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Swift observations of the possible radio supernova in NGC 3628

ATEL # 1003; S. Immler (NASA/USRA/GSFC) and P. Milne (U of Arizone) on behalf of the Swift satellite team
on 14 Feb 2007; 23:09 UT
Password Certification: Stefan Immler (immler@milkyway.gsfc.nasa.gov)

Subjects: Optical, Ultra-Violet, X-ray, Nova, Supernova

Swift Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope (XRT) observed the possible radio supernova in NGC 3628 (CBET #842) on 2006-11-12.08 UT as part of the Swift Survey of Nearby Galaxies. The following upper limits (at a 3-sigma level) to the UVOT magnitudes were measured: V > 18.1 (199 s exposure time), B > 19.2 (199 s), U > 18.9 (199 s), UVW1 [181-321nm] > 19.6 (406 s), UVM2 [166-268 nm] > 19.9 (328 s), and UVW2 [112-264 nm] > 20.5 (813 s). The magnitudes have not been corrected for extinction. We note that the SN is located along the dense central dust lane of the host galaxy which might have obscured the opt/UV emission.

An X-ray source is detected at the position of the radio source at a 5.5-sigma level of confidence in the merged 2.4 ks Swift XRT observation obtained simultaneously with the UVOT. The XRT net count rate is (1.6+/-0.3) E-02 cts/s, corresponding to an unabsorbed (0.2-10 keV band) X-ray flux of (8.1+/-1.5) E-13 ergs/cm/cm/s and a luminosity of (1.5+/-0.3) E40 ergs/s for an adopted thermal plasma spectrum with a temperature of kT = 10 keV, a Galactic foreground column density of N_H = 2.23E+20 (Dickey & Lockman, 1990, ARAA 28, 215) and a distance of 11.4 Mpc (Saha et al. 1999, ApJ 522, 802). Due to the small offset of the radio source from the nucleus of the host galaxy (~2 arcsec), the X-ray source might be due to the nucleus of the galaxy and unrelated to the radio source.

Further Swift observations are scheduled to probe the variability of the X-ray source.


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