XB-NEWS detects a new outburst of Aql X-1

ATel #17321; Kevin Alabarta, David M. Russell, Sandeep K. Rout, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), M. Cristina Baglio (INAF-OAB), Stefanie Fijma (Anton Pannekoek Institute for Astronomy, University of Amsterdam), Alessio Marino (ICE-CSIC), and Fraser Lewis (Faulkes Telescope Project & The Schools’ Observatory, LJMU)
on 5 Aug 2025; 18:55 UT
Credential Certification: Kevin Alabarta (kalabarta@nyu.edu)

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Aquila X-1 (Aql X-1) is a neutron star low-mass X-ray binary (LMXB) that undergoes outbursts with a recurrence time of approximately one per year. The previous outburst was first detected in X-rays with Einstein Probe on September 14, 2024 (MJD 60567; ATel #16821) and optical with the Las Cumbres Observatory (LCO) network (ATel #16822), being later also detected with VLA (ATel #16823), Swift/XRT (ATel #16826), SAO RAS (ATel #16840), OAN-SPM (ATel #16841), SVOM/ECLAIRs (ATel #16843) and MeerKAT (ATel #16888). This outburst lasted approximately two months, until November 12, 2024 (MJD 60626.24). After its return to quiescence, we continued to observe the source with LCO.

On August 04, 2025 (MJD 60891.27), XB-NEWS detected an increase of the optical flux in the g', r', i' and z_s bands with magnitudes 19.41 +/- 0.02, 18.46 +/- 0.01, 17.94 +/- 0.01 and 17.63 +/- 0.01, respectively. One day later, the source further brightened to g' = 19.27 +/- 0.09, V = 18.76 +/- 0.07, r' = 18.26 +/- 0.04, R = 17.99 +/- 0.04, i' = 17.80 +/- 0.04 and z_s = 17.50 +/- 0.05 mag.

Relative to the long-term quiescent averages (ATel #16822, Alabarta et al., in prep.) the optical flux increases are Δg' = 0.59 +/- 0.16, ΔV = 0.56 +/- 0.14, Δr' = 0.52 +/- 0.07, ΔR = 0.53 +/- 0.11, Δi' = 0.47 +/- 0.10 and Δz_s = 0.42 +/- 0.07. All values exceed the quiescent level by more than 3σ, confirming that the source has entered a new outburst phase. Although there is a hint of a stronger brightening at bluer bands (Δg' - Δz_s ~ 0.17 mag), the difference is less than 2σ and therefore not yet significant.

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).

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