A new, faint population of X-ray transients

Abstract

We report on the detection of a remarkable new fast high-energy transient found in the Chandra Deep Field-South, robustly associated with a faint (m R = 27.5 mag, z ph ∼2.2) host in the CANDELS survey. The X-ray event is comprised of 115 +12 −11 net 0.3-7.0 keV counts, with a light curve characterised by a ≈100 s rise time, a peak 0.3-10 keV flux of ≈5×10 −12 erg s −1 cm −2 , and a power-law decay time slope of −1.53 ± 0.27. The average spectral slope is Γ = 1.43 +0.23 −0.13 , with no clear spectral variations. The X-ray and multi-wavelength properties effectively rule out the vast majority of previously observed high-energy transients. A few theoretical possibilities remain: an "orphan" X-ray afterglow from an off-axis short-duration Gamma-ray Burst (GRB) with weak optical emission; a low-luminosity GRB at high redshift with no prompt emission below ∼20 keV rest-frame; or a highly beamed Tidal Disruption Event (TDE) involving an intermediate-mass black hole and a white dwarf with little variability. However, none of the above scenarios can completely explain all observed properties. Although large uncertainties exist, the implied rate of such events is comparable to those of orphan and low-luminosity GRBs as well as rare TDEs, implying the discovery of an untapped regime for a known transient class, or a new type of variable phenomena whose nature remains to be determined.

Binary white dwarfs and decihertz gravitational wave observations: From the Hubble constant to supernova astrophysics

“…Depending on the nature of the companion, WD coalescences could ignite the emission of different electromagnetic counterparts, as X-transients [74,75]. Moreover, binary WDs have got large attention as possible progenitors of supernovae Ia events, according to the double degenerate scenario.…”

CDF-S XT1 and XT2: White Dwarf Tidal Disruption Events by Intermediate-mass Black Holes?

“…The light curve of CDF-S XT1 shows a rise within ∼ 100 s to the peak flux of F 0.3−10 KeV ≈ 5 × 10 −12 erg s −1 cm −2 , then a power law decay in hours with a slope of −1.53 ± 0.27 (Bauer et al 2017). The light curve of CDF-S XT2 shows a long plateau, and then a sudden break at ∼ 3000 s. Its peak luminosity between 0.3 and 10 KeV is ≈ 3 × 10 45 erg s −1 .…”

Fast Extragalactic X-Ray Transients from Gamma-Ray Bursts Viewed Far Off-axis

“…To test whether the x-ray transients XRT 000519 (Jonker et al 2013) and XRT 110103 (Glennie et al 2015) and CDF-S XT1 (Bauer et al 2017) Table I, which yield a χ 2 /df = 0.39. Bottom: Comparison between the observed pulse shape of the second pulse of 000519 (Jonker et al 2013) in the 0.3-7 keV x-ray band and eq.…”

“…Table I, which yield a χ 2 /df = 1.20. Bottom: Comparison between the observed pulse shape of XDF-S XT1 (Bauer et al 2017) in the 0.3-7 keV x-ray band and eq. (5) with the best fit parameters listed in Table I, which yield χ 2 /df = 1.13. viewed far off-axis, we have fitted the 0.3-7 keV light curves of their pulses with Eq.(5).…”

“…Junker et al (2013) and Glennie et al (2015) reported the discoveries of relatively nearby fast XRT 000519 close to M86 in the Virgo cluster at a distance of ∼ 16.5 Mpc (Mei et al 2007), and XRT 110103 in the galaxy cluster ACO 3581 at a distance ∼ 94.9 Mpc (Jhohnstone et al 2005), respectively, with pulses that lasted less than few tens of seconds. XRTs at large cosmological distances that lasted more than 10 4 s were discovered in the Chandra Deep Field-South (CDF-S) observations; CDF-S XT1 associated with a faint distant galaxy at unknown redshift, that lasted more than a day was discovered by Bauer et al (2017), and CDF-S XT2 associated with a galaxy at redshift z = 0.738 that lasted more than 2 × 10 4 s was discovered by Xue et al (2019).…”