ALBERT

Description: We report the detection of a possible gamma-ray counterpart of the accreting millisecond pulsar SAX J1808.4–3658. The analysis of ~6 yr of data from the Large Area Telescope on board the Fermi gamma-ray Space Telescope ( Fermi -LAT) within a region of 15° radius around the position of the pulsar reveals a point gamma-ray source detected at a significance of ~6 (test statistic TS = 32), with a position compatible with that of SAX J1808.4–3658 within the 95 per cent confidence level. The energy flux in the energy range between 0.6 and 10 GeV amounts to (2.1 ± 0.5) x 10 –12  erg cm –2  s –1 and the spectrum is represented well by a power-law function with photon index 2.1 ± 0.1. We searched for significant variation of the flux at the spin frequency of the pulsar and for orbital modulation, taking into account the trials due to the uncertainties in the position, the orbital motion of the pulsar and the intrinsic evolution of the pulsar spin. No significant deviation from a constant flux at any time-scale was found, preventing a firm identification via time variability. Nonetheless, the association of the LAT source as the gamma-ray counterpart of SAX J1808.4–3658 would match the emission expected from the millisecond pulsar, if it switches on as a rotation-powered source during X-ray quiescence.

Description: X-ray transients, such as accreting neutron stars, periodically undergo outbursts, thought to be caused by a thermal-viscous instability in the accretion disc. Usually outbursts of accreting neutron stars are identified when the accretion disc has undergone an instability, and the persistent X-ray flux has risen to a threshold detectable by all sky monitors on X-ray space observatories. Here, we present the earliest known combined optical, UV, and X-ray monitoring observations of the outburst onset of an accreting neutron star low-mass X-ray binary (LMXB) system. We observed a significant, continuing increase in the optical i′-band magnitude starting on July 25, 12 d before the first X-ray detection with Swift/XRT and NICER (August 6), during the onset of the 2019 outburst of SAX J1808.4−3658. We also observed a 4 d optical to X-ray rise delay, and a 2 d UV to X-ray delay, at the onset of the outburst. We present the multiwavelength observations that were obtained, discussing the theory of outbursts in X-ray transients, including the disc instability model, and the implications of the delay. This work is an important confirmation of the delay in optical to X-ray emission during the onset of outbursts in LMXBs, which has only previously been measured with less sensitive all sky monitors. We find observational evidence that the outburst is triggered by ionization of hydrogen in the disc.

Description: We present an analysis of X-ray, ultraviolet and optical/near-IR photometric data of the transitional millisecond pulsar binary XSS J12270–4859, obtained at different epochs after the transition to a rotation-powered radio pulsar state. The observations, while confirming the large-amplitude orbital modulation found in previous studies after the state change, also reveal an energy dependence of the amplitudes as well as variations on time-scale of months. The amplitude variations are anticorrelated in the X-ray and the UV/optical bands. The average X-ray spectrum is described by a power law with index of 1.07(8) without requiring an additional thermal component. The power-law index varies from ~1.2 to ~1.0 between superior and inferior conjunction of the neutron star. We interpret the observed X-ray behaviour in terms of synchrotron radiation emitted in an extended intrabinary shock, located between the pulsar and the donor star, which is eclipsed due to the companion orbital motion. The G5-type donor dominates the UV/optical and near-IR emission and is similarly found to be heated up to ~6500 K as in the disc state. The analysis of optical light curves gives a binary inclination 46° i 65° and a mass ratio 0.11 q 0.26. The donor mass is found to be 0.15 M 2 0.36 M for a neutron star mass of 1.4 M . The variations in the amplitude of the orbital modulation are interpreted in terms of small changes in the mass-flow rate from the donor star. The spectral energy distribution from radio to gamma-rays is composed by multiple contributions that are different from those observed during the accretion-powered state.

