We report on International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of the soft γ-ray repeater SGR 1935+2154 performed between 2020 April 28 and May 3. Several short bursts with fluence of erg cm−2 were detected by the Imager on-board INTEGRAL (IBIS) instrument in the 20–200 keV range. The burst with the hardest spectrum, discovered and localized in real time by the INTEGRAL Burst Alert System, was spatially and temporally coincident with a short and very bright radio burst detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and Survey for Transient Astronomical Radio Emission 2 (STARE2) radio telescopes at 400–800 MHz and 1.4 GHz, respectively. Its lightcurve shows three narrow peaks separated by ∼29 ms time intervals, superimposed on a broad pulse lasting ∼0.6 s. The brightest peak had a delay of 6.5 ± 1.0 ms with respect to the 1.4 GHz radio pulse (that coincides with the second and brightest component seen at lower frequencies). The burst spectrum, an exponentially cutoff power law with photon index and peak energy , is harder than those of the bursts usually observed from this and other magnetars. By the analysis of an expanding dust-scattering ring seen in X-rays with the Neil Gehrels Swift Observatory X-ray Telescope (XRT) instrument, we derived a distance of kpc for SGR 1935+2154, independent of its possible association with the supernova remnant G57.2+0.8. At this distance, the burst 20–200 keV fluence of erg cm−2 corresponds to an isotropic emitted energy of erg. This is the first burst with a radio counterpart observed from a soft γ-ray repeater and it strongly supports models based on magnetars that have been proposed for extragalactic fast radio bursts.
We report on the serendipitous discovery of a new transient in NGC 5907, at a peak luminosity of 6.4×10 39 erg s −1 . The source was undetected in previous 2012 Chandra observations with a 3σ upper limit on the luminosity of 1.5 × 10 38 erg s −1 , implying a flux increase of a factor of > 35. We analyzed three recent 60ks/50ks Chandra and 50ks XMM-Newton observations, as well as all the available Swift/XRT observations performed between August 2017/March 2018. Until the first half of October 2017, Swift/XRT observations do not show any emission from the source. The transient entered the ULX regime in less than two weeks and its outburst was still on-going at the end of February 2018. The 0.3-10 keV spectrum is consistent with a single multicolour blackbody disc (kT∼ 1.5 keV). The source might be a ∼30 M black hole accreting at the Eddington limit. However, although we did not find evidence of pulsations, we cannot rule-out the possibility that this ULX hosts an accreting neutron star.
Calvera (1RXS J141256.0+792204) is an isolated neutron star detected only through its thermal X-ray emission. Its location at high Galactic latitude (b = +37°) is unusual if Calvera is a relatively young pulsar, as suggested by its spin period (59 ms) and period derivative (3.2 × 10−15 s s−1). Using the Neutron Star Interior Composition Explorer, we obtained a phase-connected timing solution spanning four years, which allowed us to measure the second derivative of the frequency ν ̈ = − 2.5 × 10 − 23 Hz s−2 and to reveal timing noise consistent with that of normal radio pulsars. A magnetized hydrogen atmosphere model, covering the entire star surface, provides a good description of the phase-resolved spectra and energy-dependent pulsed fraction. However, we found that a temperature map more anisotropic than that produced by a dipole field is required, with a hotter zone concentrated toward the poles. By adding two small polar caps, we found that the surface effective temperature and that of the caps are ∼0.1 and ∼0.36 keV, respectively. The inferred distance is ∼3.3 kpc. We confirmed the presence of an absorption line at 0.7 keV associated with the emission from the whole star surface, difficult to interpret as a cyclotron feature and more likely originating from atomic transitions. We searched for pulsed γ-ray emission by folding seven years of Fermi-LAT data using the X-ray ephemeris, but no evidence for pulsations was found. Our results favor the hypothesis that Calvera is a normal rotation-powered pulsar, with the only peculiarity of being born at a large height above the Galactic disk.
