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We present the evolution of the X-ray emission properties of the magnetar 1E 1547.0-5408 since February 2004 over a time period covering three outbursts. We analyzed new and archival observations taken with the Swift, NuSTAR, Chandra, and XMM-Newton X-ray satellites. The source has been observed at a relatively steady soft X-ray flux of ≈10-11 erg cm-2 s-1 (0.3-10 keV) over the last 9 years, which is about an order of magnitude fainter than the flux at the peak of the last outburst in 2009, but a factor of ∼30 larger than the level in 2006. The broad-band spectrum extracted from two recent NuSTAR observations in April 2016 and February 2019 showed a faint hard X-ray emission up to ∼70 keV. Its spectrum is adequately described by a flat power law component, and its flux is ∼7 × 10-12 erg cm-2 s-1 (10-70 keV), that is a factor of ∼20 smaller than at the peak of the 2009 outburst. The hard X-ray spectral shape has flattened significantly in time, which is at variance with the overall cooling trend of the soft X-ray component. The pulse profile extracted from these NuSTAR pointings displays variability in shape and amplitude with energy (up to ≈25 keV). Our analysis shows that the flux of 1E 1547.0-5408 is not yet decaying to the 2006 level and that the source has been lingering in a stable, high-intensity state for several years. This might suggest that magnetars can hop among distinct persistent states that are probably connected to outburst episodes and that their persistent thermal emission can be almost entirely powered by the dissipation of currents in the corona., FCZ acknowledges Matthew Baring for helpful discussions. FCZ, AB, NR and DV acknowledge support from grants SGR2017-1383 and PGC2018-095512-B-I00 and the support of the PHAROS COST Action (CA16214). FCZ is also supported by a Juan de la Cierva fellowship. NR, AB and DV are also supported by the ERC Consolidator Grant “MAGNESIA” (nr. 817661). DV acknowledges support from grants AYA2016-80289-P and AYA2017-82089-ERC. TE is supported by JSPS KAKENHI grant numbers 16H02198, 18H01246 and 18H04584. JAP acknowledges support from the Spanish Agencia Estatal de Investigación (grant PGC2018-095984-B-I00) and the Generalitat Valenciana (grant PROMETEO/2019/071). The scientific results reported in this article are based on data obtained with Swift, the Chandra X-ray Observatory, XMM–Newton and the NuSTAR mission. XMM–Newton is an ESA science mission with instruments and contributions directly funded by ESA Member States and the National Aeronautics and Space Administration (NASA). The NuSTAR mission is a project led by the California Insitute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. We made use of data supplied by the UK Swift Science Data Centre at the University of Leicester and of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. We also made use of the NuSTAR Data Analysis Software (NUSTARDAS jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA), and of softwares and tools provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory.

We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1810-197, 1RXS J1708-4009, 1E 1841-045 and the soft gamma-ray repeater SGR 1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes. . This work is aimed at identifying and/or confirming the IR counterparts of these magnetars, as well as at measuring their possible IR variability. . In order to perform photometry of objects as faint as Ks~20, we have used data taken with the largest telescopes, equipped with the most advanced IR detectors and in most of the cases with Adaptive Optics devices. The latter are critical to achieve the sharp spatial accuracy required to pinpoint faint objects in crowded fields. . We confirm with high confidence the identification of the IR counterpart to XTE J1810-197, and its IR variability. For 1E 1841-045 and SGR 1900+14 we propose two candidate IR counterparts based on the detection of IR variability. For 1RXS J1708-4009 we show that none of the potential counterparts within the source X-ray error circle can be yet convincingly associated with this AXP. . The IR variability of the AXP XTE J1810-197 does not follow the same monotonic decrease of its post-outburst X-ray emission. Instead, the IR variability appears more similar to the one observed in radio band, although simultaneous IR and radio observations are crucial to draw any conclusion in this respect. For 1E 1841-045 and SGR 1900+14, follow-up observations are needed to confirm our proposed candidates with higher confidence., Comment: 9 pages, 10 figures; accepted by A&A (high resolution images at http://staff.science.uva.nl/~nrea/NIR_AXP_testa2007.pdf)

