The EXOD search for faint transients in XMM-Newton observations: Method and discovery of four extragalactic Type I X-ray bursters

Abstract

Context. The observations carried out with XMM-Newton have produced a very extensive X-ray source catalogue in which the standard pipeline determines the variability of sufficiently bright sources through χ2 and fractional variability tests. Faint sources, however, are not automatically checked for variability, and this means that faint, short timescale transients are overlooked. From dedicated X-ray searches, as well as optical and radio archive searches, we know that some such dim sources can still be identified with high confidence. Aims. Our goal is to find new faint, fast transients in XMM-Newton EPIC-pn observations. To that end we created the EPIC-pn XMM-Newton outburst detector (EXOD) algorithm, which we run on the EPIC-pn full-frame data available in the 3XMM-DR8 catalogue. Methods. In EXOD, we computed the variability of the whole field of view by first binning in time the counts detected in each pixel of the detector. We next computed the difference between the median and maximal number of counts in each time bin and pixel to detect variability. We applied EXOD to 5751 observations in the full frame mode and compared the variability of the detected sources to the standard χ2 and Kolmogorov–Smirnov (KS) variability tests. Results. The algorithm is able to detect periodic and aperiodic variability, with both short and long flares. Of the sources detected by EXOD, 60−95% are also shown to be variable by the standard χ2 and KS tests. EXOD computes the variability over the entire field of view faster than the light curve generation takes for all the individual sources. We detect a total of 2961 X-ray variable sources. After removing the spurious detections, we obtain a net number of 2536 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, in addition to four extragalactic type I X-ray bursters that double the known neutron-star population in M 31. Conclusions. This algorithm is a powerful tool for the prompt detection of interesting variable sources in XMM-Newton observations. EXOD also detects fast transients that other variability tests would classify as non-variable due to their short duration and low number of counts. This is of increasing importance for the multi-messenger detection of transient sources. Finally, EXOD allows us to identify the nature of compact objects through their variability and to detect rare compact objects. We demonstrate this through the discovery of four extragalactic neutron-star low-mass X-ray binaries, doubling the number of known neutron stars in M 31.

LOFAR Detection of 110–188 MHz Emission and Frequency-dependent Activity from FRB 20180916B

“…Bursts I01-I08 correspond to the bursts L02-L09 presented by Pastor-Marazuela et al (2020). Our analysis did not recover their L01 burst due to periodic baseline variations affecting 2.6 hr of the LOFAR observations.…”

Emission Properties of Periodic Fast Radio Bursts from the Motion of Magnetars: Testing Dynamical Models

“…No PA swing is observed, as would generally be expected in the magnetospheric origin models (as opposed to the diverse polarization angle swing detected in FRB 180301; Luo et al 2020). Most strikingly, the active phase is observed to be chromatic, with the activity window being both narrower and earlier at higher frequencies (Pastor-Marazuela et al 2020;Pleunis et al 2020). This disfavors models explaining the periodic activity with the eclipse of a companion wind, as these theories predict a narrower activity window at lower frequencies (Ioka & Zhang 2020;Lyutikov et al 2020).…”

“…Now that we have developed a phenomenological emission model for periodic FRBs due to the motion of NSs, we apply these results to the recent multiwavelength and polarization measurements of FRB 180916. For a dynamical model to remain a plausible explanation for the periodicity of FRB 180916, it must explain the following three features of the FRB 180916 emission (Pastor-Marazuela et al 2020;Pleunis et al 2020):…”

“…Imaging from the Hubble Space Telescope shows that the location of FRB 180916 is offset by ∼250 pc from the nearest knot of star formation in the host galaxy (Tendulkar et al 2021), while young magnetars are expected to born in a star-forming region. For bursts detected in the L band, the change of linear polarization position angle (PA) is constrained to be 10°-20°across the bursts of the same phase (Nimmo et al 2020), and  50°across the L-band active phases (Pastor-Marazuela et al 2020). No PA swing is observed, as would generally be expected in the magnetospheric origin models (as opposed to the diverse polarization angle swing detected in FRB 180301; Luo et al 2020).…”

The EXTraS project: Exploring the X-ray transient and variable sky

“…We note that of the 14 EXTraS transient events that were genuinely detected by the standard analysis, only 2 are classified as variable sources in the 4XMM-DR9 catalogue. The high sensitivity of the EXTraS algorithm in detecting fast transients is also confirmed by the fact that an independent search for X-ray transients in the XMM-Newton archive, using a different approach, did not discover additional sources with respect to the 4XMM-DR9 catalogue (Pastor-Marazuela et al 2020).…”