Detection of a new X-ray outburst of Aquila X-1 by Neil Gehrels Swift Observatory

ATel #12225; Jie Lin, Shangyu Sun, and Wenfei Yu (Shanghai Astronomical Observatory)
on 18 Nov 2018; 07:56 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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Aquila X-1 is one of the neutron star LMXB transients with frequent outbursts in the past few decades. It has been inactive in the X-ray band for about 250 days since its latest outburst reported in 2018 Feb (e.g. Atel#11382).

The hard X-ray monitoring observations performed with Swift/BAT indicated that the X-ray intensity of Aquila X-1 in the 15-50 keV band has increased from 13+/-11 mCrab to 38+/-9 mCrab in the last 10 days, which suggests that Aquila X-1 may be in a new X-ray outburst again. However, the sun angle of Aquila X-1 is lower than 60 degrees. Therefore it was uncertain whether the enhance X-ray flux came from the source or not. Therefore, we proposed a Swift ToO pointed observation towards Aquila X-1 to confirm if it is indeed in a new X-ray outburst.

The Swift/XRT observation was performed with the WT mode for about 500 seconds, starting at 17:19:04 of 2018-11-17 (UTC). We did a quick analysis of the XRT data only. The net XRT X-ray count rate was 30.4+/-0.3 cps, corresponding to about 50 mCrab in the XRT band. We fitted the XRT energy spectrum in the 0.6-10 keV band with an absorbed power-law model, which gave a hydrogen column density of 3.0+/-0.2 x10^21 cm^-2 and a power-law index of 1.81+/-0.04. The X-ray flux in the 0.6-10 keV energy band is estimated as 1.65+/-0.03 x10^-9 ergs/s/cm^2, corresponding to an X-ray luminosity of 3.0x10^36 ergs/s at a source distance of 3.9 kpc. Therefore, we conclude that Aquila X-1 is in a new X-ray outburst.

We would like to thank the PI of the Neil Gehrels Swift Observatory for approving our request and scheduling the ToO observation, and the Guest Observer Facility for providing BAT monitoring data to the public.