Emergence of the Supersoft X-ray Phase of U Sco

ATel #2430; Eric M. Schlegel (UT-San Antonio); Brad Schaefer and Ashley Pagnotta (LSU); Kim Page and Julian Osborne (Leicester); Jeremy Drake (SAO); Marina Orio (Wisconsin), Dai Takei (Rikkyo Univ.), Erik Kuulkers and Jan-Uwe Ness (ESA/ESAC), and Sumner Starrfield (ASU) for a larger collaboration
on 10 Feb 2010; 20:18 UT
Credential Certification: Eric M. Schlegel (eric.schlegel@utsa.edu)

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U Sco has been detected as a supersoft source in the X-ray band using the Swift satellite following the initial discovery as an X-ray source (ATel 2419). The Swift X-ray Telescope (XRT) has been monitoring U Sco since shortly after the optical discovery with the exception of ~5 days when the field-of-view was unavailable because of Swift's lunar constraint. An observation, obtained 2010 Feb 9 13:00 UT or 12 days after the outburst, reveals strong soft emission. The XRT spectrum may be fit with a blackbody of temperature 28+/-8 eV and a column density NH of 3.8(+1.6,-1.2)x10^21 cm^-2. The detected count rate is 0.34+/-0.02 cts/s in contrast to the ~0.002 cts/s of the initial hard phase. The X-ray light curve is essentially flat from days 2 to 8. U Sco is then detected at about 100 times higher count rate at day ~12. The rise phase very likely occurred during the lunar constraint gap but a single point at day 12.1 has a count rate of ~0.1 cts/s and may be the tail of the rise. The observed flux in the 0.3-10 keV band is 9.5x10^-12 erg/s/cm^2. Adopting a column density of 3x10^21 cm^-2 (to be consistent with ATEL 2419) yields an unabsorbed flux of 1.2x10^-9 erg/s/cm^2 in the 0.3-10 keV band. Assuming a distance of 12 kpc (Schaefer 2009, ApJS in press, arXiv:0912.4426), the unabsorbed 0.3-10 keV luminosity is then 2x10^37 erg/s. Fits using WD atmosphere models yield temperatures in the range of 400-700K K, suggesting that the temperature is not tightly constrained and may be higher than the blackbody fit implies. U Sco is expected to remain in a SSS phase for the next approximately 20-30 days, based upon previous outbursts. The details are sufficiently unknown that continued monitoring is highly recommended until U Sco returns to a quiescent state. We thank Neil Gehrels for approving the TOO program and the Swift planning and operations team for their efforts to observe U Sco.