Browsing by Author "Ravasio, M. E."

Now showing 1 - 5 of 5
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    Extragalactic fast X-ray transient candidates discovered by Chandra (2014-2022)
    (2023) Quirola-Vasquez, J.; Bauer, F. E.; Jonker, P. G.; Brandt, W. N.; Yang, G.; Levan, A. J.; Xue, Y. Q.; Eappachen, D.; Camacho, E.; Ravasio, M. E.; Zheng, X. C.; Luo, B.
    Context. Extragalactic fast X-ray transients (FXTs) are short flashes of X-ray photons of unknown origin that last a few minutes to hours.
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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.
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    The Fast X-Ray Transient XRT 210423 and Its Host Galaxy
    (2023) Eappachen, D.; Jonker, P. G.; Levan, A. J.; Quirola-Vasquez, J.; Torres, M. A. P.; Bauer, F. E.; Dhillon, V. S.; Marsh, T.; Littlefair, S. P.; Ravasio, M. E.; Fraser, M.
    Fast X-ray Transients (FXTs) are X-ray flares with durations ranging from a few hundred seconds to a few hours. Possible origins include the tidal disruption of a white dwarf by an intermediate-mass black hole, a supernova shock breakout, or a binary neutron star merger. We present the X-ray light curve and spectrum as well as deep optical imaging of the FXT XRT 210423, which has been suggested to be powered by a magnetar produced in a binary neutron star merger. Our Very Large Telescope and Gran Telescopio Canarias (GTC) observations began on 2021 May 6, thirteen days after the onset of the flare. No transient optical counterpart is found in the 1 (3s) X-ray uncertainty region of the source to a depth g(s) = 27.0 AB mag. (We use the word "counterpart" for any transient light in a wave band other than the original X-ray detection wave band, whereas the word "host" refers to the host galaxy.) A candidate host lies within the 1 X-ray uncertainty region with a magnitude of 25.9 +/- 0.1 in the GTC/ HiPERCAM g(s) filter. Due to its faintness, it was not detected in other bands, precluding a photometric redshift determination. We detect two additional candidate host galaxies: one with zspec = 1.5082 +/- 0.0001 and an offset of 4 2 +/- 1 (37 +/- 9 kpc) from the FXT, and another one with = z 1.04+ (+0.22)(-0.14) and an offset of 3."6 +/- 1." (30 +/- 8 kpc). Based on the properties of all the prospective hosts, we favor a binary neutron star merger, as previously suggested in the literature, as the explanation for XRT 210423.
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    XMM-Newton-discovered Fast X-ray Transients: host galaxies and limits on contemporaneous detections of optical counterparts
    (2024) Eappachen, D.; Jonker, P. G.; Quirola-Vasquez, J.; Mata Sanchez, D.; Inkenhaag, A.; Levan, A. J.; Fraser, M.; Torres, M. A. P.; Bauer, F. E.; Chrimes, A. A.; Stern, D.; Graham, M. J.; Smartt, S. J.; Smith, K. W.; Ravasio, M. E.; Zabludoff, A. I.; Yue, M.; Stoppa, F.; Malesani, D. B.; Stone, N. C.; Wen, S.
    Extragalactic fast X-ray transients (FXTs) are a class of soft (0.3-10 keV) X-ray transients lasting a few hundred seconds to several hours. Several progenitor mechanisms have been suggested to produce FXTs, including supernova shock breakouts, binary neutron star mergers, or tidal disruptions involving an intermediate-mass black hole and a white dwarf. We present detailed host studies, including spectroscopic observations of the host galaxies of seven XMM-Newton-discovered FXTs. The candidate hosts lie at redshifts 0.0928