Searching for fast extragalactic X-ray transients in Chandra surveys. (arXiv:1906.02793v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Yang_G/0/1/0/all/0/1">Guang Yang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brandt_W/0/1/0/all/0/1">W. N. Brandt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhu_S/0/1/0/all/0/1">S. F. Zhu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bauer_F/0/1/0/all/0/1">F. E. Bauer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Luo_B/0/1/0/all/0/1">B. Luo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Xue_Y/0/1/0/all/0/1">Y. Q. Xue</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zheng_X/0/1/0/all/0/1">X. C. Zheng</a>
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.
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.
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