Swift Observations of the Luminous Transient in NGC 300

ATel #1543; Edo Berger, Alicia Soderberg (Princeton University)
on 21 May 2008; 03:17 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Alicia M. Soderberg (ams@astro.caltech.edu)

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We obtained a target-of-opportunity observation of the luminous transient in NGC 300 (IAUC # 8946 ) using the Swift satellite X-ray telescope and UV/optical telescope. The observations commenced on 2008 May 20.01 UT for a total of 6 ksec with XRT and 5.7 ksec with UVOT. We do not detect X-ray emission at the position of the transient to a limit of 1.5e-3 cps, corresponding to a flux limit of F_X < 5e-14 erg/s/cm^2 (assuming a kT=1 keV thermal bremsstrahlung model). At the distance of NGC 300 (1.9 Mpc) this corresponds to a limit of L_X < 2e37 erg/s. Similarly, we do not detect any clear source in the UVOT/UVW2 image, but we note that the location of the transient is contaminated by light from nearby sources. We place a conservative limit of UVW2 < 20 mag (<5e-17 erg/s/cm^2/A). In the context of a blackbody spectrum with R~14 mag we expect the UVW2 flux to be about 5 magnitudes brighter, indicating significant extinction along the line-of-sight or local to the source. An extinction of A_V ~ 1 mag would reproduce a thermal spectrum, increasing the reported absolute magnitude of the transient (IAUC # 8946 ) by about a magnitude. We thank the Swift ToO committee for approving these observations and note that our monitoring continues at optical and radio (Very Large Array) wavelengths.