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Faulkes Telescope observations of the optical rise of a bright outburst of Aql X-1

ATel #9306; David M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 3 Aug 2016; 15:55 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Optical, X-ray, Binary, Neutron Star, Transient, Pulsar

The neutron star X-ray binary transient, Aql X-1 has just entered a new outburst. Swift/BAT detected an increase in the hard X-ray flux starting on 2016 July 29 (MJD 57598; ATel #9287), followed by a confirmation from Swift/XRT that the source is detected and bright at 0.3-10 keV (ATel #9292) and by July 31 the optical flux had also brightened (ATel #9293).

From our long term optical monitoring of Aql X-1 with the Faulkes Telescopes North and South (e.g. ATel #1970, #2871), we find that the optical magnitude remained at a level consistent with quiescence up until July 17 (MJD 57586) from observations taken on 21 dates this year (between 2016 March 1 and July 17). The average quiescent magnitude in this period was V = 19.4, with variations spanning +- 0.1 mag. The V-band magnitude then appeared 0.4 +- 0.1 mag brighter than quiescence on our next observation on July 22 (MJD 57591.5), whereas the i'-band magnitude still remained at the quiescent level. The i'-band then brightened between July 22 and July 26 (MJD 57595.6). The first 3 sigma detection by Swift BAT from the daily average light curve was on MJD 57597, six days after the initial optical flux increase was detected.

Since the initial rise, Aql X-1 has now brightened by 2.1 mag and 1.6 mag in V and i'-bands respectively in just 11 days, to V = 17.3 +- 0.1; i' = 16.8 +- 0.1 on August 2 (MJD 57602.3). These latest magnitudes are close to the historically brightest seen since our monitoring began in 2008. The V-i' colour has become bluer while it has brightened, as expected from the relationship between colour and magnitude as the disc temperature increases (Maitra & Bailyn 2008).

We will continue to monitor the source regularly with Faulkes/LCOGT during this outburst. See the link below for our latest light curves. Multi-wavelength observations are encouraged. The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory Global Telescope Network.

Aql X-1 Faulkes Telescope light curves