Swift X-ray/UV observations of ASASSN-14ds / 1RXS J204455.9-115151
ATel #6320; C. O. Heinke, G. R. Sivakoff (Alberta), K. Z. Stanek, T. W.-S. Holoien (Ohio State)
on 16 Jul 2014; 22:47 UT
Credential Certification: Craig Heinke (cheinke@virginia.edu)
Subjects: Optical, Ultra-Violet, X-ray, Cataclysmic Variable, Transient
Referred to by ATel #: 6588
We obtained two Swift observations of the optical transient ASASSN-14ds (or 1RXS J204455.9-115151) during the later phases of its outburst (Atels #6306, #6312), on July 11, 2014, between 19:06 and 21:03 (TT time system), and on July 15, 2014, from 14:19 to 14:34 (TT).
On July 11, we observed the following UV magnitudes (on the updated UVOT photometric system, Breeveld et al. 2011, in AIP Conf. Ser. 1358, p. 373): u=15.37+/-0.04 (79 s), uvw1=15.27+/-0.04 (157 s), uvm2=15.40+/-0.05 (193 s), uvw2=15.29+/-0.05 (108 s).
On July 15, we observed the following magnitudes: uvw1=15.55+/-0.03 (317 s), uvm2: 15.76+/-0.05 (304 s), uvw2: 15.94+/-0.04 (304 s).
This photometry has not been corrected for extinction. The UV colors suggest a spectrum close to Vega on July 11, and thus a temperature close to 10,000 K, while on July 15 the object had dimmed by 0.28 to 0.65 magnitudes and become slightly redder.
ASASSN-14ds was clearly detected by Swift/XRT in both observations, with net countrates of 0.11 (±0.01) cts/s on July 11 (1145 s), and 0.13 (±0.01) cts/s on July 15 (926 s). We extracted spectra (grouped to 15 counts/bin) and created exposure maps and effective area files according to standard Swift threads (http://www.swift.ac.uk/analysis/xrt/index.php). We performed simple power-law fits to both spectra, with linked photoelectric absorption columns (using Wilms et al. 2001 abundances) and power-law indices, and found a reasonable fit (reduced chi-squared of 1.088 for 12 degrees of freedom). The fitted values were: NH=5+11-5*1020 cm-2, Γ=1.8+0.5-0.4, FX,11(0.5-10 keV, unabs)=4.8+1.5-1.1*10-12 ergs/s/cm2, FX,15(0.5-10 keV, unabs)=5.9+1.8-1.3*10-12 ergs/s/cm2. There is a suggestion of an emission line at 0.9 keV in the first spectrum, but this is only marginally significant according to an F-test. Allowing the powerlaw indices to float separately gave nearly identical values.
Unfortunately the ASAS-SN lightcurve has a gap between July 10.5 (V=13.95) and July 12.5 (V>15.83), after which it has not been detected. Thus, both Swift observations may have occurred after ASASSN-14ds returned to quiescence (where typically V ranges from 16.2-17.2). The blue UV colors and X-ray spectra and fluxes are suggestive of a cataclysmic variable, as suggested by Groot et al. (Atel #6312). At 1 kpc, the X-ray luminosity would be 6*1032 ergs/s, which is near the upper bound of cataclysmic variable X-ray luminosities, suggesting that ASASSN-14ds lies closer than 1 kpc to Earth.
