New outburst of Aql X-1 detected with XB-NEWS and MAXI

ATel #16147; Kevin Alabarta, David M. Russell, M. Cristina Baglio, Payaswini Saikia, Dan Bramich (NYU Abu Dhabi), Yasmine Heikal (UAEU, Al Ain), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU) and Jeroen Homan (Eureka Scientific)
on 25 Jul 2023; 09:48 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 (LMXB) that undergoes frequent outbursts, with a recurrence time of approximately 1 per year. The last outburst was first detected in optical on May 27, 2022 (MJD 59725.96) with the 1m SAO RAS and 0.5m RC optical telescopes (ATel #15401) and later detected in X-rays on May 28, 2022 (MJD 59727.19) with Swift/XRT (ATel #15403).

Aql X-1 is being 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 2022 outburst of Aql X-1 occurred with LCO on May 24, 2022 (MJD 59723.05) in V, R and i' bands (V = 18.37 +/- 0.02, R = 17.64 +/- 0.01 and i' = 17.41 +/- 0.02). The source reached the peak of the outburst on June 06, 2022 (MJD 59736.04), with the brightest magnitudes V = 16.87 +/- 0.01, R = 16.38 +/- 0.01 and i' = 16.32 +/- 0.01. Finally, the last LCO detection of the system before going into quiescence was on September 30, 2022 (MJD 59756.87).

Since then, the source has been in quiescence with average magnitudes of g' = 19.86 +/- 0.01, V = 19.29 +/- 0.01, r' = 18.79 +/- 0.01, R = 18.49 +/- 0.01, i' = 18.23 +/- 0.01 and z_s = 17.94 +/- 0.01. On July 23, 2023 (MJD 60148.26) XB-NEWS detected an increase of the optical flux in the g', r', i' and z_s bands with magnitudes 19.02 +/- 0.02, 18.17 +/- 0.01, 17.72 +/- 0.01 and 17.45 +/- 0.01, respectively. Thenceforward, Aql X-1 increased its optical flux up to g' = 18.19 +/- 0.01, r' = 17.52 +/- 0.01 and z_s = 16.94 +/- 0.01 on July 24, 2023 (MJD 60149.39).

The increase in flux is higher at shorter wavelengths, Δg'= 1.67 +/- 0.01, Δr'= 1.27 +/- 0.01, Δi'= 0.51 +/- 0.01 and Δz_s= 1.00 +/- 0.01, suggesting an important contribution of the accretion disc to the brightening of the source and the beginning of a new outburst (Aql X-1 is bluer when brighter; see e.g., Maitra & Bailyn 2008).

At X-ray frequencies, MAXI detected the source significantly (above 3 sigma) for the first time since the previous outburst on July 23, 2023 (MJD 60148.5), with a count rate of 0.21 +/- 0.02 in the 2-20 keV energy band, the same day as the LCO detection.

Multi-wavelength observations are encouraged to study the rise of the outburst.

A link to the light curve is below.

LCO optical light curves of Aql X-1