No X-ray detection of Nova Del 2013 with Swift
ATel #5318; K. L. Page, J. P. Osborne, A. P. Beardmore (U. Leicester) and J. A. Kennea (PSU)
on 23 Aug 2013; 21:47 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
ATel #5314 claims an X-ray detection of Nova Del 2013 on 2013 August 22. We find no significant detection in the same dataset, noting that an incorrect background estimation in the
earlier analysis may account for the claim.
The data were collected in three Windowed Timing (WT) mode snapshots with different XRT pointing directions and roll angles; the nova was located towards differing ends of the window in different snapshots. (WT data provide only one spatial dimension, but have a much shorter frame-time than Photon Counting mode, and so much lower susceptibility to optical loading.) Although the nova position was covered by the full 3.9 ks of data, most of the background area far away from the source was exposed for noticeably less than this, with some regions only exposed for half this time.
Using a small extraction region (diameter 20 pixels), suitable for a faint source, 284 counts were found at the nova position (in the full 3.9 ks). The background has to be evaluated from two different positions in order to account for the same exposure time. Using a background region of
twice the source diameter (40 pixels), located at the far ends of each snapshot (though fully contained within the window), provides 610 background counts; thus 305 counts scaled to the size of the source extraction region. Including a correction factor of 1.16 to account for PSF losses, this provides a 3-sigma upper limit of 0.012 count s-1.
