Swift detects a large flux increase and a probable period of 1.77 days in the X-ray and UV emission of the nova CSS081007:030559+054715
ATel #1873; A. P. Beardmore, J. P. Osborne, K. Page (University of Leicester), G. J. Schwarz (West Chester Unversity), S. Starrfield (Arizona State University), J.-U. Ness (ESAC, ESA)
on 11 Dec 2008; 16:02 UT
Credential Certification: Julian P Osborne (firstname.lastname@example.org)
Subjects: Ultra-Violet, X-ray, Request for Observations, Nova
Swift has continued to monitor the nova CSS081007:030559+054715 (ATEL
#1825, ATEL #1835, ATEL #1847) in the X-ray and UV wavebands. The Swift-XRT 0.3-10 keV
light curve showed a relatively constant count rate of 0.18-0.21 count/s
from 2008-Nov-10 to 2008-Nov-21. Over the same time interval the
Swift-UVOT uvw2 light curve (central wavelength 1928 A) had a mean
magnitude of 14.9, with +/-0.2 magnitude deviations from this value.
Following an 8 day hiatus, daily Swift observations were performed between
2008-Nov-30 and 2008-Dec-09. The X-ray light curve from this interval
shows a large flux increase and also large amplitude variations, with the
X-ray count rate varying from 0.2 to 2.2 count/s. A Lomb-Scargle
periodogram reveals a strong signal at a frequency of 6.53e-06 Hz, in
addition to the odd harmonics of this frequency. These suggest that a
non-sinusoidal modulation with a period of P=1.77 days is present in the
data. An analysis of test periods using the observed window function
gives us further confidence in the period determination.
When the X-ray light curve is folded at this 1.77 day period we see a
rapid decline (lasting 0.02P) to the minimum count rate level (0.2
count/s) which lasts for approximately 0.15P. Due to limited phase
coverage the duration of the maximum is more uncertain, but could
extend between 0.4 to 0.8 cycles. The modulation is not apparent in a
0.5-1.5 keV / 0.3-0.5 keV hardness ratio.
The X-ray spectrum remains essentially as reported in ATEL #1847, ie simply a very soft blackbody.
Plots of the Swift 0.3-10 keV X-ray and uvw2 UV light curve, the periodiogram of
the X-ray count rate, and the X-ray count rate folded at 1.77 days are available at http://www.swift.ac.uk/CSS081007.shtml
Swift-UVOT uvw2 photometry continued from 2008-Nov-30. It also shows that
the source has brightened, and it is also suggestive of a 1.77 day
periodicity with a modulation ranging from a magnitude of 14.6 to 14.0 in phase with the X-ray modulation.
We aim to continue Swift observations at a higher cadence over the next week to
constrain the periodicity further. Ground based photometry is encouraged.
We thank the Swift PI, Neil Gehrels, the Swift science team and the
Swift mission operations team for their support of these observations.