The XMM-Newton bright serendipitous survey - Identification and optical spectral properties

Aims.We present the optical classification and redshift of 348 X-ray selected sources from the XMM-Newton Bright Serendipitous Survey (XBS), which contains a total of 400 objects (identification level = 87%). About 240 are new identifications. In particular, we discuss in detail the classification criteria adopted for the active galactic nuclei (AGNs) population. Methods.By means of systematic spectroscopic campaigns using various telescopes and through the literature search, we have collected an optical spectrum for the large majority of the sources in the XBS survey and applied a well-defined classification "flow chart". Results.We find that the AGNs represent the most numerous population at the flux limit of the XBS survey (~10-13 erg cm-2 s-1) constituting 80% of the XBS sources selected in the 0.5-4.5 keV energy band and 95% of the "hard" (4.5-7.5 keV) selected objects. Galactic sources populate the 0.5-4.5 keV sample significantly (17%) and only marginally (3%) the 4.5-7.5 keV sample. The remaining sources in both samples are clusters/groups of galaxies and normal galaxies (i.e. probably not powered by an AGN). Furthermore, the percentage of type 2 AGNs (i.e. optically absorbed AGNs with $A_{\rm V}>2$ mag) dramatically increases going from the 0.5-4.5 keV sample ( $f=N_{\rm AGN 2}/N_{\rm AGN}=7$%) to the 4.5-7.5 keV sample (f=32%). We finally propose two simple diagnostic plots that can be easily used to obtain the spectral classification for relatively low-redshift AGNs even if the quality of the spectrum is not good

Swift follow-up of gravitational wave triggers: Results from the first aLIGO run and optimization for the future

During its first observing run, in late 2015, the advanced Laser Interferometer Gravitationalwave Observatory facility announced three gravitational wave (GW) triggers to electromagnetic follow-up partners. Two of these have since been confirmed as being of astrophysical origin: both are binary black hole mergers at ~500 Mpc; the other trigger was later found not to be astrophysical. In this paper, we report on the Swift follow-up observations of the second and third triggers, including details of 21 X-ray sources detected; none of which can be associated with the GW event. We also consider the challenges that the next GW observing run will bring as the sensitivity and hence typical distance of GW events will increase. We discuss how to effectively use galaxy catalogues to prioritize areas for follow-up, especially in the presence of distance estimates from the GW data. We also consider two galaxy catalogues and suggest that the high completeness at larger distances of the 2MASS Photometric Redshift catalogue makes it very well suited to optimize Swift follow-up observations

GRB 050410 and GRB 050412: Are they really dark gamma-ray bursts?

Aims.We present a detailed analysis of the prompt and afterglow emission of GRB 050410 and GRB 050412 detected by Swift for which no optical counterpart was observed. Methods.We analysed data from the prompt emission detected by the Swift BAT and from the early phase of the afterglow obtained by the Swift narrow field instrument XRT. Results.The 15-150 keV energy distribution of the GRB 050410 prompt emission shows a peak energy at 53 -21+40 keV. The XRT light curve of this GRB decays as a power law with a slope of $\alpha=$ 1.06 $\pm$ 0.04. The spectrum is well reproduced by an absorbed power law with a spectral index $\Gamma_{\rm x}=2.4$ $\pm$ 0.4 and a low energy absorption $N_{\rm H}$ = 4 +3-2 $\times$ 1021 cm-2 which is higher than the Galactic value. The 15-150 keV prompt emission in GRB 050412 is modelled with a hard ($\Gamma$ = 0.7 $\pm$ 0.2) power law. The XRT light curve follows a broken power law with the first slope $\alpha_1$ = 0.7 $\pm$ 0.4, the break time $T_{\rm break}$ = 254 -41+79 s and the second slope $\alpha_2$ = 2.8 -0.8+0.5. The spectrum is fitted by a power law with spectral index $\Gamma_{\rm x}=1.3$ $\pm$ 0.2 which is absorbed at low energies by the Galactic column. Conclusions.The GRB 050410 afterglow light curve reveals the expected characteristics of the third component of the canonical Swift light curve. Conversely, a complex phenomenology was detected in the GRB 050412 because of the presence of the very early break. The light curve in this case can be interpreted as being the last peak of the prompt emission. The two bursts present tight upper limits for the optical emission, however, neither of them can be clearly classified as dark. For GRB 050410, the suppression of the optical afterglow could be attributed to a low density interstellar medium surrounding the burst. For GRB 050412, the evaluation of the darkness is more difficult due to the ambiguity in the extrapolation of the X-ray afterglow light curve

