Fading of Aql X-1 to quiescent levels after its unusually short and faint brightening episode

ATel #17336; Sandeep K. Rout, Kevin Alabarta (NYU Abu Dhabi), Alessio Marino (ICE-CSIC), Guobao Zhang (YNAO, CAS), M. Cristina Baglio (INAF-OAB), David M. Russell, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), Stefanie Fijma (API, Amsterdam), and Fraser Lewis (Faulkes Telescope Project & The Schools' Observatory, LJMU)
on 16 Aug 2025; 07:39 UT
Credential Certification: Sandeep Rout (sandeep.rout@nyu.edu)

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The neutron star low-mass X-ray binary Aquila X-1 (Aql X-1) underwent an optical rise on 04 August 2025 (ATel #17321). It brightened by ~0.5 magnitudes in all optical wavebands signaling the onset of a new outburst. However, after reaching peak magnitudes of g' = 19.27 +/- 0.09, V = 18.74 +/- 0.07, r' = 18.26 +/- 0.04, R = 17.99 +/- 0.04, i' = 17.80 +/- 0.04 and z_s = 17.50 +/- 0.05 on 05 August 2025 (MJD 60892), it started to fade. As of 13 August 2025 (MJD 60900), the magnitudes are g' = 19.85 +/- 0.09, V = 19.22 +/- 0.07, r' = 18.78 +/- 0.06, R = 18.47 +/- 0.04, i' = 18.27 +/- 0.04 and z_s = 17.93 +/- 0.05, consistent with the quiescent levels.

The unusual, low-amplitude brightening was also detected in the X-rays by the Wide X-ray Telescope (WXT) on board the Einstein Probe (EP). The source was first detected on 01 August 2025, with an initial flux of (7.8 +/- 0.8)e-12 erg/cm^2/s (0.5 - 4 keV). It reached a peak flux of ~(2 - 3)e-11 erg/cm^2/s between 03 - 05 August 2025, after which the flux began to decay. The last X-ray detection of the source was on 10 August 2025, thanks to a 3 ks exposure observation performed by the Follow-up X-ray Telescope (FXT) on board EP. On that date, the source displayed a 0.5 - 10 keV flux of (1.5 +/- 0.2)e-12 erg/cm^2/s. We then conclude that the X-ray activity episode duration was approximately 10 days. Compared with all the Aql X-1 outbursts recorded in X-ray so far, this event shows the lowest flux ever measured at the outburst peak (Heinke et al. 2024, Table 7), making it exceptionally rare. The exceptional sensitivity of EP/WXT enabled us to capture this remarkably faint brightening in Aql X-1.

This is atypical of Aql X-1 which usually undergoes bright outbursts in quasi-regular intervals of approximately 1 year (but see ATel #59). Incidentally, the previous outburst had begun in September 2024, roughly a year ago (ATels #16821, #16822, #16823, #16826). To understand its origin, we evaluated the color temperatures during this activity. The de-reddened g' - i' color (A_V = 2.2) indicates that the peak temperature lies at ~7000 K, resembling the rare misfired outburst detected in Cen X-4 during 2021 (ATel #14302, #14317, Baglio et al. 2022). Alternatively, as the next outburst is also due anytime, this flare could be a precursor activity similar to other neutron star binaries such as SAX J1808.4 -3658 (Goodwin et al. 2020 and MAXI J1807+132 (Rout et al. 2025). If that is the case, the outburst should start in the next few weeks or months.

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 analysed with the real-time data analysis pipeline "X-ray Binary New Early Warning System" developed in-house (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020, Alabarta et al., in prep, and ATel #13451 for details). EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2025).

Link to the optical light curve