-ray monitoring campaign was performed with Australia Telescope Compact Array and Swift , respectively. It has been proposed that MAXI J1810−222 is a relatively distant black hole X-ray binary, albeit showing a very peculiar outburst behaviour. Here, we report on the spectral study of this source making use of a large sample of NICER observations performed between 2019 February and 2020 September. We detected a strong spectral absorption feature at ∼1 keV, which we have characterized with a physical photoionization model. Via a deep scan of the parameters space, we obtained evidence for a spectral-state dependent outflow, with mildly relativistic speeds. In particular, the soft and intermediate states point to a hot plasma outflowing at 0.05–0.15 c. This speeds rule-out thermal winds and hence, they suggest that such outflows could be radiation pressure or (most likely) magnetically driven winds. Our results are crucial to test current theoretical models of wind formation in X-ray binaries. [ABSTRACT FROM AUTHOR]

Ultraluminous X-ray sources (ULXs) are the most extreme among X-ray binaries in which the compact object, a neutron star or a black hole, accretes matter from the companion star, and exceeds a luminosity of |$10^{39} \ \rm erg \, s^{-1}$| in the X-ray energy band alone. Despite two decades of studies, it is still not clear whether ULX spectral transitions are due to stochastic variability in the wind or variations in the accretion rate or in the source geometry. The compact object is also unknown for most ULXs. In order to place constraints on to such scenarios and on the structure of the accretion disc, we studied the temporal evolution of the spectral components of the variable source NGC 55 ULX-1. Using recent and archival data obtained with the XMM- Newton satellite, we modelled the spectra with two blackbody components which we interpret as thermal emission from the inner accretion flow and the regions around or beyond the spherization radius. The luminosity–temperature (L–T) relation of each spectral component agrees with the L ∝ T4 relationship expected from a thin disc model, which suggests that the accretion rate is close to the Eddington limit. However, there are some small deviations at the highest luminosities, possibly due to an expansion of the disc and a contribution from the wind at higher accretion rates. Assuming that such deviations are due to the crossing of the Eddington or supercritical accretion rate, we estimate a compact object mass of 6–14 M⊙, favouring a stellar-mass black hole as the accretor. [ABSTRACT FROM AUTHOR]

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 1039 erg s−1. The average number of ULXs per galaxy is still not well-constrained, especially given the uncertainty on the fraction of ULX transients. Here, we report the identification of a new transient ULX in the galaxy NGC 55 (which we label as ULX-2), thanks to recent XMM–Newton and the Neil Gehrels Swift Observatory observations. This object was previously classified as a transient X-ray source with a luminosity around a few 1038 erg s−1 in a 2010 XMM–Newton observation. Thanks to new and deeper observations (∼130 ks each), we show that the source reaches a luminosity peak >1.6 × 1039 erg s−1. The X-ray spectrum of ULX-2 is much softer than in previous observations and fits in the class of soft ULXs. It can be well-described using a model with two thermal components, as often found in ULXs. The time-scales of the X-ray variability are of the order of a month and are likely driven by small changes in the accretion rate or due to super-orbital modulations, attributed to precession of the accretion disc, which is similar to other ULXs. [ABSTRACT FROM AUTHOR]

We present results from radio and X-ray observations of the X-ray transient MAXI J1810−222. The nature of the accretor in this source has not been identified. In this paper, we show results from a quasi-simultaneous radio and X-ray monitoring campaign taken with the Australia Telescope Compact Array, the Neil Gehrels Swift Observatory X-ray Telescope (XRT) , and the Swift Burst Alert Telescope. We also analyse the X-ray temporal behaviour using observations from the Neutron star Interior Composition Explorer. Results show a seemingly peculiar X-ray spectral evolution of MAXI J1810−222 during this outburst, where the source was initially only detected in the soft X-ray band for the early part of the outburst. Then, ∼200 d after MAXI J1810−222 was first detected the hard X-ray emission increased and the source transitioned to a long-lived (∼1.5 yr) bright, harder X-ray state. After this hard state, MAXI J1810−222 returned back to a softer state, before fading and transitioning again to a harder state and then appearing to follow a more typical outburst decay. From the X-ray spectral and timing properties, and the source's radio behaviour, we argue that the results from this study are most consistent with MAXI J1810−222 being a relatively distant (≳6 kpc) black hole X-ray binary. A sufficiently large distance to source can simply explain the seemingly odd outburst evolution that was observed, where only the brightest portion of the outburst was detectable by the all-sky XRTs. [ABSTRACT FROM AUTHOR]

We studied the spectral changes of the high-mass X-ray binary system LMC X-4 to understand the origin and mechanisms beyond its superorbital modulation (30.4 d). To this aim, we obtained a monitoring campaign with Swift /XRT (0.3–10 keV) and complemented these data with the years-long Swift /BAT survey data (15–60 keV). We found a self-consistent, physically motivated, description of the broad-band X-ray spectrum using a Swift /XRT and a NuSTAR observation at the epoch of maximum flux. We decomposed the spectrum into the sum of a bulk + thermal Comptonization, a disc reflection component, and a soft contribution from a standard Shakura–Sunyaev accretion disc. We applied this model to 20 phase-selected Swift spectra along the superorbital period. We found a phase-dependent flux ratio of the different components, whereas the absorption column does not vary significantly. The disc emission is decoupled with respect to the hard flux. We interpret this as a geometrical effect in which the inner parts of the disc are tilted with respect to the obscuring outer regions. [ABSTRACT FROM AUTHOR]

Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems, dips have been observed. The mechanism behind this state transition and the dips nature are still debated. To investigate these issues, we obtained a long XMM–Newton monitoring campaign of a member of this class, NGC 247 ULX-1. We computed the hardness–intensity diagram for the whole data set and identified two different branches: the normal branch and the dipping branch, which we study with four and three hardness–intensity resolved spectra, respectively. All seven spectra are well described by two thermal components: a colder (kT bb ∼ 0.1–0.2 keV) blackbody, interpreted as emission from the photosphere of a radiatively driven wind, and a hotter (kT disc ∼ 0.6 keV) multicolour disc blackbody, likely due to reprocessing of radiation emitted from the innermost regions. In addition, a complex pattern of emission and absorption lines has been taken into account based on previous high-resolution spectroscopic results. We studied the evolution of spectral parameters and flux of the two thermal components along the two branches and discuss two scenarios possibly connecting the state transition and the dipping phenomenon. One is based on geometrical occultation of the emitting regions, the other invokes the onset of a propeller effect. [ABSTRACT FROM AUTHOR]

Most ultraluminous X-ray sources (ULXs) are believed to be powered by super-Eddington accreting neutron stars and, perhaps, black holes. Above the Eddington rate the disc is expected to thicken and to launch powerful winds through radiation pressure. Winds have been recently discovered in several ULXs. However, it is yet unclear whether the thickening of the disc or the wind variability causes the switch between the classical soft and supersoft states observed in some ULXs. In order to understand such phenomenology and the overall super-Eddington mechanism, we undertook a large (800 ks) observing campaign with XMM-Newton to study NGC 247 ULX-1, which shifts between a supersoft and classical soft ULX state. The new observations show unambiguous evidence of a wind in the form of emission and absorption lines from highly ionized ionic species, with the latter indicating a mildly relativistic outflow (−0.17 c) in line with the detections in other ULXs. Strong dipping activity is observed in the light curve and primarily during the brightest observations, which is typical among soft ULXs, and indicates a close relationship between the accretion rate and the appearance of the dips. The latter is likely due to a thickening of the disc scale height and the wind as shown by a progressively increasing blueshift in the spectral lines. [ABSTRACT FROM AUTHOR]

Most ultraluminous X-ray sources (ULXs) are believed to be stellar mass black holes or neutron stars accreting beyond the Eddington limit. Determining the nature of the compact object and the accretion mode from broad-band spectroscopy is currently a challenge, but the observed timing properties provide insight into the compact object and details of the geometry and accretion processes. Here, we report a timing analysis for an 800 ks XMM–Newton  campaign on the supersoft ultraluminous X-ray source, NGC 247 ULX-1. Deep and frequent dips occur in the X-ray light curve, with the amplitude increasing with increasing energy band. Power spectra and coherence analysis reveals the dipping preferentially occurs on ∼5 and ∼10 ks time-scales. The dips can be caused by either the occultation of the central X-ray source by an optically thick structure, such as warping of the accretion disc, or from obscuration by a wind launched from the accretion disc, or both. This behaviour supports the idea that supersoft ULXs are viewed close to edge-on to the accretion disc. [ABSTRACT FROM AUTHOR]

The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW) triggers from the LIGO/Virgo collaboration during the O3 observing run in 2019/2020, performing approximately 6500 pointings in total. Of these events, four were finally classified (if real) as binary black hole (BH) triggers, six as binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, one an unmodelled (Burst) trigger, and the remaining three were subsequently retracted. Thus far, four of these O3 triggers have been formally confirmed as real gravitational wave events. While no likely electromagnetic counterparts to any of these GW events have been identified in the X-ray data (to an average upper limit of 3.60 × 10−12 erg cm−2 s−1 over 0.3–10 keV), or at other wavelengths, we present a summary of all the Swift -XRT observations performed during O3, together with typical upper limits for each trigger observed. The majority of X-ray sources detected during O3 were previously uncatalogued; while some of these will be new (transient) sources, others are simply too faint to have been detected by earlier survey missions such as ROSAT. The all-sky survey currently being performed by eROSITA will be a very useful comparison for future observing runs, reducing the number of apparent candidate X-ray counterparts by up to 95 per cent. [ABSTRACT FROM AUTHOR]