Description: In 2013 April a new magnetar, SGR 1745–2900, was discovered as it entered an outburst, at only 2.4 arcsec angular distance from the supermassive black hole at the centre of the Milky Way, Sagittarius A*. SGR 1745–2900 has a surface dipolar magnetic field of ~2 x 10 14  G, and it is the neutron star closest to a black hole ever observed. The new source was detected both in the radio and X-ray bands, with a peak X-ray luminosity L X ~ 5 x 10 35  erg s –1 . Here we report on the long-term Chandra (25 observations) and XMM–Newton (eight observations) X-ray monitoring campaign of SGR 1745–2900 from the onset of the outburst in 2013 April until 2014 September. This unprecedented data set allows us to refine the timing properties of the source, as well as to study the outburst spectral evolution as a function of time and rotational phase. Our timing analysis confirms the increase in the spin period derivative by a factor of ~2 around 2013 June, and reveals that a further increase occurred between 2013 October 30 and 2014 February 21. We find that the period derivative changed from 6.6 x 10 –12 to 3.3 x 10 –11  s s –1 in 1.5 yr. On the other hand, this magnetar shows a slow flux decay compared to other magnetars and a rather inefficient surface cooling. In particular, starquake-induced crustal cooling models alone have difficulty in explaining the high luminosity of the source for the first ~200 d of its outburst, and additional heating of the star surface from currents flowing in a twisted magnetic bundle is probably playing an important role in the outburst evolution.

Description: The ultraluminous accreting pulsar M82-X2 (NuSTAR J095551+6940.8) offers an unprecedented opportunity to study the disc-magnetosphere interaction in a new regime of supercritical accretion. The source X-ray emission has been highly variable during the last 15 yrs. It ranged from a maximum of ~2 x 10 40 erg s –1 through intermediate values ~ a few x 10 39 erg s –1 , and down to a minimum below 2 x 10 38  erg s –1 that we have determined here, by analysing archival Chandra HRC observations of the source at an epoch at which it was undetected. We interpret the source variability via a magnetically threaded disc model: when at peak luminosity, the neutron star (NS) is close to spin equilibrium, its inner disc edge r m ~ 10 8  cm is approximately half the corotation radius r co , and radiation pressure dominates the disc out to r tr 10 9  cm. In the radiation-pressure-dominated regime, r m grows very slowly as the mass inflow rate drops: as a result, r m 〈 r co remains valid until $\dot{M} \gtrsim$ $\dot{M}_E$ , the Eddington accretion rate, allowing a wide range of accretion luminosities to the NS. Once $\dot{M} 〈 \dot{M}_E$ accretion on to the NS is inhibited because r m 〉 r co , and the source luminosity is expected to drop by a large factor. We conclude that a magnetically threaded accretion disc surrounding a highly magnetized NS ( B 10 13  G), and transitioning between the radiation-pressure and gas-pressure dominated regimes, offers the best interpretation for all the currently observed properties of NuSTAR J095551+6940.8.

Description: Iron emission lines at 6.4–6.97 keV, identified with Kα radiative transitions, are among the strongest discrete features in the X-ray band. These are one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper, we present a recent Suzaku observation, 100-ks effective exposure, of the atoll source and X-ray burster 4U 1705–44, where we clearly detect signatures of a reflection component which is distorted by the high-velocity motion in the accretion disc. The reflection component consists of a broad iron line at about 6.4 keV and a Compton bump at high X-ray energies, around 20 keV. All these features are consistently fitted with a reflection model, and we find that in the hard state the smearing parameters are remarkably similar to those found in a previous XMM–Newton observation performed in the soft state. In particular, we find that the inner disc radius is R in  = 17 ± 5 R g (where R g is the gravitational radius, GM / c 2 ), the emissivity dependence from the disc radius is r –2.5 ± 0.5 , the inclination angle with respect to the line of sight is i  = 43° ± 5°, and the outer radius of the emitting region in the disc is R out  〉 200 R g . We note that the accretion disc does not appear to be truncated at large radii, although the source is in a hard state at ~3 per cent of the Eddington luminosity for a neutron star. We also find evidence of a broad emission line at low energies, at 3.03 ± 0.03 keV, compatible with emission from mildly ionized argon (Ar XVI–XVII). Argon transitions are not included in the self-consistent reflection models that we used and we therefore added an extra component to our model to fit this feature. The low-energy line appears compatible with being smeared by the same inner disc parameters found for the reflection component.

