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Aql X-1 in brightest outburst since 2003

ATel #2871; D. M. Russell (Univ. of Amsterdam), P. Roche, F. Lewis (Faulkes Telescope Project, Open Univ., Univ. of Glamorgan), D. Maitra (Univ. of Michigan)
on 25 Sep 2010; 11:21 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

Subjects: Infra-Red, Optical, X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 2891, 2902, 2911, 9306

In July, the transient neutron star low-mass X-ray binary Aql X-1 was detected in outburst at X-ray, UV, optical and near-infrared wavelengths (ATel #2742, #2744, #2748). On 1st September the X-rays re-brightened (ATel #2850). Here we report on our ongoing optical monitoring campaign (ATel #1218, #1970, #2288) with the Faulkes Telescopes North and South.

The outburst started between 5th July (MJD 55382.5; when its V and i' magnitudes were consistent with quiescent levels) and 8th July (MJD 55385.4; when it had brightened by 0.6 mag in V and 0.3 mag in i' above quiescent levels). The source then continued to brighten by 1.4 mag in V and 1.0 mag in i' in two weeks (the spectrum getting bluer; see Maitra & Bailyn 2008, ApJ, 688, 537), reaching V = 18.0 +- 0.1; i' = 17.3 +- 0.1 on 17th July (MJD 55394.3). Aql X-1 then faded by ~0.5 mag but very likely did not return to quiescence before re-brightening by 7th September (MJD 55446.4). The X-ray flux had risen above X-ray all-sky monitor detection limits by 1st September (ATel #2850) but our optical monitoring shows this is not a new outburst, but rather a further brightening of the one that started in July. Aql X-1 remained ~ 1 mag brighter than quiescence in optical and near-infrared from July to September; see also SMARTS monitoring light curves here. By 15th September (MJD 55454.3) the optical magnitudes were the brightest seen since our Faulkes monitoring began in August 2006, with V = 17.1 +- 0.1; i' = 16.6 +- 0.1. The RXTE ASM X-ray flux was the highest since the outburst in March 2003, with ~ 28 cps (1.5-12 keV) on 16th September, now fading to 18 cps (1.5-12 keV) on 23rd September (MJD 55462). We expect a state transition back to the hard state from the current soft state (ATel #2850) in the coming days/weeks.

We also report on timing studies conducted on 28th July (MJD 55405.5; ten images in i'-band) and 30th July (MJD 55407.5; five images in V-band). The source in outburst (V ~ 18.4; i' ~ 17.8) was consistent with being non-variable within errors, with a rms variability of < 0.06 mag in V-band and < 0.07 mag in i'-band (3 sigma upper limits; the time resolution was 245 sec in V and 124 sec in i'). This is similar to other neutron star transients in outburst (e.g. Lewis et al. 2010, A&A, 517, A72) but much less variable than some black hole transients in the hard state (ATel #2547; Gandhi 2009, ApJ, 697, L167).

The Faulkes Telescope observations are part of an ongoing monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0712.2751). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). DMR acknowledges support from a Netherlands Organisation for Scientific Research (NWO) Veni Fellowship. FL acknowledges support from the Dill Faulkes Educational Trust.

Faulkes Telescope light curves of Aql X-1