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 (firstname.lastname@example.org)
Subjects: X-ray, Cataclysmic Variable, Nova
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.