We report on our observing campaign of the compact binary merger GW190814, detected by the Advanced LIGO and Advanced Virgo detectors on 2019 August 14. This signal has the best localization of any observed gravitational wave (GW) source, with a 90 per cent probability area of 18.5 deg2, and an estimated distance of ≈240 Mpc. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering 88 per cent of the probability area down to a limiting magnitude of w = 19.9 AB. Nearby galaxies within the high probability region were targeted with the Lowell Discovery Telescope (LDT), whereas promising candidate counterparts were characterized through multicolour photometry with the Reionization and Transients InfraRed (RATIR) and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical and near-infrared limits in conjunction with the upper limits obtained by the community to constrain the possible electromagnetic counterparts associated with the merger. A gamma-ray burst seen along its jet's axis is disfavoured by the multiwavelength data set, whereas the presence of a burst seen at larger viewing angles is not well constrained. Although our observations are not sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (>0.1 M⊙) fast-moving (mean velocity ≥0.3 c) wind ejecta for a possible kilonova associated with this merger. [ABSTRACT FROM AUTHOR]

Multiwavelength spectral energy distributions of low-mass X-ray binaries (LMXBs) in the hard state are determined by the emission from a jet, for frequencies up to mid-infrared, and emission from the accretion flow in the optical to X-ray range. In the last years, the flat radio-to-mid-IR spectra of black hole (BH) X-ray binaries was described using the internal shocks model, which assumes that the fluctuations in the velocity of the ejecta along the jet are driven by the fluctuations in the accretion flow, described by the X-ray power density spectrum (PDS). In this work, we attempt to apply this model for the first time to a neutron star (NS) LMXB, i.e. 4U 0614 + 091. We used the multiwavelength data set obtained in 2006, comprising data from radio to X-ray, and applied a model that includes an irradiated disc model for the accretion flow and an updated version of the internal shocks code for the ejection. The new version of the code allows to change the geometry of the jet for the case of non-conical jets. Only two alternative scenarios provide a satisfactory description of the data: using the X-ray PDS but in a non-conical geometry for the jet, or either using a conical geometry but with a 'flicker-noise' PDS. Both scenarios would imply some differences with the results obtained with similar models on BH X-ray binaries, shedding light on the possibility that jets in NS and BH binaries might somehow have a different geometry or a different coupling with the accretion flow. [ABSTRACT FROM AUTHOR]

We analysed 13 yr of the Neil Gehrels Swift Observatory survey data collected on the high-mass X-ray binary IGR J18214-1318. Performing the timing analysis, we detected a periodic signal of 5.42 d. From the companion star characteristics, we derived an average orbital separation of |$\sim 41 \rm R_{\odot }\simeq 2 R_{\star }$|⁠. The spectral type of the companion star (O9) and the tight orbital separation suggest that IGR J18214-1318 is a wind-accreting source with eccentricity lower than 0.17. The intensity profile folded at the orbital period shows a deep minimum compatible with an eclipse of the source by the companion star. In addition, we report on the broad-band 0.6–100 keV spectrum using data from XMM–Newton , NuSTAR , and Swift , applying self-consistent physical models. We find that the spectrum is well fitted either by a pure thermal Comptonization component, or, assuming that the source is a neutron star accreting above the critical regime, by a combined thermal and bulk motion Comptonization model. In both cases, the presence of a local neutral absorption (possibly related to the thick wind of the companion star) is required. [ABSTRACT FROM AUTHOR]

The black hole transient GRS 1716−249 was monitored from the radio to the γ-ray band during its 2016–2017 outburst. This paper focuses on the spectral energy distribution (SED) obtained in 2017 February–March, when GRS 1716−249 was in a bright hard spectral state. The soft γ-ray data collected with the INTEGRAL /SPI telescope show the presence of a spectral component that is in excess of the thermal Comptonization emission. This component is usually interpreted as inverse Compton emission from a tiny fraction of non-thermal electrons in the X-ray corona. We find that hybrid thermal/non-thermal Comptonization models provide a good fit to the X-/γ-ray spectrum of GRS 1716−249. The best-fitting parameters are typical of the bright hard state spectra observed in other black hole X-ray binaries. Moreover, the magnetized hybrid Comptonization model belm provides an upper limit on the intensity of the coronal magnetic field of about 106 G. Alternatively, this soft γ-ray emission could originate from synchrotron emission in the radio jet. In order to test this hypothesis, we fit the SED with the irradiated disc plus Comptonization model combined with the jet internal shock emission model ishem. We found that a jet with an electron distribution of p ≃ 2.1 can reproduce the soft γ-ray emission of GRS 1716−249. However, if we introduce the expected cooling break around 10 keV, the jet model can no longer explain the observed soft γ-ray emission, unless the index of the electron energy distribution is significantly harder (p < 2). [ABSTRACT FROM AUTHOR]

1RXS J180408.9–342058 is a low-mass X-ray binary hosting a neutron star, which shows X-ray activity at very different mass-accretion regimes, from very faint to almost the Eddington luminosity. In this work, we present a comprehensive X-ray study of this source using data from the Neil Gehrels Swift Observatory, NuSTAR , and INTEGRAL /JEM-X. In order to follow the spectral evolution, we analysed the 2015 outburst using Swift data and three NuSTAR observations. Besides the canonical hard and soft spectral states, we identified the rarely observed intermediate state. This was witnessed by the appearance of the accretion disc emission in the spectrum (at kT disc ∼0.7 keV) and the simultaneous cooling of the hot corona. In addition, we also unveiled a hard tail above 30 keV in this state. In the hard state, a thermal Comptonization model with two seed photons populations (kT s,1 ∼ 1.5 keV and kT s,2 ∼ 0.4 keV, respectively) and a hot Comptonizing plasma, represents the physically best motivated scenario to describe the data. We also estimated a reflection fraction below 20 per cent in all states, while no constraints on the inclination and only lower limits on the inner disc radius could be inferred. Finally, we studied a number of type-I X-ray bursts displayed from the source, one of them at the Eddington limit (observed with JEM-X). Their characteristics, combined with the clocked behaviour observed during the intermediate state, point out H/He composition for the accreted material, which makes unlikely the helium dwarf nature for the companion. [ABSTRACT FROM AUTHOR]

