Pili Esquej

A tidal disruption-like X-ray flare from the quiescent galaxy SDSS J120136.02+300305.5

Aims. The study of tidal disruption flares from galactic nuclei has historically been hampered by a lack of high quality spectral observations taken around the peak of the outburst. Here we introduce the first results from a program designed to identify tidal disruption events at their peak by making near-real-time comparisons of the flux seen in XMM-Newton slew sources with that seen in ROSAT. Methods. Flaring extragalactic sources, which do not appear to be AGN, are monitored with Swift and XMM-Newton to track their temporal and spectral evolution. Timely optical observations are made to monitor the reaction of circumnuclear material to the X-ray flare. Results. SDSS J120136.02+300305.5 was detected in an XMM-Newton slew from June 2010 with a flux 56 times higher than an upper limit from ROSAT, corresponding to LX ∼ 3 × 10 44 erg s −1 . It has the optical spectrum of a quiescent galaxy (z = 0.146). Overall the X-ray flux has evolved consistently with the canonical t −5/3 model, expected for returning stellar debris, fading by a factor ∼300 over 300 days. In detail the source is very variable and became invisible to Swift between 27 and 48 days after discovery, perhaps due to self-absorption. The X-ray spectrum is soft but is not the expected tail of optically thick thermal emission. It may be fit with a Bremsstrahlung or double-power-law model and is seen to soften with time and declining flux. Optical spectra taken 12 days and 11 months after discovery indicate a deficit of material in the broad line and coronal line regions of this galaxy, while a deep radio non-detection implies that a jet was not launched during this event.

Swift follow-up of unidentified X-ray sources in the XMM–Newton Slew Survey

We present deep Swift follow-up observations of a sample of 94 unidentified X-ray sources from the XMM―Newton Slew Survey. The X-ray Telescope (XRT) on-board Swift detected 29 per cent of the sample sources; the flux limits for undetected sources suggest the bulk of the Slew Survey sources are drawn from one or more transient populations. We report revised X-ray positions for the XRT-detected sources, with typical uncertainties of 2.9 arcsec, reducing the number of catalogued optical matches to just a single source in most cases. We characterize the sources detected by Swift through their X-ray spectra and variability and via Ultraviolet-Optical Telescope photometry and using catalogued near-infrared, optical and radio observations of potential counterparts. Six sources can be associated with known objects and eight sources may be associated with unidentified ROSAT sources within the 3σ error radii of our revised X-ray positions. We find 10 of the 30 XRT- and/or Burst Alert Telescope (BAT)-detected sources are clearly stellar in nature, including one periodic variable star and two high proper motion stars. For 11 sources we propose an active galactic nucleus (AGN) classification, among which four are detected in hard X-rays and three have redshifts spanning z = 0.2-0.9 obtained from the literature or from optical spectroscopy presented here. A further three sources are suspected AGN and one is a candidate Galactic hard X-ray flash, while five sources remain unclassified. The 67 Slew Survey sources we do not detect with Swift XRT or BAT are studied via their characteristics in the Slew Survey observations and by comparison with the XRT- and BAT-detected population. We suggest that these are mostly if not all extragalactic, though unlikely to be highly absorbed sources in the X-rays such as Compton thick AGN. A large number of these are highly variable soft X-ray (0.2-2 keV) sources and a smaller number are highly variable hard (2-12 keV) sources. A small fraction of mainly hard-band Slew Survey detections may be spurious. This follow-up programme brings us a step further to completing the identifications of a substantial sample of XMM―Newton Slew Survey sources, important for understanding the nature of the transient sky and allowing flux-limited samples to be constructed.

Tidal disruption candidates discovered by XMM‐Newton

The paradigm that the cores of most, if not all, galaxies are occupied by supermassive black holes (SMBHs) was predicted long ago by theory. While it has been demonstrated that active nuclei powered by gas accretion onto SMBHs populate the cores of a number of galaxies, confirmation of their lurking in a dormant state within non‐active galaxies is difficult to obtain. An unavoidable consequence of the existence of remnant SMBHs is the detection off are radiation produced when a star is tidally disrupted and accreted by a SMBH, these are the so‐called tidal disruption events. Although active galactic nuclei can also be susceptible to these phenomena, the most unambiguous cases for a stellar disruption come from host galaxies with no—or only faint—permanent activity. A number of these exceptional events have been hitherto detected, being the two most recent ones discovered by XMM‐Newton during slew observations.

The XMM‐Newton Slew Survey: Towards The Whole X‐ray Sky and the Rarest X‐ray Events

The data collected by XMM-Newton as it slews between pointings currently cover almost half the entire sky, and many familiar features and new sources are visible. The soft-band sensitivity limit of the Slew is close to that of the RASS, and a large-area Slew-RASS comparison now provides the best opportunity for discovering extremely rare high-variability objects.