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Register a new userThe EXOD search for faint transients in XMM-Newton observations: Method and discovery of four extragalactic Type I X-ray Bursters
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In\\'es Pastor-Marazuela Ms.
Publication date
2020
fields
Physics
and research's language is
English
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XMM-Newton has produced an extensive X-ray source catalogue in which the standard pipeline determines the variability of sufficiently bright sources through chi-square and fractional variability tests. Faint sources, however, are not automatically checked for variability, thus overlooking faint, short timescale transients. Our goal is to find new faint, fast transients in XMM-Newton EPIC-pn observations. To that end we have created the EPIC-pn XMM-Newton Outburst Detector (EXOD) algorithm, which we run on the EPIC-pn data available in the 3XMM-DR8 catalogue. In EXOD, we compute the whole-field variability by binning in time the counts in each detector pixel. We next compute the maximum-to-median count difference in each pixel to detect variability. We applied EXOD to 5,751 observations and compared the variability of the detected sources to the standard chi-square and Kolmogorov-Smirnov (KS) variability tests. The algorithm is able to detect periodic and aperiodic variability, short and long flares. Of the sources detected by EXOD, 60-95% are also shown to be variable by the chi-square and KS tests. We obtain a net number of 2,536 variable sources. Of these we investigate the nature of 35 sources with no previously confirmed classification. Amongst the new sources, we find stellar flares and AGNs; but also four extragalactic type I X-ray bursters that double the known neutron-star population in M31. This algorithm is a powerful tool to promptly detect variable sources in XMM-Newton observations. EXOD detects fast transients that other variability tests classify as non-variable due to their short duration and low number of counts. Finally, EXOD allows us to detect and identify the nature of rare compact objects through their variability. We demonstrate this through the discovery of four extragalactic neutron-star low mass X-ray binaries, doubling the number of known neutron stars in M31.
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Recent works have discovered two fast ($approx 10$ ks) extragalactic X-ray transients in the Chandra Deep Field-South (CDF-S XT1 and XT2). These findings suggest that a large population of similar extragalactic transients might exist in archival X-ray observations. We develop a method that can effectively detect such transients in a single Chandra exposure, and systematically apply it to Chandra surveys of CDF-S, CDF-N, DEEP2, UDS, COSMOS, and E-CDF-S, totaling 19~Ms of exposure. We find 13 transient candidates, including CDF-S XT1 and XT2. With the aid of available excellent multiwavelength observations, we identify the physical nature of all these candidates. Aside from CDF-S XT1 and XT2, the other 11 sources are all stellar objects, and all of them have $z$-band magnitudes brighter than 20. We estimate an event rate of $59^{+77}_{-38} rm{evt yr^{-1} deg^{-2}}$ for CDF-S XT-like transients with 0.5-7 keV peak fluxes $log F_{rm peak} gtrsim -12.6$ (erg cm$^{-2}$ s$^{-1}$). This event rate translates to $approx 15^{+20}_{-10}$ transients existing among Chandra archival observations at Galactic latitudes $|b|>20^{circ}$, which can be probed in future work. Future missions such as Athena and the Einstein Probe with large grasps (effective area $times$ field of view) are needed to discover a large sample ($sim$ thousands) of fast extragalactic X-ray transients.