New information on short/hard gamma-ray bursts (GRBs) is being gathered thanks to the discovery of their optical and X-ray afterglows. However, some key aspects are still poorly understood, including the collimation level of the outflow, the duration of the central engine activity, and the properties of the progenitor systems. We want to constrain the physical properties of the short GRB 050724 and of its host galaxy, and make some inferences on the global short GRB population. We present optical observations of the afterglow of GRB 050724 and of its host galaxy, significantly expanding the existing dataset for this event. We compare our results with models, complementing them with available measurements from the literature. We study the afterglow light curve and spectrum including X-ray data. We also present observations of the host galaxy. The observed optical emission was likely related to the large flare observed in the X-ray light curve. The apparent steep decay was therefore not due to the jet effect. Available data are indeed consistent with low collimation, in turn implying a large energy release, comparable to that of long GRBs. The flare properties also constrain the internal shock mechanism, requiring a large Lorentz factor contrast between the colliding shells. This implies that the central engine was active at late times, rather than ejecting all shells simultaneously. The host galaxy has red colors and no ongoing star formation, consistent with previous findings on this GRB. However, it is not a pure elliptical, and has some faint spiral structure. GRB 050724 provides the most compelling case for association between a short burst and a galaxy with old stellar population. It thus plays a pivotal role in constraining progenitors models, which should allow for long delays between birth and explosion., 8 pages, 5 figures, 4 tables, accepted for publication in A&A, typo fixed

Optical spectroscopic identification of the nature of 21 unidentified southern hard X-ray objects is reported here in the framework of our campaign aimed at determining the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 5 of these objects are magnetic Cataclysmic Variables (CVs), 4 are High-Mass X-ray Binaries (HMXBs; one of which is in the Large Magellanic Cloud) and 12 are Active Galactic Nuclei (AGNs). When feasible, the main physical parameters for these hard X-ray sources are also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in the study of the hard X-ray spectrum of AGNs, HMXBs and CVs, and the usefulness of a strategy of catalogues cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of the X-ray sources detected with INTEGRAL., 14 pages, 7 figures, 6 tables, accepted for publication on A&A, main journal. Figures 1 and 2 and Table 1 will only appear on the on-line version of the paper

We report here gamma-ray, X-ray and near-infrared observations of GRB040223 along with gamma-ray and optical observations of GRB040624. GRB040223 was detected by INTEGRAL close to the Galactic plane and GRB040624 at high Galactic latitude. Analyses of the prompt emission detected by the IBIS instrument on INTEGRAL are presented for both bursts. The two GRBs have long durations, slow pulses and are weak. The gamma-ray spectra of both bursts are best fit with steep power-laws, implying they are X-ray rich. GRB040223 is among the weakest and longest of INTEGRAL GRBs. The X-ray afterglow of this burst was detected 10 hours after the prompt event by XMM-Newton. The measured spectral properties are consistent with a column density much higher than that expected from the Galaxy, indicating strong intrinsic absorption. We carried out near-infrared observations 17 hours after the burst with the NTT of ESO, which yielded upper limits. Given the intrinsic absorption, we find that these limits are compatible with a simple extrapolation of the X-ray afterglow properties. For GRB040624, we carried out optical observations 13 hours after the burst with FORS 1 and 2 at the VLT, and DOLoRes at the TNG, again obtaining upper limits. We compare these limits with the magnitudes of a compilation of promptly observed counterparts of previous GRBs and show that they lie at the very faint end of the distribution. These two bursts are good examples of a population of bursts with dark or faint afterglows that are being unveiled through the increasing usage of large diameter telescopes engaged in comprehensive observational programmes., Comment: 13 pages, 12 figures, accepted for publication in A&A

We consider the Unipolar Inductor Model (Goldreich & Lynden-Bell 1969) applied to Double Degenerate Binaries (DDBs) with ultrashort periods (Wu et al. 2002). In this model a magnetized primary white dwarf has a slight asynchronism between its spin and orbital motion, so that the (non-magnetic) secondary experiences a motional electric field when moving through the primary field lines. This induces a current flow between the two stars and provides an electric spin-orbit coupling mechanism for the primary. We study the combined effect of Gravitational Wave emission and electric spin-orbit coupling on the evolution of the primary degree of asynchronism and the associated rate of electric current dissipation in such systems, assuming that the primary's spin is not affected by any other mechanisms. In particular, we show that in ultrashort period binaries the emission of GW pumps energy in the electric circuit as to keep it steadily active. This happens despite the fact that spin-orbit coupling can rapidly synchronize the primary, because GW represent a slow desynchronizing mechanism steadily substracting orbital angular momentum to the system. A slightly asynchronous steady-state is thus achieved, determined by the balance between these two competing effects. This can be shown to correspond to a condition where the total available electric energy is conserved, because of GW emission, while dissipation, synchronization and orbital shrinking continue.

