Broad UV and probable X-ray eclipses in nova U Sco

ATel #2442; Julian P. Osborne, Kim L. Page, Graham Wynn, Andrew Beardmore (Leicester), Bradley E. Schaefer, Ashley Pagnotta (LSU), Eric Schlegel (UT-San Antonio), Erik Kuulkers, Jan-Uwe Ness (ESA/ESAC), Matt Darnley (Liverpool John Moores), Margarita Hernanz (CSIC-IEEC), Sumner Starrfield (Arizona State), Jeremy Drake (SAO), Stewart Eyres (UC Lancashire)
on 17 Feb 2010; 14:44 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

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Swift has started a campaign of high cadence observations of the recent outburst of the recurrent nova U Sco, following the emergence first of a hard, and then a super-soft X-ray source (ATEL #2419, ATEL #2430).

Observations spanning 2010 Feb 16:34 to Feb 14 23:40UT (14.25 - 17.54 days after outburst discovery on Jan 28.4385, IAUC # 9111 ) were made mostly at 96 minute intervals by the XRT and by the UVOT with the u (3465A), uvw1 (2600A) and uvw2 (1928A) filters (also with the uv grism). The UVOT photometry shows two well covered, and one partly covered, eclipses at approximately 15.00, 16.26 & 17.47 days after outburst discovery. These eclipses have a depth of ~0.7 mag in uvw1 and ~0.5 mag in u, and a FWHM ~0.22 day in both filters. The uneclipsed magnitudes are ~13.4 in all three filters.

The X-ray spectrum continues to be super-soft. The X-ray light curve does not show a clear, sharp, deep or total eclipse, but does typically have lower flux around the time of eclipse. The apparently random variation in the X-ray brightness makes measurement difficult, but an eclipse count rate of ~0.5 c/s compared to an out-of-eclipse rate of ~0.8 c/s is a reasonable estimate of the folded lightcurve. It is very difficult to constrain the full width, but it might be 0.3 days (based on one continuously observed cycle). No XRT count rate around the time of eclipse is consistent with zero flux.

This is the first time that an eclipse phase of a known eclipsing erupting nova has been observed in X-rays (although Sala et al. 2008 ApJ 675, L93 may have observed an eclipse by the disk in nova V5116 Sgr). That the U Sco probable eclipse in X-rays is partial and broad is a surprise, suggesting a large scattering region. The absence of a total X-ray eclipse, in combination with the >300k K temperature (ATEL #2430) and the system geometry of Thoroughgood et al. (2001, MNRAS 327, 1323) shows that we are not observing the white dwarf photosphere directly, as the luminosity implied by the Stefan-Boltzmann law would be >300 L_Edd.

The strong UV eclipse is much broader than the quiescent optical eclipse, which has a FWHM in B of 0.08-0.09 days (Schaefer 2009, ApJS in press, arXiv:0912.4426), implying that the UV emission region is much larger than the quiescent accretion disk.

The X-ray and UV eclipses may be the same width, consistent with electron scattering, and so implying continuous ejection of mass from the white dwarf. Integrating down to unit scattering optical depth given by the eclipse width then suggests an average mass loss rate of 2x10^21 g/s and a total ejecta mass of order 10^-6 M_sun.

A light curve plot showing the data referred to here is available here.

We are grateful to the Swift PI, Neil Gehrels, and the Swift mission operations team for their support of these observations.

Swift XRT and UVOT lightcurves of the recurrent nova U Sco 2010 taken during its super-soft phase.