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Swift observations of SN2011ja

ATel #3820; R. Margutti, A. M. Soderberg, D. Milisavljevic (Harvard Univ.)
on 22 Dec 2011; 16:12 UT
Credential Certification: Raffaella Margutti (rmargutti@cfa.harvard.edu)

Subjects: Ultra-Violet, X-ray, Supernovae

SN2011ja has been recently classified as a type IIP supernova with spectroscopic similarities to SN2004et (Milisavljevic, CBET 2946). SN2011ja exploded in NGC4945 at a distance of about 4 Mpc (from NED). A Swift-ToO was executed to observe the field of SN2011ja starting from 2011-12-18T19:52:30 UT. Statistically significant but faint X-ray emission is detected at the SN position by Swift-XRT (total exposure of 7.9 ks) at the level of (2.2+\-0.7)d-3 c/s. Assuming a spectral power-law spectral model, this translates into an unabsorbed 0.3-10 keV flux of ~1.d-13 erg/s/cm2, corresponding to a luminosity of ~2d38 erg/s at a distance of 4 Mpc. The Galactic neutral hydrogen column density in the direction of the SN is 1.38d21 cm-2 (Kalberla 2005). Interestingly, type IIP SN2004et was also detected in the X-rays at a level of ~1d38 erg/s at comparable epoch (Misra 2007 MNRAS 381 280). Using pre-explosion images of NGC4945 by Chandra acquired in January 2000, we note however the presence of two sources in a 20" radius around the SN position: Swift-XRT cannot resolve the two sources which therefore contribute to the flux inferred by XRT. The total flux from the two sources is ~3d-14 erg/s/cm2, formally lower than the flux measured by XRT but still consistent with the Swift measurement at 2-sigma. Detection of fading associated to the Swift-XRT source would clarify if SN2011ja contributes to the measured flux. Alternatively, the detection of a third source, distinct from the two previously detected by Chandra and consistent with SN2011ja would clarify this issue. Given the radio detection and monitoring of the SN with the ATCA (Ryder et al. CBET 2946) additional X-ray observations will provide an opportunity for a joint X-ray and radio study Swift-UVOT observations have also been obtained in the w1, m2 and w2 filters: due to the presence of a bright star in the field, u, b and v observations could not be performed. No source is detected in the UV bands at the SN position, consistent with the relatively old age (about 1 week after maximum light) of SN2011ja as inferred from optical spectroscopy and the likely large extinction (Milisavljevic, CBET 2946). We thank the Swift team for scheduling these ToO observations.