We present ESO-NTT low resolution spectroscopy of the neutron star X-ray transient Cen X-4 in quiescence over a complete orbital cycle. Our data reveal the presence of a K3-7 V companion which contributes 63% to the 5600-6900A flux and orbits the neutron star with a velocity semi-amplitude of K_2=145.8 +/- 1.0 km s^{-1}. This, combined with a previous determination of the inclination angle and mass ratio, yields a neutron star and companion mass of M_1=1.5 +/- 1.0 M_Sun and M_2=0.31 +/- 0.27 M_Sun, respectively. The mass donor is thus undermassive for the inferred spectral type indicating it is probably evolved, in agreement with previous studies. Doppler tomography of the H_alpha line shows prominent emission located on the companion and a slightly asymmetric accretion disc distribution similar to that seen in systems with precessing eccentric discs. Strong H_alpha emission from the companion can be explained by X-ray irradiation from the primary. No evidence is found for a hot spot in H_alpha, whereas one is revealed via Doppler tomography of the HeI lines. This can be interpreted as the hot spot and outer regions of the disc being at a higher temperature than in other systems., 9 pages, 5 figures, accepted for publication in A&A

We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X-ray Pulsar (AXP) XTE J1810−197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004a, ApJ, 603, L97) was fainter by ∆H = 0.7 ± 0.2 and ∆Ks = 0.5 ± 0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810−197. We also report on an XMM-Newton observation carried out the day before the VLT observations. The 0.5−10 keV absorbed flux of the source was 2.2 × 10 −11 erg cm −2 s −1 , which is less by a factor of about two compared to the previous XMM-Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X-ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of AXPs.

We report on spectral and timing analysis of BeppoSAX data of the 13.6 s period transient X-ray pulsar EXO 0531-6609.2. Observations were carried out in March 1997 and October 1998, catching the source during a high and a low emission state, respectively. Correspondingly, the X-ray luminosity is found at a level of 4.2x10^37 erg/s and 1.5x10^36 erg/s in the two states. In the high state the X-ray emission in the energy range 1-100 keV is well fitted by an absorbed power-law with photon index Gamma ~1.7 plus a blackbody component with a characteristic temperature of ~3.5 keV. Moreover, we find an evidence of an iron emission at ~6.8 keV, typical feature in this class of sources but never revealed before in the EXO 0531-6609.2 spectrum. In the low state an absorbed power-law with Gamma ~0.4 is sufficient to fit the 1-10 keV data. During BeppoSAX observations EXO 0531-6609.2 display variations of the pulse profile with the X-ray flux: it showed single peaked and double peaked profiles in the low and high state, respectively. Based on these two observations we infer a spin-up period derivative of -(1.14+/-0.08)x10^-10 s/s. By comparing these with other period measurements reported in literature we find an alternating spin-up and spin-down behaviour that correlates well with the X-ray luminosity., 6 pages, 8 figures, A&A

We report the discovery of the new dipping X-ray source, XMMU J004308.6+411247, in M31, during a systematic search for periodicities in XMM-Newton archival observations. During the 2002 January 6 observation, the dips recur with a 107 min period and the source count rate is consistent with zero at the dip minimum. Dips with the same modulation and period are also observed during the XMM-Newton observations carried out on 2000 June 25, 2001 June 29 and the Chandra observation of 2001 October 5. The dips of XMMU J004308.6+411247 show no evidence of energy dependence. The average X-ray flux of XMMU J004308.6+411247 is nearly constant across different observations (~1.e37 erg/s for an assumed M31 distance of 780 kpc in the 0.3-10 keV band); the spectrum is well fit by an absorbed power law with a photon index ~0.8 or an absorbed Comptonization model. The photo-electric absorption is consistent with the Galactic value in the source direction. If XMMU J004308.6+411247 is located in M31 its properties are consistent with those of dipping low mass X-ray binaries in the Galaxy. Present observations do not allow to distinguish between dips and eclipses. The possibility that XMMU J004308.6+411247 is a foreground X-ray source cannot be ruled out at present; in this case the source might be a magnetic cataclysmic variable., Comment: Accepted for publication in A&A, 16 pages, 8 postscript figures

