The neutron star LMXB EXO0748-676 has returned to quiescence

ATel #17191; N. Degenaar (U. of Amsterdam), J. Homan (independent), E. Cackett (Wayne State U.), R. Wijnands (U. of Amsterdam), M. T. Wolff (NRL), D. J. K. Buisson (independent)
on 19 May 2025; 05:01 UT
Credential Certification: Nathalie Degenaar (degenaar@uva.nl)

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The eclipsing and bursting transient neutron star X-ray binary EXO 0748-676 has has been exhibiting an accretion outburst that was first detected in X-rays in 2024 June (GCN #36653), with an optical rise starting in 2024 April (ATel #16648). Swift/XRT observations suggest that the source started to decline to quiescence some time between 2025 February and April.

Data taken with Swift on 2025 April 13 (ObsID 00031272064) and May 8, 11 and 15 (ObsIDs 00019755001-3) show a steady decrease in XRT count rate from 5.6E-2 c/s to 3.5E-2 c/s (0.3-10 keV). These rates are two orders of magnitude lower than the values of 4.5-5.2 c/s detected with the XRT on 2025 February 7 and 12 (ObsIDs 00031272062-63). At that time, the source was still seen active in X-rays (ATels #17015, #17024), although the optical emission was reported to show a decline (ATel #17004).

We utilized the online XRT tool (Evans et al. 2009) to extract and fit the X-ray spectra for the three observations performed in 2025 May, which had exposure times of 3.1, 4.5 and 1.4 ks respectively. A simple black body model adopting the Galactic hydrogen column density of 1.5E21 cm^-2 (Willingale et al. 2013) results in black body temperatures of 0.26+/-0.03, 0.27+/-0.02, and 0.25+/-0.02 keV on May 8, 11 and 15, respectively. The corresponding unabsorbed 0.3-10 keV fluxes inferred from these automated fits are (1.3+/-0.3)E-12, (1.0+/-0.2)E-12 and (0.8+/-0.1)E-12 erg/cm2/s, respectively. For a distance of 7 kpc (Galloway et al. 2008), these translate into 0.3-10 keV luminosities of (4.7-7.6)E33 erg/s.

The temperatures and fluxes measured by Swift/XRT in the three May observations are typical for neutron star X-ray binaries in quiescence and similar to those measured shortly after the previous outburst of EXO 0748-676 (Degenaar et al. 2009). This strongly suggests that the source has returned to quiescence after an X-ray outburst that lasted for 11 months. This is much shorter than the previous outburst, which ended in 2008 after having persisted for >24 years (Degenaar et al. 2009).

A multi-year Swift and Chandra monitoring program has been triggered to observe potential cooling of the accretion-heated neutron star, as seen after the previous outburst (Parikh et al. 2020), now that the source has returned to quiescence again.

References:
Degenaar et al. 2009, MNRAS 396, L26
Evans et al. 2009, MNRAS 397, 1177
Galloway et al. 2008, MNRAS 387, 268
Parikh et al. 2020, A&A 638, L2
Willingale et al. 2013, MNRAS 31, 394