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Swift X-ray flux limits for the recent novae V1280 and V1281 Sco

ATel #1011; J P Osborne, K Page (Leicester), S Starrfield, J-U Ness (Arizona State), J Krautter (Heidelberg), M Bode (Liverpool John Moores), M Orio (INAF, Padova), G Schwarz (West Chester), T O'Brien (Manchester), T Belloni (INAF, Brera)
on 23 Feb 2007; 17:22 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

Subjects: X-ray, Nova

Referred to by ATel #: 2063

Swift observed V1280 Sco and V1281 Sco shortly after their discovery by Nakamura & Sakurai (IAUC # 8803 ) and by Nakamura & Nakano (IAUC # 8810 ), respectively. No X-ray flux was detected in either case.

V1280 Sco was observed by Swift at 21-Feb-2007 12:17 UT for 1.0 ksec, this was 16.7 days after discovery and ~2.5 days from visual maximum. Because of the brightness of this object at the time (www.aavso.org V~4.7), the UVOT filter wheel was set to the blank position. The X-ray telescope was set to observe in Windowed Timing mode (resolution = 1.8 msec) to minimise the effects of optical light in the on-board X-ray event recognition process. The one-dimensional nature of WT mode data reduces the sensitivity of the XRT to faint point sources however. V1280 Sco was not detected, we derive a 3-sigma upper limit to the 0.3-10 keV WT grade 0-2 count rate of 26 c/ks. Assuming an optically thin thermal spectrum with kT=10 keV absorbed by NH=1x10^20 cm^-2, this corresponds to an unabsorbed 0.3-10 keV flux of <1.7x10^-12 erg/cm^2/s and a luminosity limit at 1 kpc of L<2.9x10^32 erg/s.

V1281 Sco was observed by Swift at 22-Feb-2007 17:20 UT for 3.9 ksec, this was 2.9 days after discovery. This object was at V~9-10 at the time (www.aavso.org), the UVOT UV grism was used and the XRT was in its normal Photon Counting mode. V1281 was not detected, we derive a 3-sigma upper limit to the 0.3-10 keV PC grade 0-12 count rate of 0.87 c/ks. Assuming an optically thin thermal spectrum with kT=10 keV absorbed by NH=1x10^20 cm^-2, this corresponds to a 0.3-10 keV flux of <5.0x10^-14 erg/cm^2/s and a luminosity limit at 1 kpc of L<6.1x10^30 erg/s.

The luminosity limits we derive, while admittedly uncertain because of the unknown distances, are orders of magnitude below the luminosity of 4.5x10^34 erg/s measured from V382 Vel at 20.5 days by Mukai & Ishida (ApJ, 551, 1024, 2001). RXTE has also measured an X-ray flux limit for V1280 Sco (Swank 2007, ATEL #1010).

We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. We thank the Swift PI, Neil Gehrels, the Swift science team, and the Swift mission operations team for their support of these observations.