X-ray upper limits from the M82 optical transient

ATel #1503; Thomas J Maccarone, Rob Fender (Southampton), Rob Beswick, Simon Garrington, Ralph Spencer, Tom Muxlow, Peter Thomasson (Manchester)
on 2 May 2008; 10:50 UT
Credential Certification: Tom Maccarone (tjm@phys.soton.ac.uk)

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Swift observed the recent optical transient (ATel #1501) in M82 for 4900 sec and made no detection of X-rays from the source. The upper limit corresponds to approximately 2*10^-3 cts/sec (99% confidence level), which corresponds to a .1-10 keV X-ray luminosity of 1.5 * 10^38 ergs/sec for a 1 keV blackbody spectrum or for a Gamma=1.7 power law, assuming the foreground absoprtion of 5*10^20 cm^-3. The flux limit is only very weakly dependent on absorption below 10^22 cm^-3 (in which case the optical transient would like have been heavily extincted). If the transient truly is located in M82 (i.e. if it is neither a foreground nor background source), then the two most likely explanations of its M_V of approximately -7.5 are that it is a classical nova or it is a long orbital period black hole X-ray binary. Black hole X-ray binaries in outburst should have X-ray luminosities at least as large as their optical luminosities, and the outburst decays of the long orbital period black hole X-ray binaries which could be bright enough to produce the observed optical luminosity tend to be slow. For a 1 keV blackbody, an X-ray source of 3*10^38 ergs/sec -- the bare minimum expected from a black hole X-ray binary at this optical luminosity --would have been expected to produce 15 counts with Swift in these observations, so the non-detection rules this out possibility. A classical nova origin remains plausible. Hard X-ray emission from novae generally peaks at ~10^35 ergs/sec or less (Mukai, Orio & Della Valle 2008). About 20% of novae have been found to show supersoft phases (L_X ~ 10^36-38 ergs/sec, T~2*10^5-10^6 K). The brightest and hottest of these could have been detected with these Swift data, provided that the foreground absorption column to the source is dominated by the Galactic column, but it is worth noting that the supersoft phase is not generally seen at the very beginning of the nova explosion, and can last for years afterwards. Additional soft X-ray observations of this source over the next 6-12 months should help determine whether this source is a nova with a supersoft phase. Additional observations of this source are encouraged, especially given the point raised in ATel #1501 that it may be located in a globular cluster. We note also that MERLIN observations were made late yesterday, but the analysis of these data is not yet complete.