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Swift follow-up observations of the optical transient AT2018cow/ATLAS18qqn

ATel #11737; L. E. Rivera Sandoval, T. Maccarone (Texas Tech University)
on 19 Jun 2018; 22:54 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Liliana Rivera Sandoval (liliana.rivera@ttu.edu)

Subjects: Optical, Ultra-Violet, X-ray, Transient

Referred to by ATel #: 11738, 11739, 11742, 11749, 11753, 11767, 11776, 12067

We report on 0.5 ks Swift observations of the optical transient AT2018cow/ATLAS18qqn (ATel #11734, #11732, #11729, #11727). The observations were taken on 19 June 2018 using the XRT and UVOT instruments. Fitting an absorbed powerlaw model to the X-ray spectrum at z=0 we obtained nH= 6.8 +/- 0.08 e20 cm2, photon index = 1.48 +/- 0.2 and a flux of 3.4 +/- 0.5 e-11 ergs/cm2/s in the 0.3-10 keV band.

We determine UVOT magnitudes in the following 6 filters (all in Vega system)

v = 13.79 +/- 0.04 mags

b = 13.82 +/- 0.02 mags

u = 12.53 +/- 0.03 mags

uvw1 = 11.75 +/- 0.02 mags

uvw2 = 11.70 +/- 0.01 mags

uvm2 = 11.60 +/- 0.02 mags

If the object is a CV with typical M_V= 5, the object would be at a distance of around 600pc. This means that the X-ray luminosity would be Lx ~1.4e33 erg/sec, suggesting that the WD would be heavy, both to allow such high X-ray luminosity and to have the X-ray spectrum be so hard. However, the fast evolution of the outburst suggests it is a short orbital period system.

If as suggested in Atel #11736, the source is associated with CGCG 137-068, Lx would be about 10^10 times higher, or Lx ~1.4e43 erg/sec. In this case, the mostly featureless spectrum and this X-ray luminosity combine to suggest a gamma-ray burst afterglow, perhaps from an off-axis GRB

.