Browsing by Author "Levan, A. J."

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    Investigating the off-axis GRB afterglow scenario for extragalactic fast X-ray transients
    (2024) Wichern, H. C. I.; Ravasio, M. E.; Jonker, P. G.; Quirola-Vasquez, J. A.; Levan, A. J.; Bauer, F. E.; Kann, D. A.
    Context. Extragalactic fast X-ray transients (FXTs) are short-duration (similar to ks) X-ray flashes of unknown origin, potentially arising from binary neutron star (BNS) mergers, tidal disruption events, or supernova shock breakouts. Aims. In the context of the BNS scenario, we investigate the possible link between FXTs and the afterglows of off-axis merger-induced gamma-ray bursts (GRBs). Methods. By modelling well-sampled broadband afterglows of 13 merger-induced GRBs, we make predictions for their X-ray light curve behaviour had they been observed off-axis, considering both a uniform jet with core angle theta(C) and a Gaussian-structured jet whose edge lies at an angle theta(W) = 2 theta(C). We compare their peak X-ray luminosity, duration, and temporal indices alpha (where F proportional to t(alpha)) with those of the currently known extragalactic FXTs. Results. Our analysis reveals that a slightly off-axis observing angle of theta(obs) approximate to (2.2 - 3)theta(C) and a structured jet are required to explain the shallow (|alpha|less than or similar to 0.3) temporal indices of the FXT light curves, which cannot be reproduced in the uniform-jet case at any viewing angle. In the case of a structured jet with truncation angle theta(W) = 2 theta(C), the distributions of the duration of the FXTs are consistent with those of the off-axis afterglows for the same range of observing angles, theta obs approximate to (2.2 - 3)theta(C). While the distributions of the off-axis peak X-ray luminosity are consistent only for theta(obs) = 2.2 theta(C), focussing on individual events with different intrinsic luminosities reveals that the match of all three properties (peak X-ray luminosity, duration and temporal indices) of the FXTs at the same viewing angle is possible in the range theta(obs) similar to (2.2 - 2.6)theta(C). Despite the small sample of GRBs analysed, these results show that there is a region of the parameter space - although quite limited - where the observational properties of off-axis GRB afterglow can be consistent with those of the newly discovered FXTs. Future observations of FXTs discovered by the recently launched Einstein Probe mission and GRB population studies combined with more complex afterglow models will shed light on this possible GRB-FXT connection, and eventually unveil the progenitors of some FXTs.
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    Probing a magnetar origin for the population of extragalactic fast X-ray transients detected by Chandra
    (2024) Quirola-Vasquez, J.; Bauer, F. E.; Jonker, P. G.; Brandt, W. N.; Eappachen, D.; Levan, A. J.; Lopez, E.; Luo, B.; Ravasio, M. E.; Sun, H.; Xue, Y. Q.; Yang, G.; Zheng, X. C.
    Context. Twenty-two extragalactic fast X-ray transients (FXTs) have now been discovered from two decades of Chandra data (analyzing similar to 259 Ms of data), with 17 associated with distant galaxies (greater than or similar to 100 Mpc). Different mechanisms and progenitors have been proposed to explain their properties; nevertheless, after analyzing their timing, spectral parameters, host -galaxy properties, luminosity function, and volumetric rates, their nature remains uncertain. Aims. We interpret a sub -sample of nine FXTs that show a plateau or a fast -rise light curve within the framework of a binary neutron star (BNS) merger magnetar model. Methods. We fit their light curves and derive magnetar (magnetic field and initial rotational period) and ejecta (ejecta mass and opacity) parameters. This model predicts two zones: an orientation -dependent free zone (where the magnetar spin -down X-ray photons escape freely to the observer) and a trapped zone (where the X-ray photons are initially obscured and only escape freely once the ejecta material becomes optically thin). We argue that six FXTs show properties consistent with the free zone and three FXTs with the trapped zone. Results. This sub -sample of FXTs has a similar distribution of magnetic fields and initial rotation periods to those inferred for short gamma -ray bursts, suggesting a possible association. We compare the predicted ejecta emission fed by the magnetar emission (called merger -nova) to the optical and near -infrared upper limits of two FXTs, XRT 141001 and XRT 210423 where contemporaneous optical observations are available. The non -detections place lower limits on the redshifts of XRT 141001 and XRT 210423 of z greater than or similar to 1.5 and greater than or similar to 0.1, respectively. Conclusions. If the magnetar remnants lose energy via gravitational waves (GWs), it should be possible to detect similar objects with the current advanced LIGO detectors out to a redshift z less than or similar to 0.03, while future GW detectors will be able to detect them out to z approximate to 0.5.