A possible optical counterpart and refined position for the X-ray transient Swift J1713.4-4219

ATel #13226; Maria Cristina Baglio (NYU Abu Dhabi), Francesco Coti Zelati (Institute of Space Science, CSIC-IEEC), Dave M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Payaswini Saikia (NYU Abu Dhabi)
on 24 Oct 2019; 19:44 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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A new outburst of the X-ray transient Swift J1713.4-4219 was detected on October 7th 2019 by INTEGRAL (ATel #13169).
We report here on Swift XRT and UVOT observations of the current outburst of Swift J1713.4-4219, together with optical observations obtained with the Las Cumbres Observatory (LCO) 2-m Faulkes and 1-m telescopes (i' band).
The XRT observed for 1466s in photon counting mode, starting on October 23rd 2019 at 01:03UTC. A source is significantly detected (net count rate of 0.71 +/- 0.02 cts/s in the 0.3-10 keV energy band; signal-to-noise ratio of ~ 30), within the INTEGRAL IBIS/ISGRI error circle (ATel 13169), at a position of:

RA = 17h 13m 41.01s

Dec = -42° 18' 37.5''.

The error radius associated to this position is 2.2 arcsec (at the 90% confidence level).
Fitting an absorbed power law model to the X-ray spectrum yields the following best-fitting parameters (uncertainties are reported at a confidence level of 1 sigma):

nH = (1.3 +/- 0.3) x 10^22 cm^-2

Gamma = 1.4 +/- 0.2

Unabs flux (0.3-10 keV) = (7.8 +/- 0.6) x 10^-11 erg/s/cm^-2,

where Gamma is the index of the power law. The reduced chi-squared of the fit is 1.651 for 23 degrees of freedom, giving a null hypothesis probability of 2.6e-02. These X-ray properties are suggestive of an X-ray binary in the hard state.
The value derived for the column density is larger than the Galactic column density, which is about 8.4 x 10^21 cm^-2 (Willingale et al. 2013, MNRAS 431 394).
The UVOT observed for 1457 s in the image mode using the UVW1 filter. No counterpart was detected, with a 3sigma upper limit on the magnitude of 20.46 (Vega).

We also report on optical observations with the LCO 2-m Faulkes and 1-m telescopes, obtained on October 9th, 10th, 12th.
On October 9th, four 200s exposure time images were obtained with the 1-m telescopes. Once averaged together, we detected one faint source inside the XRT error circle (star A in the linked finding chart) at the position (0.2' uncertainty):

RA = 17h 13m 40.993s

Dec = -42° 18' 38.20''.

The measured magnitude, evaluated through PSF photometry and calibrated using the APASS catalogue, is i'=19.738 +/- 0.075.

On October 10th, two 200s exposure time observations were acquired with the 2-m LCO telescope in Australia. Also in this epoch, after averaging the images to increase the signal-to-noise, we detect the same star (star A) in the XRT error circle with magnitude i'=20.035 +/- 0.069. Since the field is very crowded, we repeated the analysis for two stars (B and C in the finding chart), which are located just outside the XRT error circle. For star B, we measure a magnitude of i'=20.452+/- 0.125 on Oct 9th, and i'=20.673+/-0.113 on Oct 10th. Its position is:

RA = 17h 13m 41.069s

Dec = -42° 18' 40.40''.

For star C, we obtain a calibrated magnitude of i'=20.042+/-0.085 on October 9th, and i'=19.972+/-0.066 on October 10th. Its position is:

RA = 17h 13m 41.338s

Dec = -42° 18' 36.10''.

All positions have an uncertainty of 0.2''.
We do not detect significant variability for any of the stars between the two epochs. However, star A seems to be slightly fainter on October 10th, with a variation of 0.3+/-0.1 magnitudes.
None of the possible counterparts are in the 2MASS catalogue, and neither the PanSTARRS or the VPHAS catalogues cover the field of view.

We will continue monitoring the region to look for variations in these candidate counterparts. Further multiwavelength observations are encouraged to investigate the nature of the transient.

The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO).

Finding chart of Swift J1713.4-4219