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X-ray and Optical follow-up of the mid-2014 Outburst of Aql X-1 at peak and at low activity

ATel #6447; Poshak Gandhi (Durham), Vik S. Dhillon (Sheffield), John A. Tomsick (Berkeley), Tim Butterley (Durham), Stuart M. Littlefair (Sheffield), Richard W. Wilson (Durham) and Jamie A. Kennea (Penn State)
on 5 Sep 2014; 20:57 UT
Credential Certification: Poshak Gandhi (poshak.gandhi@durham.ac.uk)

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

Referred to by ATel #: 7088

Following reports of optical and X-ray brightening of the soft X-ray transient Aql X-1 (ATel #6280, #6286), we obtained monitoring observations of the source with the Swift X-ray mission, and with the 0.5 m Durham/Sheffield robotic optical telescope located on La Palma. Here we report and compare observations at outburst peak with recent observations at a lower activity level.

The MAXI lightcurve shows that the source peaked in X-rays around mid-July 2014. A Swift/XRT observation (ObsID 00080757001) occurred starting at 2014 Jul 17 UT 17.15 h, with an exposure time of about 17 min after standard filtering. The 0.5-10 keV XRT count rate was 183.7+/-0.4 c/s. The spectrum is reasonably well-described (reduced-chi2 = 1.4 for 565 dof) by a model with a 0.74+/-0.01 keV blackbody and a power-law with a photon index of 1.27+/-0.06 (the errors on the X-ray fit parameters are 90% confidence). Both components are absorbed with a column density of (4.5+/-0.3)E21 cm-2 (using Wilms et al. 2000 abundances). The unabsorbed 0.5-10 keV flux is 1.2E-8 erg/cm2/s, corresponding to an isotropic luminosity of L(0.5-10)~3.6E37 erg/s for a distance of 5 kpc. The absorbed 0.5-10 keV flux is 1.0E-8 erg/cm2/s.

On the same night, approximately 2 hours of imaging was obtained in the R band with the 0.5 m telescope starting at ~UT 21.43 h. Conditions were photometric with seeing of ~2 arcsec. A bright field comparison star of known brightness was used for photometric calibration. The source was clearly detected with an observed R mag = 15.94+/-0.04 (1-sigma error).

The source has steadily faded in X-rays since mid-July and is not detectable by MAXI at present. A Swift/XRT observation (ObsID 00032888046) occurred starting at 2014 Sep 01 UT 16.75 h, and an exposure time of about 32 min was obtained. The 0.5-10 keV count rate was 0.220+/-0.011 c/s. The spectrum is well-described by an absorbed power-law with a column density of (9.5+/-2.0)E21 cm-2, a steep photon index of 2.6+/-0.3, and an unabsorbed 0.5-10 keV flux of (2.0+/-0.5)E-11 erg/cm2/s, yielding an isotropic luminosity of L(0.5-10)~6.0E34 erg/s. The absorbed 0.5-10 keV flux is ~1E-11 erg/cm2/s.

In conjunction, a short optical observation was also obtained the following night with the 0.5 m telescope starting on 2014 Sep 02 at ~UT 20.66 h. Conditions were photometric with a seeing of ~1.6 arcsec. The source mag on this occasion was measured to be R = 17.86+/-0.15.

These observations provide a measure of the amplitude of the present outburst in X-rays and in optical, with the following implications.

1) At peak, the source X-ray luminosity as measured by Swift was more than 4 orders of magnitudes higher than typical quiescence levels (Cackett et al. 2011). Since then, the source has faded by a factor of ~600, and now lies at least an order of magnitude higher than its typical quiescent flux level, i.e. the source is still active in X-rays;

2) The quiescent optical mags of r~18.9 and i~18.2 reported in ATel #6280 correspond to R~18.6 according to the photometric transformation equation of Lupton 2005). By comparing with the optical mag of R=15.9 at peak, the amplitude of the present outburst is determined to be ~2.7 mag in the R band (a factor of ~12 in flux). The source has since faded by 1.92+/-0.16 mag (a factor of ~6 in flux), but is still ~2 times brighter than optical quiescence.

Continued monitoring of the source is encouraged to obtain full coverage of the present activity and to understand the nature of its broadband emission. We thank Neil Gehrels for approving the Swift ToO request.