Swift confirms KS 1947+300 is in X-ray outburst

ATel #5441; J. A. Kennea (PSU), P. A. Evans (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano, V. Mangano (INAF-IASFPA), P. Curran (CEA-Saclay), K. Yamaoka (Waseda U) and H. Negoro (Nihon U.)
on 3 Oct 2013; 18:45 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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As a follow-up to the MAXI reported outburst of the Be/X-ray binary KS 1947+300 (Kawagoe et al., ATEL #5438), we initiated a Swift target-of-opportunity observation of the source. Starting at 23:03UT on October 2nd, 2013, Swift/XRT took a 1ks exposure of KS 1947+300 utilizing Photon Counting mode. We find in the field one bright X-ray source at the following location: Ra/Dec(J2000) = 297.3978, 30.2085, which is equivalent to:

RA(J2000) = 19h 49m 35.48s
Dec(J2000) = +30d 12' 30.5''

with an estimated uncertainty of 2.6 arc-sec radius (90% confidence), which lies 1.3 arc-sec from the Simbad cataloged position of the optical counterpart of KS 1947+300. This position was derived using the UVOT corrected spacecraft attitude, to correct for systematic astrometry errors.

Performing a Z²₁ search on the XRT event list (with barycentric correction applied), we strongly detect a modulation with a period of 18.79 s, unambiguously confirming that the XRT source is indeed KS 1947+300, which had an RXTE reported period of 18.76 s in October 2000 (Swank & Morgan, 2000, IAU Circ. 7531).

The light-curve during the outburst appears to be flat, with an average count rate of 2.9 +/- 0.1 c/s. We note that archival observations of this field taken by Swift/XRT in 2008 detect this source, at a much lower count rate of ~0.02 XRT count s^-1, confirming that the source is currently in outburst.

The outburst spectrum in XRT is relatively hard,well fit by a power-law model with a photon index = 0.72 +/- 0.20, with an absorption N_H = 4.5 +/- 1.8 x 10²¹ cm^-2. The average flux during the observation was 2.6 +/- 0.3 x 10^-10 erg s^-1 cm^-2 (0.3-10 keV, uncorrected for absorption), roughly a factor of 100 times bright than the average flux measured by Swift in 2008, ~ 2.2 x 10^-12 erg s^-1 cm^-2 (0.5 -10 keV). The fit to the 2008 quiescent spectrum is consistent within errors with the outburst spectrum with photon index = 0.86 +/- 0.35.

The outburst of KS 1947+300 is also detected in the Swift/BAT hard X-ray transient monitor in the 15-50 keV band. The event was first detected on 2013 October 1 (MJD 56566) at a rate of 0.0054 +/- 0.001 ct/sec/cm² (~25 mCrab) and has continued to brighten since that time. On 2013 October 3, it was at 0.0075 +/- 0.002 (35 mCrab).