We report on the X-ray properties of four rotation-powered pulsars with characteristic ages in the range 0.3 − 5 Myr, derived from the analysis of XMM-Newton archival observations. We found convincing evidence of thermal emission only in the phase-averaged spectrum of PSR B0114+58, that is well fitted by a blackbody with temperature kT = 0.17 ± 0.02 keV and emitting radius R = 405 +110 −90 m, consistent with the size of its polar cap. The other three considered pulsars, PSR B0628−28, PSR B0919+06 and PSR B1133+16, have phase-averaged spectra well described by single power-laws with photon index Γ ∼ 3. The 3σ upper limits on the bolometric luminosity of a possible thermal component with temperature in the range ∼ 0.05 − 2 keV are L bol 3.2 × 10 28 erg s −1 and L bol 2.4 × 10 29 erg s −1 , for PSR B0628−28 and PSR B0919+06, respectively. On the other hand, we found possible evidence that the pulsed emission of PSR B0628−28 is thermal. Two absorption lines at ∼ 0.22 keV and ∼ 0.44 keV are detected in the spectrum of PSR B1133+16. They are best interpreted as proton cyclotron features, implying the presence of multipolar components with a field of a few 10 13 G at the neutron star polar caps. We discuss our results in the context of high-energy emission models of old rotation-powered pulsars.
Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 10 39 erg s −1 . The ULX population counts several hundreds objects but only a minor fraction is well studied. Here we present a detailed analysis of all ULXs hosted in the galaxy NGC 7456. It was observed in X-rays only once in the past (in 2005) by XMM-Newton but the observation was short and strongly affected by high background. In 2018, we obtained a new, deeper (∼ 90 ks) XMM-Newton observation that allowed us to perform a detailed characterization of the ULXs hosted in the galaxy. ULX-1 and ULX-2, the two brightest objects (L X ∼ 6 − 10 × 10 39 erg s −1 ), have spectra that can be described by a two-thermal component model as often found in ULXs. ULX-1 shows also one order of magnitude in flux variability on short-term timescales (hundreds to thousand ks). The other sources (ULX-3 and ULX-4) show flux changes of at least an order of magnitude, and these objects may be candidate transient ULXs although longer X-ray monitoring or further studies are required to ascribe them to the ULX population. In addition, we found a previously undetected source that might be a new candidate ULX (labelled as ULX-5) with a luminosity of ∼ 10 39 erg s −1 and hard power-law spectral shape, whose nature is still unclear and for which a background Active Galactic Nucleus cannot be excluded. We discuss the properties of all the ULXs in NGC 7456 within the framework of super-Eddington accretion onto stellar mass compact objects. Although no pulsations were detected, we cannot exclude that the sources host neutron stars.
After 15 yr, in late 2018, the magnetar XTE J1810−197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM–Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of 2 change in its spin-down rate from ∼7.2 × 10−12 to ∼1.5 × 10−11 s s−1 after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810−197 is well modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810−197 has likely evolved relative to that found for the 2003 event.
Most isolated neutron stars have been discovered thanks to the detection of their pulsed non-thermal emission, at wavelengths spanning from radio to gamma-rays. However, if the beamed non-thermal radiation does not intercept our line of sight or it is too faint or absent, isolated neutron stars can also be detected through their thermal emission, which peaks in the soft X-ray band and is emitted nearly isotropically. In the past thirty years, several thermally-emitting isolated neutron stars have been discovered thanks to X-ray all-sky surveys, observations targeted at the center of supernova remnants, or as serendipitous X-ray sources. Distinctive properties of these relatively rare X-ray sources are very soft spectra and high ratios of X-ray to optical flux. The recently released 4XMM-DR10 catalog contains more than half a million X-ray sources detected with the XMM-Newton telescope in the 0.2–10 keV range in observations carried out from 2000 to 2019. Based on a study of the spectral properties of these sources and on cross-correlations with catalogs of possible counterparts, we have carried out a search of isolated neutron stars, finding four potential candidates. The spectral and long-term variability analysis of these candidates, using also Chandra and Swift-XRT data, allowed us to point out the most interesting sources deserving further multiwavelength investigations.
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