Swift/XRT Detects Hard-State X-ray Emission from the 2020 Outburst of Aquila X-1

ATel #13961; Arash Bahramian (Curtin University/ICRAR), Payaswini Saikia (NYU Abu Dhabi), Gregory Sivakoff (U. Alberta), David Russell (NYU Abu Dhabi) on behalf of a larger multi-team collaboration
on 21 Aug 2020; 18:46 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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The XB-NEWS team recently reported optical emission from Aquila X-1 (Aql X-1), the prototypical recurrent neutron star low-mass X-ray binary (distance d=5.2+/-0.7 kpc; Jonker & Nelemans 2004, MNRAS, 354, 355), that indicated a new outburst of Aql X-1 was beginning (ATel #13953). We thus arranged X-ray, UV, and optical observations with the Neil Gehrels Swift Observatory using its X-ray telescope (XRT) and UV/optical telescope (UVOT). Here, we report preliminary results from an observation on 2020 Aug 20 at 16:25-16:30 (MJD 59081.684-59081.688). We clearly detect X-ray emission from the source consistent with the beginning of a new outburst of Aql X-1.

The XRT observed the source for 598 seconds in Auto mode. Since the source was bright, the observations remained in Windowed Timing mode. From initial analysis of the X-ray data, we clearly detect the source, at a count rate of 12.19+/-0.15 cnt s^-1 in the 0.3-10 keV range. There was no clear evidence of a strongly variable light curve, suggesting that no Type I bursts occurred and a single spectral fit is sufficient for measuring the source flux. After grouping 50 counts per bin of grade 0 events, we find a count rate of 10.42 +/- 0.13 cnt s^-1 in the 0.5-10 keV. The spectra of these data are fit well (chi-square of 80.9 for 99 degrees of freedom) by an absorbed (TBABS) power-law with a column density of (3.5 +0.3 / -0.4) *1e21 cm^-2, a photon index of 1.99+/-0.07, and a 2-10 keV flux of (3.22+0.16/-0.15)*1e-10 erg cm^-2 s^-1. The errors in the spectra represent 90% confidence intervals. At 5.2+/-0.7 kpc, this corresponds to an approximate 2-10 keV luminosity of (1.0 +/- 0.3)*1e36 erg s^-1. This flux also corresponds to about 13 mCrab. The source is clearly in the early stages of a hard accretion state, as expected for the beginning of an outburst.

The UVOT observed the source in the U, B, UVW1, and UVW2 filters for exposures of 78.5, 78.5, 157.2, and 255.86 seconds, respectively. Using a 3” aperture centered on the GAIA DR2 position (19:11:16.057 +00:35:05.876), and a circular source-free region with a 15” radius for background correction, we see marginal evidence of emission in the B filter (4.6 sigma), but no other filter detects emission at more than a 2.3 sigma significance. We therefore conservatively place 5-sigma upper limits of >19.73, >19.01, >19.95, and >20.48 in AB mag (>18.71, >19.14, >18.44, >18.75 in Vega mag) for U, B, UVW1, and UVW2, respectively. This radius captures light from the donor, accretion disk, and an interloper star approximately 0.5” from the source.

This 2020 outburst at optical was detected about 380 days after Swift/BAT detected the second-to-last outburst at X-ray wavelengths (ATel #13016, #13953) and about 390 days after Las Cumbres Observatory (LCO) detected the start of the latter outburst at optical wavelengths (ATel #13953). The beginning of the last outburst was not observed by LCO. Based on past outbursts, the XB-NEWS team predicted that the first MAXI (X-ray) detection of this outburst around 2020 Aug 20.6 +/- 2.2. Given typical MAXI daily sensitivity, a 5-sigma detection corresponds to around 20 mCrab and 16 mCrab in the 2-20 and 2-4 keV bands, respectively. With our Swift/XRT detection at 13 mCrab, it is very likely that MAXI will detect the source in the predicted window. The source may take a few days to reach the 30 mCrab, 7-sigma limit typically used by Swift/BAT at 15-50 keV.

Further multiwavelength observations are encouraged to trace the evolution of this relatively-early detected rise of the system.

We thank the staff of the Neil Gehrels Swift Observatory for rapidly approving and scheduling these observations.