Aql X-1 reaches the peak of its 2023 outburst making it the brightest outburst since 2016

ATel #16187; Kevin Alabarta, David M. Russell, M. Cristina Baglio, Payaswini Saikia and D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU) and Jeroen Homan (Eureka Scientific)
on 14 Aug 2023; 11:40 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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Aquila X-1 (Aql X-1) is a transient neutron-star low-mass X-ray binary (NS LMXB) undergoing approximately one outburst per year. Its latest outburst started in July 2023 (ATel #16147, #16153) and since then Aql X-1 has been found to increase its flux in X-rays (ATel #16153, #16158, #16174) and optical (ATel #16147, #16170) wavelengths.

Aql X-1 is regularly monitored with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network (see e.g. ATel #1970, #9306, #13953, #14563), as part of an ongoing monitoring 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).

The first optical detection of the current outburst of Aql X-1 occurred with LCO on July 23, 2023 (MJD 60148.26) in the bands g'=19.02 +/- 0.02, r'=18.17 +/- 0.01, i' = 17.72 +/- 0.01 and z_s = 17.45 +/- 0.01 (ATel #16147). Then, the source increased its optical flux up to magnitudes g'=16.31 +/- 0.01, V = 16.05 +/- 0.01, r'=15.87 +/- 0.01, R= 15.62 +/- 0.01, i' = 15.56 +/- 0.01 and z_s = 15.39 +/- 0.01 on August 07, 2023 (MJD 60163.03), when the source reached the peak of the outburst. These were the brightest magnitudes of Aql X-1 since August 13, 2016 (V = 15.66 +/- 0.01, R = 15.28 +/- 0.01 and i' = 15.29 +/- 0.01), making this the brightest outburst since that year. Thenceforward, we detected a decrease in the optical flux of Aql X-1 down to g'=16.70 +/- 0.01, V = 16.44 +/- 0.01, r'=16.20 +/- 0.01, R= 15.97 +/- 0.01, i' = 15.97 +/- 0.01 and z_s = 15.75 +/- 0.01, on August 12, 2023 (MJD 60168.03).

At X-ray frequencies, Aql X-1 reached the peak of the outburst on August 5, 2023 (MJD 60161.5) at a MAXI 2-20 keV count rate of 3.58 +/- 0.06 c/s/cm-2, two days before the outburst peak in optical frequencies. This is the highest MAXI count rate of Aql X-1 since August 11, 2016 (count rate of 3.58 +/- 0.04 c/s/cm^-2). We will continue monitoring Aql X-1 at optical wavelengths with LCO. Multi-wavelength observations are encouraged to study the decay of the outburst.

A link to the LCO light curve is below.

LCO optical light curves of Aql X-1