We present a multiband search for X-ray, optical, and γ-ray emission of the radio binary millisecond pulsar J1836-2354A, hosted in the globular cluster M22. X-ray emission is significantly detected in two Chandra observations, performed in 2005 and 2014, at a luminosity of ∼2–3 × 1030 erg s−1, in the 0.5–8 keV energy range. The radio and the X-ray source positions are found consistent within 1σ error box. No detection is found in archival XMM–Newton and Swift /XRT observations, compatible with the Chandra flux level. The low statistics prevents us to assess if the X-ray source varied between the two observations. The X-ray spectrum is consistent with a power-law of photon index ∼1.5. We favour as the most probable origin of the X-ray emission an intrabinary shock scenario. We searched for optical and γ-ray counterparts to the radio source using data from Hubble Space Telescope and Fermi –LAT catalogues, respectively. No optical counterpart down to V = 25.9 and I = 24.7 (3σ) is detected, which suggests a companion mass of 0.1–0.2 M ⊙. Combined with the low X-ray luminosity, this is consistent with a black widow nature of PSR J1636-2354A. Inspecting the 8-year Fermi –LAT catalogue, we found a γ-ray source, 4FGL J1836.8–2354, with a positional uncertainty consistent with the globular cluster, but not with the radio position of the millisecond pulsar. [ABSTRACT FROM AUTHOR]

We present the spectral and timing analysis of X-ray observations performed on the Galactic black hole transient GRS 1716–249 during the 2016–2017 outburst. The source was almost continuously observed with the Neil Gehrels Swift Observatory from 2016 December until 2017 October. The X-ray hardness ratio and timing evolution indicate that the source approached the soft state three times during the outburst, even though it never reached the canonical soft spectral state. Thus, GRS 1716–249 increases the number of black hole transients showing outbursts with 'failed' state transition. During the softening events, XRT and BAT broad-band spectral modelling, performed with thermal Comptonization plus a multicolour disc black-body, showed a photon index (Γ < 2) and an inner disc temperature (kT in= 0.2–0.5 keV) characteristic of the hard intermediate state. This is in agreement with the root mean square amplitude of the flux variability (rms > 10 |${{\ \rm per\ cent}}$|⁠). We find that, coherently with a scenario in which the disc moves closer to the compact object, the accretion disc inner radius decreases with the increase of the inner disc temperature, until a certain point when the temperature starts to increase at constant radius. This, in addition with the spectral analysis results, suggests that either the accretion disc reached the innermost stable circular orbit during the hard intermediate state or the hot accretion flow might recondensate in an inner mini-disc. We report on the radio observations performed during the outburst finding that GRS 1716–249 is located on the radio-quiet 'outlier' branch of the radio/X-ray luminosity plane. [ABSTRACT FROM AUTHOR]

MXB 1659-298 is a transient neutron-star low-mass X-ray binary system that shows eclipses with a periodicity of 7.1 h. MXB 1659-298 went to outburst in 2015 August, after 14 years of quiescence. We investigate the orbital properties of this source with a baseline of 40 years, obtained by combining the eight eclipse arrival times present in the literature with 51 eclipse arrival times collected during the last two outbursts. A quadratic ephemeris does not fit the delays associated with the eclipse arrival times and the addition of a sinusoidal term with a period of 2.31 ± 0.02 yr is required. We infer a binary orbital period of P = 7.1161099(3) h and an orbital period derivative of P = -8.5(1.2) × 10-12 s s-1. We show that the large orbital period derivative can be explained with a highly non-conservative mass-transfer scenario, in which more than 98 per cent of the mass provided by the companion star leaves the binary system. We predict an orbital period derivative value of P = -6(3) × 10-12 s s-1 and constrain the companion-star mass between 0.3 and 1.2 M⊙. Assuming that the companion star is in thermal equilibrium, the periodic modulation can be due to either a gravitational quadrupole coupling arising from variations of the oblateness of the companion star or the presence of a third body of mass M3 > 21 Jovian masses. [ABSTRACT FROM AUTHOR]

