Swift/BAT detection of hard X-rays from Tycho's Supernova Remnant: evidence for Titanium-44 emission

ATel #6044; E. Troja (UMCP & NASA/GSFC), A. Segreto (INAF-IASFPa), W. Baumgartner (UMBC & NASA/GSFC), V. La Parola (INAF-IASFPa), C. Markwardt (NASA/GSFC), G. Cusumano (INAF-IASFPa), S. Barthelmy (NASA/GSFC), D. Hartmann (Clemson University), & N. Gehrels (NASA/GSFC)
on 5 Apr 2014; 20:13 UT
Credential Certification: Eleonora Troja (eleonora.troja@nasa.gov)

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We report on Swift/BAT survey observations of Tycho's SNR, spanning a period of 104 months since Swift mission's launch in November 2004. The total on-axis equivalent exposure in the direction of the SNR is 19.6 Ms. Our preliminary analysis shows significant hard X-ray emission up to ~100 keV.

Two independent analyses based on products from the BatImager software (Segreto et al 2010 A&A 510 47; Cusumano et al 2010 A&A 524 64) and from the GSFC standard pipeline (Baumgartner et al 2013 ApJS 207 19; Markwardt et al. 2005 ApJL 633 77; Tueller et al 2010 ApJS, 186, 378) both show a significant excess above the continuum in the 60--85 keV region attributable to the 68 and 78 keV lines from the decay of 44Ti-44Sc-44Ca.

The significance of the signal in the 60--85 keV bin has been growing with time, from 3.4 sigma (using the first 25 months of survey data), to 3.9 sigma (50 months), and to 4.7 sigma (75 months). By modeling the SNR spectrum with a simple power-law function, we measure a photon index Gamma=3.0 +/- 0.3 (90% confidence level). The observed emission above 60 keV is in excess with respect to the underlying continuum. If attributed to the lines at 68 keV and 78 keV,  the emission detected by BAT yields a flux in each line of (1.2 +/- 0.6)E-5 ph/cm2/s (90% confidence level). These results from BatImager are consistent with the Goddard BAT Survey analysis. Assuming a SNR distance of 3 kpc, the derived Titanium-44 yield is approximately 2.4E-4 solar masses. This value is sensitive to the  assumed SNR distance, and should be considered as a preliminary estimate.

By applying the same data reduction and analysis to the Cas A supernova remnant, we detect the Ti-44 lines as a high significance excess (~7 sigma) in the 60--85 keV energy band.  We measure a preliminary flux in each of the 68 and 78 keV lines of (2.6 +/- 0.6)E-5 ph/cm2/s (90% confidence level), consistent with previous measurements by Beppo-SAX and  INTEGRAL (Vink et al 2001 ApJL 560 79; Renaud et al. 2006 ApJL 647 41).

Further analysis is ongoing.