[vsnet-alert 10825] OT_J030558.6+054715 = CSS081007:030559+054715 brightening (ATEL)

[Taichi Kato]

   Forgot to forward this.  This object (=OT_J030558.6+054715) has
brightened to 14 mag.

===

http://www.astronomerstelegram.org/?read=1873 

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 

Password Certification: Julian P Osborne (julo at star.le.ac.uk) 

Subjects: Ultra-Violet, X-ray, Request for Observations, Novae 

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.