aims: We obtained phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its outburst in 2008 to find a signature of the donor star, constrain its radial velocity semi-amplitude (K_2), and derive estimates on the pulsar mass. methods: Using Doppler images of the Bowen region we find a significant (>8sigma) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure K_em=248+/-20 km/s (1sigma confidence) which represents a strict lower limit to K_2. Also, the Doppler map of He II lambda4686 shows the characteristic signature of the accretion disk, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disk that are expected in very low mass ratio interacting binaries. results: The lower-limit on K_2 leads to a lower-limit on the mass function of f(M_1)>0.10M_sun. Applying the maximum K-correction gives 228, Comment: 4 pages, 5 figures, accepted for publication in A&A Letters

Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries associated with OB supergiant companions and characterised by an X-ray flaring behaviour whose dynamical range reaches 5 orders of magnitude on timescales of a few hundred to thousands of seconds. Current investigations concentrate on finding possible mechanisms to inhibit accretion in SFXTs and explain their unusually low average X-ray luminosity. We present the Swift observations of an exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014 October 10 when the source achieved a peak luminosity of $3\times10^{38}$ erg s$^{-1}$. This extends the total source dynamic range to $\gtrsim$10$^6$, the largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence for pulsations at a period of 11.6 s is also reported. We show that these observations challenge, for the first time, the maximum theoretical luminosity achievable by an SFXT and propose that this giant outburst was due to the formation of a transient accretion disc around the compact object., Comment: Accepted for publication in Astronomy and Astrophysics Letters. 5 pages, 5 figures, 2 tables

The system RX J0806.3+1527 (HM Cnc) is a pulsating X-ray source with 100 per cent modulation on a period of 321.5 s (5.4 min). This period reflects the orbital motion of a close binary consisting of two interacting white dwarfs. Here we present a series of simultaneous X-ray (0.2-10 keV) and near-ultraviolet (2600 angstrom and 1928 angstrom) observations that were carried out with the Swift satellite. In the near-ultraviolet, the counterpart of RX J0806.3+1527 was detected at flux densities consistent with a blackbody with temperature 27E+3 K. We found that the emission at 2600 angstrom is modulated at the 321.5-s period with the peak ahead of the X-ray one by 0.28 cycles and is coincident within 0.05 cycles with the optical. This phase-shift measurement confirms that the X-ray hot spot (located on the primary white dwarf) is at about 80-100 degrees from the direction that connects the two white dwarfs. Albeit at lower significance, the 321.5-s signature is present also in the 1928-angstrom data; at this wavelength, however, the pulse peak is better aligned with that observed at X-rays. We use the constraints on the source luminosity and the geometry of the emitting regions to discuss the merits and limits of the main models for RX J0806.3+1527., Comment: Minor changes to match final version (Astronomy & Astrophysics)

Context. Supergiant fast X-ray transients are a subclass of high mass X-ray binaries that host a neutron star accreting mass from the wind of its OB supergiant companion. They are characterized by an extremely pronounced and rapid variability in X-rays, which still lacks an unambiguous interpretation. A number of deep pointed observations with XMM-Newton have been carried out to study the quiescent emission of these sources and gain insight into the mechanism that causes their X-ray variability. Aims. We continued this study by using three XMM-Newton observations of the two supergiant fast X-ray transient prototypes XTEJ1739-302 and IGR J08408-4503 in quiescence. Methods. An in-depth timing and spectral analysis of these data have been carried out. Results. We found that the quiescent emission of these sources is characterized by both complex timing and spectral variability, with multiple small flares occurring sporadically after periods of lower X-ray emission. Some evidence is found in the XMM-Newton spectra of a soft component below ~2 keV, similar to that observed in the two supergiant fast X-ray transients AXJ1845.0-0433 and IGRJ16207-5129 and in many other high mass X-ray binaries. Conclusions.We suggest some possible interpretations of the timing and spectral properties of the quiescent emission of XTEJ1739- 302 and IGR J08408-4503 in the context of the different theoretical models proposed to interpret the behavior of the supergiant fast X-ray transients., 13 pages, 14 figures. Accepted for publication in A&A. V2: Corrected few typos