Early X-ray and radio observations of Nova Sco 2015 implicate strong shocks against a red giant wind

ATel #7085; T. Nelson (Minnesota), J. Linford (Michigan State), L. Chomiuk (Michigan State), J. Sokoloski (Columbia), K. Mukai (UMBC/NASA GSFC), T. Finzell (Michigan State), J. Weston (Columbia), M. Rupen (NRC-HIA) and A. Mioduszewski (NRAO)
on 16 Feb 2015; 23:22 UT
Credential Certification: Thomas Nelson (tnelson@physics.umn.edu)

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We report the first observations of Nova Sco 2015 (PNV J17032620-3504140) at X-ray, UV and radio wavelengths. The X-ray observations were carried out with the Swift satellite between 2015 February 15.5 and 16.3 UT (roughly 4 days after discovery) and resulted in a total exposure time with the XRT instrument of 4065 s. An X-ray source is clearly detected at the position of the nova with a count rate in the 0.3-10 keV range of 0.14 cts/s. The spectrum is hard and can be modeled as a highly absorbed, hot thermal plasma (N(H) ~ 6+/-1 x 10^22 cm^-2; kT > 41 keV). However, a significant excess of counts over the model prediction is observed between 1 and 2 keV, possibly indicating the presence of a second, softer emission component. The total observed flux in the 0.3 - 10 keV range is 1.9 (+1.0, -1.8) x 10^-11 erg/s/cm^2 (90% confidence).

An image of the nova in the near-UV was obtained with the UVOT instrument on Swift. The magnitude in the UVM2 filter (central wavelength 2246 Angstroms) is 13.34 +/- 0.03 in the Vega system.

We also observed Nova Sco 2015 at radio wavelengths with the Karl G. Jansky Very Large Array (VLA) on 2015 February 14.5, approximately 3 days after optical discovery. Observations were carried out in B configuration (maximum baseline of 11.1 km) at C-band and Ka-band, with a total bandwidth of 2 GHz in each band split between two sidebands. The nova was detected at frequencies from 4.55 to 36.5 GHz with a spectrum typical of non-thermal synchrotron emission (spectral index between -0.7 and -0.9). The flux densities are as follows:

Frequency | Flux density
4.55 (GHz) | 4.13 +/- 0.02 (mJy)
7.38 (GHz) | 2.79 +/- 0.01 (mJy)
28.2 (GHz) | 0.82 +/- 0.06 (mJy)
36.5 (GHz) | 0.68 +/- 0.08 (mJy)

The presence of hard, absorbed X-rays and synchrotron radio emission at this early stage of the outburst suggest that the nova-producing white dwarf is embedded within the wind of a red-giant companion, with collisions between the ejecta and this wind shock-heating plasma and accelerating particles (as in, e.g. RS Oph, V407 Cyg and V745 Sco). This interpretation supports a similar suggestion made in ATel #7060. Further X-ray and radio observations are planned, and continued monitoring at other wavelengths is strongly encouraged.