The prompt-afterglow connection in gamma-ray bursts: a comprehensive statistical analysis of Swift X-ray light curves

We present a comprehensive statistical analysis of Swift X-ray light curves of gamma-ray bursts (GRBs) collecting data from more than 650 GRBs discovered by Swift and other facilities. The unprecedented sample size allows us to constrain the rest-frame X-ray properties of GRBs from a statistical perspective, with particular reference to intrinsic time-scales and the energetics of the different light-curve phases in a common rest-frame 0.3–30 keV energy band. Temporal variability episodes are also studied and their properties constrained. Two fundamental questions drive this effort: (i) Does the X-ray emission retain any kind of ‘memory’ of the prompt γ-ray phase? (ii) Where is the dividing line between long and short GRB X-ray properties? We show that short GRBs decay faster, are less luminous and less energetic than long GRBs in the X-rays, but are interestingly characterized by similar intrinsic absorption. We furthermore reveal the existence of a number of statistically significant relations that link the X-ray to prompt γ-ray parameters in long GRBs; short GRBs are outliers of the majority of these two-parameter relations. However and more importantly, we report on the existence of a universal three-parameter scaling that links the X-ray and the γ-ray energy to the prompt spectral peak energy of both long and short GRBs: EX, iso∝E[Superscript: 1.00 ± 0.06]γ, iso/E[Superscript: 0.60 ± 0.10]pk

Evidence for the magnetar nature of 1E 161348-5055 in RCW 103

We report on the detection of a bright, short, structured X-ray burst coming from the supernova remnant RCW 103 on 2016 June 22 caught by the Swift/Burst Alert Telescope (BAT) monitor, and on the follow-up campaign made with Swift/X-ray Telescope, Swift/UV/Optical Telescope, and the optical/near-infrared (NIR) Gamma-Ray burst Optical and Near-infrared Detector. The characteristics of this flash, such as duration and spectral shape, are consistent with typical short bursts observed from soft gamma repeaters. The BAT error circle at 68 per cent confidence range encloses the point-like X-ray source at the centre of the nebula, 1E 161348−5055. Its nature has been long debated due to a periodicity of 6.67 h in X-rays, which could indicate either an extremely slow pulsating neutron star, or the orbital period of a very compact X-ray binary system. We found that 20 min before the BAT trigger, the soft X-ray emission of 1E 161348−5055 was a factor of ∼100 higher than measured 2 yr earlier, indicating that an outburst had already started. By comparing the spectral and timing characteristics of the source in the 2 yr before the outburst and after the BAT event, we find that, besides a change in luminosity and spectral shape, also the 6.67 h pulsed profile has significantly changed with a clear phase shift with respect to its low-flux profile. The UV/optical/NIR observations did not reveal any counterpart at the position of 1E 161348−5055. Based on these findings, we associate the BAT burst with 1E 161348−5055, we classify it as a magnetar, and pinpoint the 6.67 h periodicity as the magnetar spin period

Relativistic jet activity from the tidal disruption of a star by a massive black hole

Supermassive black holes have powerful gravitational fields with strong gradients that can destroy stars that get too close1,2, producing a bright flare in ultraviolet and X-ray spectral regions from stellar debris that forms an accretion disk around the black hole3–7. The aftermath of this process may have been seen several times over the past two decades in the form of sparsely sampled, slowly fading emission from distant galaxies8–14, but the onset of the stellar disruption event has not hitherto been observed. Here we report observations of a bright X-ray flare from the extragalactic transient Swift J164449.31573451. This source increased in brightness in the X-ray band by a factor of at least 10,000 since 1990 and by a factor of at least 100 since early 2010. We conclude that we have captured the onset of relativistic jet activity from a supermassive black hole. A companion paper15 comes to similar conclusions on the basis of radio observations. This event is probably due to the tidal disruption of a star falling into a supermassive black hole, but the detailed behaviour differs from current theoretical models of such events