We present a new method to identify luminous off-nuclear X-ray sources in the outskirts of galaxies from large public redshift surveys, distinguishing them from foreground and background interlopers. Using the 3XMM-DR5 catalog of X-ray sources and the SDSS DR12 spectroscopic sample of galaxies, with the help of this off-nuclear cross-matching technique, we selected 98 sources with inferred X-ray luminosities in the range $10^{41} < L_{rm X} < 10^{44},{rm erg,s}^{-1}$, compatible with hyperluminous X-ray objects (HLX). To validate the method, we verify that it allowed us to recover known HLX candidates such as ESO 243$-$49 HLX$-$1 and M82 X$-$1. From a statistical study, we conservatively estimate that up to $71 pm 11$ of these sources may be fore- or background sources, statistically leaving at least 16 that are likely to be HLXs, thus providing support for the existence of the HLX population. We identify two good HLX candidates and using other publicly available datasets, in particular the VLA FIRST in radio, UKIDSS in the near-infrared, GALEX in the ultra-violet and CFHT Megacam archive in the optical, we present evidence that these objects are unlikely to be foreground or background X-ray objects of conventional types, e.g. active galactic nuclei, BL Lac objects, Galactic X-ray binaries or nearby stars. However, additional dedicated X-ray and optical observations are needed to confirm their association with the assumed host galaxies and thus secure their HLX classification.
Most violent and energetic processes in our universe, including mergers of compact objects, explosions of massive stars and extreme accretion events, produce copious amounts of X-rays. X-ray follow-up is an efficient tool for identifying transients because (1) X-rays can quickly localize transients with large error circles, and (2) X-rays reveal the nature of transients that may not have unique signatures at other wavelengths. In this white paper, we identify key science questions about several extragalactic multi-messenger and multi-wavelength transients, and demonstrate how X-ray follow-up helps answer these questions
We report on a 40 ks long, uninterrupted X-ray observation of the candidate supergiant fast X-ray transient (SFXT) IGRJ16418-4532 performed with XMM-Newton on February 23, 2011. This high mass X-ray binary lies in the direction of the Norma arm, at an estimated distance of 13 kpc. During the observation, the source showed strong variability exceeding two orders of magnitudes, never observed before from this source. Its X-ray flux varied in the range from 0.1 counts/s to about 15 counts/s, with several bright flares of different durations (from a few hundreds to a few thousands seconds) and sometimes with a quasi-periodic behavior. This finding supports the previous suggestion that IGRJ16418-4532 is a member of the SFXTs class. In our new observation we measured a pulse period of 1212+/-6 s, thus confirming that this binary contains a slowly rotating neutron star. During the periods of low luminosity the source spectrum is softer and more absorbed than during the flares. A soft excess is present below 2 keV in the cumulative flares spectrum, possibly due to ionized wind material at a distance similar to the neutron star accretion radius. The kind of X-ray variability displayed by IGRJ16418-4532, its dynamic range and time scale,together with the sporadic presence of quasi-periodic flaring, all are suggestive of a transitional accretion regime between pure wind accretion and full Roche lobe overflow. We discuss here for the first time this hypothesis to explain the behavior of IGRJ16418-4532 and, possibly, of other SFXTs with short orbital periods.
We report the first broad-band (0.5-150 keV) simultaneous X-ray observations of the very faint X-ray transient IGRJ17285-2922/XTEJ1728-295 performed with XMM-Newton and INTEGRAL satellites during its last outburst, started on 2010, August 28. XMM-Newton observed the source on 2010 September 9-10, for 22ks. INTEGRAL observations were part of the publicly available Galactic Bulge program, and overlapped with the times covered by XMM-Newton. The broad-band spectroscopy resulted in a best-fit with an absorbed power law displaying a photon index of 1.61+/-0.01, an absorbing column density of (5.10+/-0.05)E21 cm-2, and a flux of 2.4E-10 erg/cm2/s (1-100 keV), corrected for the absorption. The data did not require either a spectral cut-off (E>50 keV) or an additional soft component. The slopes of the XMM-Newton and INTEGRAL separate spectra were compatible, within the uncertainties. The timing analysis does not show evidence either for X-ray pulsations or for type I X-ray bursts. The broad band X-ray spectrum as well as the power density spectrum are indicative of a low hard state in a low mass X-ray binary, although nothing conclusive can be said about the nature of the compact object (neutron star or black hole). The results we are reporting here allow us to conclude that IGRJ17285-2922 is a low mass X-ray binary, located at a distance greater than 4 kpc.
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