Description: We report on the quiescent state of the soft gamma repeater SGR 0501+4516 observed by XMM–Newton on 2009 August 30. The source exhibits an absorbed flux ~75 times lower than that measured at the peak of the 2008 outburst, and a rather soft spectrum, with the same value of the blackbody temperature observed with ROSAT back in 1992. This new observation is put into the context of all existing X-ray data since its discovery in 2008 August, allowing us to complete the study of the timing and spectral evolution of the source from outburst until its quiescent state. The set of deep XMM–Newton observations performed during the few years time-scale of its outburst allows us to monitor the spectral characteristics of this magnetar as a function of its rotational period, and their evolution along these years. After the first ~10 d, the initially hot and bright surface spot progressively cooled down during the decay. We discuss the behaviour of this magnetar in the context of its simulated secular evolution, inferring a plausible dipolar field at birth of 3 10 14 G, and a current (magnetothermal) age of ~10 kyr.

Description: The origin of the strong magnetic fields measured in magnetars is one of the main uncertainties in the neutron star field. On the other hand, the recent discovery of a large number of such strongly magnetized neutron stars is calling for more investigation on their formation. The first proposed model for the formation of such strong magnetic fields in magnetars was through alpha-dynamo effects on the rapidly rotating core of a massive star. Other scenarios involve highly magnetic massive progenitors that conserve their strong magnetic moment into the core after the explosion, or a common envelope phase of a massive binary system. In this work, we do a complete re-analysis of the archival X-ray emission of the supernova remnants (SNRs) surrounding magnetars, and compare our results with all other bright X-ray emitting SNRs, which are associated with compact central objects (which are proposed to have magnetar-like B-fields buried in the crust by strong accretion soon after their formation), high-B pulsars and normal pulsars. We find that emission lines in SNRs hosting highly magnetic neutron stars do not differ significantly in elements or ionization state from those observed in other SNRs, neither averaging on the whole remnants, nor studying different parts of their total spatial extent. Furthermore, we find no significant evidence that the total X-ray luminosities of SNRs hosting magnetars, are on average larger than that of typical young X-ray SNRs. Although biased by a small number of objects, we found that for a similar age, there is the same percentage of magnetars showing a detectable SNR than for the normal pulsar population.

Description: XSS J12270–4859 is the only low-mass X-ray binary (LMXB) with a proposed persistent gamma-ray counterpart in the Fermi -Large Area Telescope domain, 2FGL 1227.7–4853. Here, we present the results of the analysis of recent INTEGRAL observations, aimed at assessing the long-term variability of the hard X-ray emission, and thus the stability of the accretion state. We confirm that the source behaves as a persistent hard X-ray emitter between 2003 and 2012. We propose that XSS J12270–4859 hosts a neutron star in a propeller state, a state we investigate in detail, developing a theoretical model to reproduce the associated X-ray and gamma-ray properties. This model can be understood as being of a more general nature, representing a viable alternative by which LMXBs can appear as gamma-ray sources. In particular, this may apply to the case of millisecond pulsars performing a transition from a state powered by the rotation of their magnetic field to a state powered by matter infall, such as that recently observed from the transitional pulsar PSR J1023+0038. While the surface magnetic field of a typical neutron star (NS) in an LMXB is lower by more than four orders of magnitude than the much more intense fields of neutron stars accompanying high-mass binaries, the radius at which the matter inflow is truncated in an NS-LMXB system is much smaller. The magnetic field at the magnetospheric interface is then orders of magnitude larger at this interface, and as consequence, so is the power to accelerate electrons. We demonstrate that the cooling of the accelerated electron population takes place mainly through synchrotron interaction with the magnetic field permeating the interface, and through inverse Compton losses due to the interaction between the electrons and the synchrotron photons they emit. We found that self-synchrotron Compton processes can explain the high-energy phenomenology of XSS J12270–4859.