A Unified Binary Neutron Star Merger Magnetar Model for the Chandra X-Ray Transients CDF-S XT1 and XT2

Abstract

Two bright X-ray transients were reported from the Chandra Deep Field South archival data, namely CDF-S XT1 and XT2. Whereas the nature of the former is not identified, the latter was suggested as an excellent candidate for a rapidly spinning magnetar born from a binary neutron star (BNS) merger. Here we propose a unified model to interpret both transients within the framework of the BNS merger magnetar model. According to our picture, CDF-S XT2 is observed from the "free zone" where the magnetar spindown powered X-ray emission escapes freely, whereas CDF-S XT1 originates from the "trapped zone" where the X-ray emission is initially blocked by the dynamical ejecta and becomes transparent after the ejecta is pushed to a distance where Thomson optical depth drops below unity. We fit the magnetar model to the light curves of both transients and derived consistent parameters for the two events, with magnetic field, initial spin period and X-ray emission efficiency being (B p = 10 16 G, P = 1.2 ms, η = 0.001) and (B p = 10 15.8 G, P = 4.4 ms, η = 0.001) for XT1 and XT2, respectively. The "isotropic equivalent" ejecta mass of XT1 is M ej ∼ 10 −3 M , while it is not constrained for XT2. Our results suggest that more extreme magnetar parameters are required to have XT1 detected from the trapped zone. The model parameters for both events are generally consistent with those derived from SGRB X-ray plateau observations. The host galaxy properties of both transients are also consistent with those of SGRBs. The event rate densities of both XT1 and XT2 are consistent with that of BNS mergers.

Neutron star mergers and how to study them

“…It may be possible to detect similar signatures from proto-magnetar winds outside of the observable prompt GRB line of sight (Sun et al 2017). There are potentially two such detections already (Xue et al 2019;Sun et al 2019).…”

Extragalactic fast X-ray transient candidates discovered by Chandra (2000-2014)

“…In the case of CDF-S XT2, its multi-wavelength constraints and host galaxy properties are consistent with the expected features of off-axis sGRBs (Xue et al 2019), although other possibilities may not be completely ruled out (e.g., a TDE origin; Peng et al 2019). CDF-S XT2 is particularly intriguing because it exhibits a flat, extended X-ray light curve suggesting a magnetar wind origin (Sun et al 2019;Xiao et al 2019;Lü et al 2019), similar to GRB 160821B (Troja et al 2019) and others, in line with the aforementioned predictions of Zhang (2013). The X-ray afterglows of GRBs also show similar plateaus in their light curves (e.g., Lyons et al 2010;Rowlinson et al 2013;Yi et al 2014), suggestive of a central engine related to a magnetar wind or an accreting black hole (BH; Troja et al 2007;Li et al 2018).…”

“…On the other hand, the luminosities and light curve shapes of FXRTs 7, 10, and 12 share remarkable similarities to X-ray flashes XRF 060218/SN 2006aj and XRF 100316D/SN 2010bh (which may be related to shock breakout from choked GRB jets; Campana et al 2006;Bromberg et al 2012;, as well as CDF-S XT2 (which is consistent with being powered by a millisecond magnetar; Xue et al 2019;Sun et al 2019). The light curves of FXRTs 7, 10, and 12 follow the expected shape for IMBH-WD TDEs (e.g.…”

“…The available data are consistent with expectations for off-axis sGRBs, although other possibilities might not be ruled out. For instance, Peng et al (2019) argue for an IMBH-WD TDE, Sun et al (2019) explain the X-ray emission considering a magnetar remnant after a BNS merger observer at an off-axis viewing angle, while Sarin et al (2021) discuss an association with an off-axis afterglow of a BNS merger, without discarding that its X-ray properties could be related to compact object such as an asteroid hitting an isolated foreground neutron star (Colgate & Petschek 1981;van Buren 1981;Campana et al 2011).…”

“…On the other hand, CDF-S XT1 could be associated with a few possible scenarios: (i) an "orphan" X-ray afterglow from an off-axis sGRB with weak optical emission (Bauer et al 2017;Sarin et al 2021), (ii) a low-luminosity GRB at high redshift with no prompt gamma-ray emission below ∼20 keV rest-frame (Bauer et al 2017), or (iii) a highly beamed IMBH-WD TDE (Bauer et al 2017;Peng et al 2019). More recently, Sun et al (2019) proposed a possible origin as a magnetar remnant of a NS merger, viewed at a larger off-axis angle than CDF-S XT2 and strongly obscured by ejecta material at early times. While none of these scenarios completely explain all observed properties, the large redshift uncertainty makes it difficult to discard them outright.…”