Recurrent Nova U Sco Has Sharp Decline in X-ray/UV/Optical/IR

ATel #2477; Bradley E. Schaefer, Ashley Pagnotta (Louisiana State Univ.), Julian P. Osborne, Kim L. Page, Andrew Beardmore (Leicester), Eric Schlegel (UT-San Antonio), Gerald Handler (Univ. Vienna), M. F. Bode (Liverpool JMU), Erik Kuulkers, Jan-Uwe Ness (ESA/ESAC), Jeremy J. Drake (CfA), Greg Schwarz (AAS), James Truran (Univ. Chicago), and Sumner Starrfield (Arizona State)
on 12 Mar 2010; 22:56 UT
Credential Certification: Bradley E. Schaefer (schaefer@lsu.edu)

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U Sco is a recurrent nova with a fast decline leveling off to a plateau in the optical light curve (IAUC # 9111 , ATEL #2452), and it has become visible as a super-soft source (SSS) at the start of the plateau phase (ATEL #2430, ATEL #2442). Here, we report that the plateau phase has ended and the brightness of U Sco is fast falling in the UV/optical/IR bands. The flux during the plateau did not vary greatly up until 1 March, when all bands (X-ray to IR) started to smoothly turn over and began fading with an accelerating rate. The table below gives characteristic fluxes during the various phases of the plateau and sharp drop. The column labeled "DAD" is the days after the discovery date of 2010 Jan 28.4385 UT. The link below gives graphs of the full optical and X-ray light curves. The X-ray fluxes are the Swift 0.3-10 keV count rates, the W2 magnitude is for the Swift W2 filter centered at 2030 A, and the V and K magnitudes are the standard optical and IR bands (outside eclipses).

PHASE:	2010 date	DAD	X-ray (ct/s)	W2 (mag)	V (mag)	K (mag)
Start of Plateau	Feb 12	15	0.8	13.6	14.0	12.3
Middle of Plateau	Feb 20	23	0.9	13.6	14.3	12.9
End of Plateau	Mar 1	32	2.0	15.0	15.2	13.9
After sharp drop	Mar 9	40	0.10	16.8	16.5	>15

In the optical bands, the eclipses started with the plateau phase, deepening from 0.6 mag to 1.4 mag in depth, and slightly increased in total duration from 0.29+-0.06 to 0.38+-0.06 in phase. In the ultraviolet bands, the eclipses deepened from 0.6 mag at the start of the plateau to 1.5 mag near the end of the plateau. In the 0.3-10 keV band, a shallow ~30% eclipse with substantial variations or flickering was seen during the first and last thirds of the plateau phase, but there is little evidence for eclipses either in the middle of the plateau or now after the recent decline.

With the end of the SSS, the light is returning to its quiescent level. (This final fading has never been observed for U Sco, so photometric and spectroscopic observations are still needed.) From the now-ten known eruption years (1863, 1906, 1917, 1936, 1945, 1969, 1979, 1987, 1999, and 2010), the inter-eruption intervals (with missed eruptions ~1927 and ~1957) range from 7.9 to 11.8 years, with an average of 10.3 years. So we predict the next eruption in 2020+-2. This predicted date can be made much more accurate (Schaefer 2005, ApJLett, 621, L53) by following the accretion rate (through the B-band flux) over the next decade.

X-ray and V-band light curves of U Sco