Optical and X-ray observations of Aql X-1 indicate its return to quiescence

ATel #16284; Kevin Alabarta (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), David M. Russell, Payaswini Saikia, Sandeep Rout and D. M. Bramich (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB) and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 12 Oct 2023; 06:32 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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Aql X-1 is a neutron-star low-mass X-ray binary (NS LMXB) that undergoes approximately one outburst per year. We have been monitoring the source with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network. Its latest outburst started in July 2023 (ATel #16147, #16153) and, after that, Aql X-1 increased its flux in X-rays (ATel #16153, #16158, #16174) and optical (ATel #16147, #16170) reaching the peak of the outburst on August 05 (MJD 60161.5) and August 07 (MJD 60163.03) in X-rays and optical, respectively (ATel #16187). The following LCO observations showed that the optical flux started to decay (ATel #16187).

We have continued our LCO optical monitoring of Aql X-1 since then. The flux kept decreasing and, on October 5, 2023 (MJD 60222.02), the optical magnitudes (V = 19.23 +/- 0.06, R = 18.45 +/- 0.03 and i' = 18.23 +/- 0.03) were consistent with those of the quiescence state prior to the current outburst (V ~ 19.3, R ~ 18.5, i' ~ 18.2). The latest LCO observations occurring on October 8, 2023 (MJD 60225.43) showed an optical magnitude of V = 19.29 +/- 0.01, R = 18.49 +/- 0.01, i' = 18.23 +/- 0.01. These observations imply that Aql X-1 has probably ended its outburst, which lasted ~74 days. This outburst has also been the brightest of the source since 2016 (ATel #16187).

Monitoring observations of Aql X-1 with NICER suggest that the source is rapidly approaching quiescence in X-rays as well. Between September 14 and October 3, the background subtracted count rate (0.5-10 keV) dropped from ~800 cts/s to ~1.2 cts/s. For the October 3 observation, we measured an absorbed 0.5-10 keV flux of ~4e-12 erg/cm^2/s. This is a factor of a few above the fluxes typically seen during the initial quiescent phase after outbursts of Aql X-1 (Degenaar et al. 2019). More recent NICER observations have been made, but the data are not available yet.

Multi-wavelength observations of Aql X-1 during quiescence are encouraged. We will continue to observe the system with LCO.

The LCO observations of Aql X-1 are part of an ongoing monitoring campaign of ~50 LMXBs (Lewis et al. 2008). All the obtained images are analyzed with the real-time data analysis pipeline "X-ray Binary New Early Warning System"(XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details).

This material is based upon work supported by Tamkeen under the NYU Abu Dhabi Research Institute grant CASS (Center for Astrophysics and Space Science).

We thank the NICER team for their efforts to cover the outburst of Aql X-1.

A link to the LCO and NICER light curves can be found below.

LCO (top) and X-ray (bottom) light curves of Aql X-1