XB-NEWS detects a new outburst from Aquila X-1

ATel #13953; Payaswini Saikia (NYU Abu Dhabi), Yasmine Heikal (Sorbonne University Abu Dhabi), David M. Russell, D. M. Bramich, Maria Cristina Baglio, Saarah F. Pirbhoy (NYU Abu Dhabi) and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 19 Aug 2020; 14:04 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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Aquila X-1 (Aql X-1) is a transient neutron star low-mass X-ray binary system discovered in 1973 (Kunte et al. 1973). The system shows frequent outbursts with an irregular recurrence rate of roughly one year, with each outburst typically lasting from a few weeks to a few months (e.g. Degenaar et al. 2019). We have been closely monitoring the source since 2006 April with the Las Cumbres Observatory (LCO) 2-m (Faulkes) and 1-m robotic telescopes (see ATel #1218, #2871, #9306, Diaz Trigo et al. 2018).

The last outburst of Aql X-1 was reported in 2019 August with Swift/BAT transient monitor (ATel #13016), during which it was also detected in the radio band at 15.5 GHz with the Arcminute Microkelvin Imager Large Array (AMI-LA, ATel #13016) and in optical with the 1-m SAO RAS optical telescope (ATel #13020). Our optical monitoring of the source with LCO shows the start of the outburst to be around MJD 58691 (2019 July 27; i'=17.95+-0.01, V=18.93+-0.02), reaching a peak around MJD 58720 (2019 August 25; i'=16.04+-0.01, V=16.52+-0.01), and ending the outburst approximately around MJD 58756 (2019 September 30; i'=18.15+-0.02, V=19.30+-0.03). The source then faded at optical frequencies, going back to the quiescent level, and became Sun constrained.

We resumed our LCO observations on MJD 58925 (2020 March 17; i'=17.18+-0.01, V=17.82+-0.02) after the Sun constraint ended, and found the source to be in the middle of a new outburst. This outburst was comparatively fainter than the 2019 outburst, and peaked around MJD 58932-58934 (with i'=16.79+-0.05 on 2020 March 26, and V=17.52+-0.01 on 2020 March 24). It ended on approximately MJD 58967.6 (2020 April 28; i'=18.20+-0.01, V=19.22+-0.02).

Very recently, a new brightening of the source was observed by LCO, with magnitudes going from i'=18.15+-0.01 and V=19.06+-0.02 on MJD 59074.5 (2020 August 13) to i'=17.70+-0.01 and V=18.61+-0.01 on MJD 59079.3 (2020 August 18), detecting the source in an early outburst phase. After studying the rise rate and start dates of previous outbursts of this source, we find a fairly constant delay between the beginning of the optical V-band rise of an Aql X-1 outburst, and the first MAXI detection, of 10.7 +- 2.2 days. By fitting the rise slope, the V-band rise started on MJD 59070.9 (2020 Aug 9), so we predict the first MAXI detection around 2020 Aug 20.6 +- 2.2. Hence, we expect a MAXI rise any day now.

The optical monitoring of the source with LCO is still ongoing. Further multiwavelength observations are encouraged in order to confirm the brightening, trace the early rise of the system and determine its accretion state.

This work makes use of observations from the Las Cumbres Observatory (LCO). The LCO observations are part of an on-going monitoring campaign of ~ 50 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. The analysis of the LCO data are performed with a new real-time data analysis pipeline, the ''X-ray Binary New Early Warning System'' (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical LCO light curves of Aql X-1