The accretion-powered X-ray pulsar 4U 1626-67 is one of the few highly magnetized pulsars that accretes through Roche lobe overflow from a low-mass companion. The characteristics of its broad-band spectrum are similar to those of X-ray pulsars hosted in a high-mass X-ray binary system, with a broad resonant cyclotron scattering feature (CRSF) at ~37 keV. In this work, we examine the pulse-resolved and the pulse-averaged broad-band spectrum using data from NuSTAR and Swift. We use the Becker &Wolff model of bulk+thermal Comptonization to infer key physical parameters of the accretion column flow and a broad-band model for the disc-reflected spectrum. In the softer X-ray band, we need to add a soft blackbody component with kTbb ~ 0.5 keV, whose characteristics indicate a possible origin from the neutron star surface. Residuals suggest that the shape of the cyclotron line could be more satisfactorily fitted using a narrow core and broader wings and, at higher energies, a second harmonic could be present at ~61 keV. [ABSTRACT FROM AUTHOR]

We present the spectral and timing analysis of the X-ray pulsar GRO J1744-28 during its 2014 outburst using data collected with the X-ray satellites Swift, INTEGRAL, Chandra, and XMM-Newton. We derived, by phase-connected timing analysis of the observed pulses, an updated set of the source ephemeris.We were also able to investigate the spin-up of the X-ray pulsar as a consequence of the accretion torque during the outburst. Relating the spin-up rate and the mass accretion rate as ̇v ∞ ̇M β, we fitted the pulse phase delays obtaining a value of β = 0.96(3). Combining the results from the source spin-up frequency derivative and the flux estimation, we constrained the source distance to be between 3.4 and 4.1 kpc, assuming a disc viscous parameter α to be in the range of 0.1-1. Finally, we investigated the presence of a possible spin-down torque by adding a quadratic component to the pulse phase delay model. The marginal statistical improvement of the updated model does not allow us to firmly confirm the presence of this component. [ABSTRACT FROM AUTHOR]

We report on the analysis of the peculiar X-ray variability displayed by the accreting millisecond X-ray pulsar IGR J00291+5934 in a 80 ks-long joint NuSTAR and XMM-Newton observation performed during the source outburst in 2015. The light curve of the source is characterized by a flaring-like behaviour, with typical rise and decay time-scales of ~120 s. The flares are accompanied by a remarkable spectral variability, with the X-ray emission being generally softer at the peak of the flares. A strong quasi-periodic oscillation (QPO) is detected at ~8 mHz in the power spectrum of the source and clearly associated with the flaring-like behaviour. This feature has the strongest power at soft X-rays (≲3 keV). We carried out a dedicated hardness-ratio-resolved spectral analysis and a QPO phase-resolved spectral analysis, together with an in-depth study of the source-timing properties, to investigate the origin of this behaviour. We suggest that the unusual variability of IGR J00291+5934 observed by XMM-Newton and NuSTAR could be produced by a heartbeat-like mechanism, similar to that observed in black hole X-ray binaries. The possibility that this variability, and the associated QPO, are triggered by phases of quasi-stable nuclear burning, as sustained in the literature for a number of other neutron star binaries displaying a similar behaviour, cannot be solidly tested in the case of IGR J00291+5934 due to the paucity of type I X-ray bursts detected from this source. [ABSTRACT FROM AUTHOR]

We report on the spectral and timing properties of the accreting millisecond X-ray pulsar IGR J00291+5934 observed by XMM-Newton and NuSTAR during its 2015 outburst. The source is in a hard state dominated at high energies by a Comptonization of soft photons (~0.9 keV) by an electron population with kTe ~30 keV, and at lower energies by a blackbody component with kT ~0.5 keV. A moderately broad, neutral Fe emission line and four narrow absorption lines are also found. By investigating the pulse phase evolution, we derived the best-fitting orbital solution for the 2015 outburst. Comparing the updated ephemeris with those of the previous outbursts, we set a 3σ confidence level interval -6.6 × 10-13 s s-1 < Porb < 6.5 × 10-13 s s-1 on the orbital period derivative. Moreover, we investigated the pulse profile dependence on energy finding a peculiar behaviour of the pulse fractional amplitude and lags as a function of energy. We performed a phase-resolved spectroscopy showing that the blackbody component tracks remarkably well the pulse profile, indicating that this component resides at the neutron star surface (hotspot). [ABSTRACT FROM AUTHOR]

We study the behaviour of the cyclotron resonant scattering feature (CRSF) of the high-mass X-ray binary Vela X-1 using the long-term hard X-ray monitoring performed by the Burst Alert Telescope (BAT) on board Swift. High-statistics, intensity-selected spectra were built along 11 years of BAT survey. While the fundamental line is not revealed, the second harmonic of the CRSF can be clearly detected in all the spectra, at an energy varying between ∼53 and ∼58 keV, directly correlated with the luminosity. We have further investigated the evolution of the CRSF in time, by studying the intensity-selected spectra built along four 33-month time intervals along the survey. For the first time, we find in this source a secular variation in the CRSF energy: independent of the source luminosity, the CRSF second harmonic energy decreases by ∼0.36 keV yr−1 between the first and the third time intervals, corresponding to an apparent decay of the magnetic field of ∼3 × 1010 G yr−1. The intensity-cyclotron energy pattern is consistent between the third and the last time intervals. A possible interpretation for this decay could be the settling of an accreted mound that produces either a distortion of the poloidal magnetic field on the polar cap or a geometrical displacement of the line forming region. This hypothesis seems supported by the correspondence between the rate of the line shift per unit accreted mass and the mass accreted on the polar cap per unit area in Vela X-1 and Her X-1, respectively. [ABSTRACT FROM AUTHOR]

