Detection of U Sco in X-rays

ATel #2419; 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.), and Erik Kuulkers and Jan-Uwe Ness (ESA/ESAC)
on 3 Feb 2010; 19:07 UT
Credential Certification: Eric M. Schlegel (eric.schlegel@utsa.edu)

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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.), and Erik Kuulkers and Jan-Uwe Ness (ESA/ESAC), representing a large collaboration, report that U Sco has been detected in the X-ray band using the Swift satellite following the optical outburst discovery at V=8.05 on 2010 January 28.4743 (www.aavso.org/publications/alerts/alert415.shtml). The Swift X-ray Telescope (XRT) has been monitoring U Sco since shortly after the optical discovery. In a total exposure time of 15.5 ksec obtained During 2010 Jan 29-30, no source was detected with a 90% confidence upper limit in the 0.3-10 keV band of 1.9x10^-3 cts/s or a flux upper limit of 1.1x10^-13 erg/s/cm^2. During 2010 Jan 31 to Feb 1, a total of 23 counts (4 background) in 15.2 ksec in the 0.3-10 keV band were obtained at the position of U Sco. The detection significance is 4 sigma; the corresponding count rate is ~2.5(+/-0.3)x10^-3 cts/s. The arrival times of the X-ray events do not show evidence of a burst nor a ramp-up. The spectrum at this stage shows all but 2 events with energies above 3 keV; the mean event energy is ~4.8 keV requiring that the observed spectrum is rather hard. Adopting an optically thin thermal model with kT = 5 keV, the observed flux is 7.7(+5.9,-4.0)x10^-14 erg/s/cm^2 in the XRT (0.3-10 keV) band. Given the few counts, the temperature range is not yet well constrained. Adopting a column density of 3x10^21 cm^-2 yields an unabsorbed flux of 9.5(+7.3,-4.8)x10^-14 erg/s/cm^2. Assuming a distance of 12 kpc (Schaefer 2009, ApJS in press, arXiv:0912.4426), the unabsorbed 0.3-10 keV luminosity is then 1.7(+1.3,-0.8)x10^33 erg/s. Additional observations with Swift are scheduled. We thank Neil Gehrels for approving our TOO program and the Swift planning and operations team for their efforts to observe U Sco.