Swift observations of the binary white dwarf SDSS J0651+2844
ATel #5062; S. Campana (Osservatorio astronomico di Brera), G. L. Israel (Osservatorio astronomico di Roma)
on 13 May 2013; 15:06 UT
Credential Certification: Sergio Campana (campana@merate.mi.astro.it)
Subjects: Ultra-Violet, X-ray, Binary, Gravitational Waves
We observed the 12.75-minute detached binary white dwarf SDSSJ065133.338+284423.37 (hereafter J0651)
system with Swift (Brown et al. 2011, ApJ, 737, L23). J0651 was observed successfully 6 times.
All UVOT observations carried out with the M2 filter, the one retaining the best UV purity (i.e. not
contaminated by a red leak). J0651 is detected in all the observations with magnitudes reported below.
Observation |Exp.(s) | Magnitude
03 Dec 2011 | 5777 |17.96+/-0.03
07 Dec 2011 | 2744 |17.95+/-0.04
08 Dec 2011 | 2740 |18.01+/-0.03
14 Dec 2011 | 1256 |17.98+/-0.06
16 Dec 2011 | 2431 |17.89+/-0.03
20 Dec 2011 | 723 |17.95+/-0.07
The first UVOT observation was carried out in event mode, allowing us to search for eclipses observed
in the optical band. We do not detect any periodicity at the known orbital period of 765.21 s.
A variation of 0.2 mag is observed in the g filter (~20%) data from the GTC (10.8 m) telescope (Hermes et al. 2012, ApJ, 757, L21). Nothing similar is observed in the UVOT data, due to the low
statistics, with the lowest point in the phased light curve being 0.87+/-0.07 (1 sigma).
X-ray observations with the XRT telescope collected a total exposure time of 15,738 s.J0651 was
undetected with a 3 sigma upper limit of 1.26x10^-3 c s^-1. Assuming a power law with photon index
Gamma=2 and a column density of 4x10^19 cm^-2, as derived from the optical data (Brown et al. 2011),
one obtains a limiting flux of <4x10^-14 erg cm^-2 s^-1. At a distance of 1 kpc this equals to
<5x10^30 erg s^-1.
Given the hottest white dwarf temperature (T~16530 K, R~0.04 Rsun; Hermes et al. 2012) we do not
expect to detect any X-ray emission from the system. This indicates that the white dwarfs of J0651
are not not interacting, making it the cleanest system for gravitational wave studies.