We analysed a 115-ks XMM-Newton observation and the stacking of 8 d of INTEGRAL observations, taken during the raise of the 2015 outburst of the accreting millisecond X-ray pulsar SAX J1748.9-2021. The source showed numerous type-I burst episodes during the XMM-Newton observation, and for this reason we studied separately the persistent and burst epochs. We described the persistent emission with a combination of two soft thermal components, a cold thermal Comptonization component (~2 keV) and an additional hard X-ray emission described by a power law (г ~ 2.3). The continuum components can be associated with an accretion disc, the neutron star (NS) surface and a thermal Comptonization emission coming out of an optically thick plasma region, while the origin of the high-energy tail is still under debate. In addition, a number of broad (σ = 0.1-0.4 keV) emission features likely associated with reflection processes have been observed in the XMM-Newton data. The estimated 1.0-50 keV unabsorbed luminosity of the source is ~5 × 1037 erg s -1, about 25 per cent of the Eddington limit assuming a 1.4 M☉ NS. We suggest that the spectral properties of SAX J1748.9-2021 are consistent with a soft state, differently from many other accreting X-ray millisecond pulsars which are usually found in the hard state. Moreover, none of the observed type-I burst reached the Eddington luminosity. Assuming that the burst ignition and emission are produced above the whole NS surface, we estimate an NS radius of ~7-8 km, consistent with previous results. [ABSTRACT FROM AUTHOR]

IGR J16195-4945 is a hard X-ray source discovered by INTEGRAL during the Core Programme observations performed in 2003. We analysed the X-ray emission of this source exploiting the Swift-Burst Alert Telescope (BAT) survey data from 2004 December to 2015 March, and all the available Swift-X-ray Telescope (XRT)-pointed observations. The source is detected at a high significance level in the 123-month BAT survey data, with an average 15-150 keV flux of the source of ~1.6 mCrab. The timing analysis on the BAT data reveals with a significance higher than six standard deviations the presence of a modulated signal with a period of 3.945 d, that we interpret as the orbital period of the binary system. The folded light curve shows a flat profile with a narrow full eclipse lasting ~3.5percent of the orbital period. We requested phase-constrained XRT observations to obtain a more detailed characterization of the eclipse in the soft X-ray range. Adopting reasonable guess values for the mass and radius of the companion star, we derive a semimajor orbital axis of ~31R⊙, equivalent to ~1.8 times the radius of the companion star. From these estimates and from the duration of the eclipse, we derive an orbital inclination between 55 and 60 deg. The broad-band time-averaged XRT+BAT spectrum is well modelled with a strongly absorbed flat power law, with absorbing column NH = 7 ? 1022 cm-2 and photon index Γ = 0.5, modified by a high energy exponential cutoff at Ecut = 14 keV. [ABSTRACT FROM AUTHOR]

Broad emission features of abundant chemical elements, such as iron, are commonly seen in the X-ray spectra of accreting compact objects and their studies can provide useful information about the geometry of the accretion processes. In this work, we focus our attention on GX 3+1, a bright, persistent accreting low-mass X-ray binary, classified as an atoll source. Its spectrum is well described by an accretion disc plus a stable Comptonizing, optically thick corona which dominates the X-ray emission in the 0.3-20 keV energy band. In addition, four broad emission lines are found and we associate them with reflection of hard photons from the inner regions of the accretion disc, where Doppler and relativistic effects are important. We used self-consistent reflection models to fit the spectra of the 2010 XMM-Newton observation and the stacking of the whole data sets of 2010 INTEGRAL observations. We conclude that the spectra are consistent with reflection produced at ~10 gravitational radii by an accretion disc with an ionization parameter of ξ ~ 600 erg cm s-1 and viewed under an inclination angle of the system of ~35°. Furthermore, we detected for the first time for GX 3+1, the presence of a power-law component dominant at energies higher than 20 keV, possibly associated with an optically thin component of non-thermal electrons. [ABSTRACT FROM AUTHOR]

The bursting pulsar, GRO J1744-28, went again in outburst after ~18 yr of quiescence in 2014 mid-January. We studied the broad-band, persistent, X-ray spectrum using X-ray data from a XMM-Newton observation, performed almost at the peak of the outburst, and from a close INTEGRAL observation, performed 3 d later, thus covering the 1.3-70.0 keV band. The spectrum shows a complex continuum shape that cannot be modelled with standard high-mass X-ray pulsar models, nor by two-components models. We observe broad-band and peaked residuals from 4 to 15 keV, and we propose a self-consistent interpretation of these residuals, assuming they are produced by cyclotron absorption features and by a moderately smeared, highly ionized, reflection component. We identify the cyclotron fundamental at ~4.7 keV, with hints for two possible harmonics at ~10.4 and ~15.8 keV. The position of the cyclotron fundamental allows an estimate for the pulsar magnetic field of (5.27 ± 0.06) × 1011 G, if the feature is produced at its surface. From the dynamical and relativistic smearing of the disc reflected component, we obtain a lower limit estimate for the truncated accretion disc inner radius (≳100 Rg) and for the inclination angle (18°-48°). We also detect the presence of a softer thermal component that we associate with the emission from an accretion disc truncated at a distance from the pulsar of 50-115 Rg. From these estimates, we derive the magnetospheric radius for disc accretion to be ~0.2 times the classical Alfvén radius for radial accretion. [ABSTRACT FROM AUTHOR]

IGR J19149+1036 is a high-mass X-ray binary detected by INTEGRALin 2011 in the hard X-ray domain. We have analysed the Burst Alert Telescope (BAT) survey data of the first 103 months of the Swift mission detecting this source at a significance level of ∼30 standard deviations. The timing analysis on the long-term BAT light curve reveals the presence of a strong sinusoidal intensity modulation of 22.25 ± 0.05 d, that we interpret as the orbital period of this binary system. A broad-band (0.3–150 keV) spectral analysis was performed combining the BAT spectrum and the X-Ray Telescope (XRT) spectra from the pointed follow-up observations. The spectrum is adequately modelled with an absorbed power law with a high-energy cutoff at ∼24 keV and an absorption cyclotron feature at ∼31 keV. Correcting for the gravitational redshift, the inferred magnetic field at the neutron star surface is Bsurf ∼ 3.6 × 1012 G. [ABSTRACT FROM AUTHOR]

When the EPIC-pn instrument on board XMM–Newton is operated in Timing mode, high count rates (>100 counts s−1) of bright sources may affect the calibration of the energy scale, resulting in a modification of the real spectral shape. The corrections related to this effect are then strongly important in the study of the spectral properties. Tests of these calibrations are more suitable in sources which spectra are characterized by a large number of discrete features. Therefore, in this work, we carried out a spectral analysis of the accreting neutron star GX 13+1, which is a dipping source with several narrow absorption lines and a broad emission line in its spectrum. We tested two different correction approaches on an XMM–Newton EPIC-pn observation taken in Timing mode: the standard rate-dependent charge transfer inefficiency (RDCTI or epfast) and the new, rate-dependent pulse height amplitude (RDPHA) corrections. We found that, in general, the two corrections marginally affect the properties of the overall broad-band continuum, while hints of differences in the broad emission line spectral shape are seen. On the other hand, they are dramatically important for the centroid energy of the absorption lines. In particular, the RDPHA corrections provide a better estimate of the spectral properties of these features than the RDCTI corrections. Indeed the discrete features observed in the data, applying the former method, are physically more consistent with those already found in other Chandra and XMM–Newton observations of GX 13+1. [ABSTRACT FROM AUTHOR]

We present a study of the accretion flow to the intermittent accreting millisecond pulsar, HETE J1900.1−2455, based on observations made simultaneously by XMM–Newton and RXTE. The 0.33–50 keV energy spectrum is described by the sum of a hard Comptonized component produced in an optically thin τ ≃ 1 corona, a soft thermal kTin ≃ 0.2 keV component interpreted as accretion disc emission, and of disc reflection of the hard component. Two emission features are detected at energies of 0.98(1) and 6.58(7) keV, respectively. The latter is identified as Kα transition of Fe xxiii–xxv. A simultaneous detection in the European Photon Imaging Camera (EPIC-pn), EPIC-MOS2 and Reflection Grating Spectrometer (RGS) spectra favours an astrophysical origin also for the latter, which has an energy compatible with Fe-Lα and helium-like Ne-Kα transitions. The broadness of the two features, σ/E ≃ 0.1, suggests a common origin, resulting from reflection in an accretion disc with inclination of (30+ 4− 3)°, and extending down to Rin = 25+ 16− 11 gravitational radii from the compact object. However, the strength of the feature at lower energy measured by EPIC-pn cannot be entirely reconciled with the amplitude of the Fe-Kα line, hampering the possibility of describing it in terms of a broad-band reflection model, and preventing a firm identification. Pulsations at the known 377.3 Hz spin frequency could not be detected with an upper limit of 0.4 per cent at 3σ confidence level on the pulsed fractional amplitude.We interpret the value of the inner disc radius estimated from spectral modelling and the lack of significant detection of coherent X-ray pulsations as an indication of a disc accretion flow truncated by some mechanism connected to the overall evolution of the accretion disc, rather than by the neutron star magnetic field. This is compatible with the extremely close similarity of spectral and temporal properties of this source with respect to other, non-pulsing atoll sources in the hard state. [ABSTRACT FROM AUTHOR]

ABSTRACT We analyse the spectral and pulse properties of the 11 Hz transient accreting pulsar, IGR J17480−2446, in the globular cluster Terzan 5, considering all the available Rossi X-Ray Timing Explorer, Swift and INTEGRAL observations performed during the outburst shown between 2010 October and November. By measuring the pulse phase evolution we conclude that the neutron star (NS) spun up at an average rate of Hz s−1, compatible with the accretion of the Keplerian angular momentum of matter at the inner disc boundary. This confirms the trend previously observed by Papitto et al., who considered only the first few weeks of the outburst. Similar to other accreting pulsars, the stability of the pulse phases determined by using the second harmonic component is higher than that of the phases based on the fundamental frequency. Under the assumption that the second harmonic is a good tracer of the NS spin frequency, we successfully model its evolution in terms of a luminosity-dependent accretion torque. If the NS accretes the specific Keplerian angular momentum of the in-flowing matter, we estimate the inner disc radius to lie between 47 and 93 km when the luminosity attains its peak value. Smaller values are obtained if the interaction between the magnetic field lines and the plasma in the disc is considered. The phase-averaged spectrum is described by thermal Comptonization of photons with energy of ≈1 keV. A hard to soft state transition is observed during the outburst rise. The Comptonized spectrum evolves from a Comptonizing cloud at an electron temperature of ≈20 keV towards an optically denser cloud at kTe≈ 3 keV. At the same time, the pulse amplitude decreases from 27 per cent to few per cent, as already noted by Papitto et al., and becomes strongly energy dependent. We discuss various possibilities to explain such a behaviour, proposing that at large accretion luminosities a significant fraction of the in-falling matter is not channelled towards the magnetic poles, but rather accretes more evenly on to the NS surface. [ABSTRACT FROM AUTHOR]

The newly discovered 11-Hz accreting pulsar, IGR J17480−2446, located in the globular cluster Terzan 5, has shown several bursts with a recurrence time as short as a few minutes. The source shows the shortest recurrence time ever observed from a neutron star. Here we present a study of the morphological, spectral and temporal properties of 107 bursts observed by the Rossi X-ray Timing Explorer. The recurrence time and the fluence of the bursts clearly anticorrelate with the increase in the persistent X-ray flux. The ratio between the energy generated by the accretion of mass and that liberated during bursts indicates that helium is ignited in a hydrogen-rich layer. Therefore, we conclude that all the bursts shown by IGR J17480−2446 are Type I X-ray bursts. Pulsations could be detected in all the brightest bursts and no drifts of the frequency are observed within 0.25 Hz of the spin frequency of the neutron star. These are also phase-locked with respect to the pulsations observed during the persistent emission and no rise in the rms associated to the pulse frequency is observed during the burst. This behaviour would favour a scenario where the Type I burst, possibly ignited at the polar caps, immediately propagates to the entire neutron star surface. [ABSTRACT FROM AUTHOR]

We report on a 70 ks XMM–Newton Target of Opportunity (ToO) observation of the newly discovered accreting millisecond pulsar, IGR J 3057. Pulsations at 244.833 9512(1) Hz are observed throughout the outburst with an rms-pulsed fraction of 14.4(3) per cent. Pulsations have been used to derive a precise solution for the binary system. The measured mass function indicates a main-sequence companion star with a mass between 0.15 and . The XMM–Newton 0.5–11 keV spectrum of IGR J 3057 can be modelled by at least three components, which we interpret, from the softest to the hardest, as multi-coloured disc emission, thermal emission from the neutron star surface and thermal Comptonization emission. Spectral fit of the XMM–Newton data and of the Rossi X-ray Timing Explorer ( RXTE) data, taken in a simultaneous temporal window, well constrain the Comptonization parameters: the electron temperature, , is rather high, while the optical depth ( ) is moderate. The energy dependence of the pulsed fraction supports the interpretation of the cooler thermal component as coming from the accretion disc, and indicates that the Comptonizing plasma surrounds the hot spots on the neutron star surface, which in turn provides the seed photons. Signatures of reflection, such as a broadened iron Kα emission line and a Compton hump at ∼30 keV, are also detected. We derive from the smearing of the reflection component an inner disc radius of ≳40 km for a neutron star, and an inclination between 38° and 68°. XMM–Newton also observed two type I X-ray bursts, whose fluence and recurrence time suggest that the bursts are ignited in a nearly pure helium environment. No photospheric radius expansion is observed, thus leading to an upper limit on the distance to the source of 10 kpc. A lower limit of 6.5 kpc can be also set if it is assumed that emission during the decaying part of the burst involves the whole neutron star surface. Pulsations are observed during the burst decay with an amplitude similar to the persistent emission. They are also compatible with being phase locked to pre-burst pulsations, suggesting that the location on the neutron star surface where they are formed does not change much during bursts. [ABSTRACT FROM AUTHOR]

We have analysed the Swift data relevant to the high-mass X-ray binary Swift J1816.7–1613. The timing analysis of the Burst Alert Telescope survey data unveiled a modulation at a period of P0 = 118.5 ± 0.8 d, which we interpret as the orbital period of the X-ray binary system. The modulation is the result of a sequence of bright flares, lasting ∼30 d, separated by long quiescence intervals. This behaviour is suggestive of a Be binary system, where periodic or quasi-periodic outbursts are the consequence of an enhancement of the accretion flow from the companion star at the periastron passage. The position of Swift J1816.7–1613 on the Corbet diagram strengthens this hypothesis. The broad-band 0.2–150 keV spectrum is well modelled with a strongly absorbed power law with a flat photon index Γ ∼ 0.2 and a cut-off at ∼10 keV. [ABSTRACT FROM PUBLISHER]