Swift Discovers a New Hard X-ray Bright Polar
ATel #1309; K. Mukai (GSFC and UMBC), W. Baumgartner (GSFC and UMBC), J. Tueller (GSFC), R. Mushotzky (GSFC), J. Kennea (PSU), B. Cenko (Caltech), A. Rau (Caltech), C. Markwardt (GSFC and UMD) and the BAT team.
on 27 Nov 2007; 22:54 UT
Credential Certification: Koji Mukai (email@example.com)
Subjects: Optical, X-ray, Cataclysmic Variable
Referred to by ATel #: 1318
We have identified Swift/BAT detected source as a new hard X-ray bright
The BAT source, Swift J2319.4+2619, is located at 23 19 29.1 +26 19 03
(J2000) with an uncertainty of 6 arcmin radius. The BAT 15-100 keV
spectrum can be fit with a power law (photon index 2.7) or a
bremsstrahlung (kT=19 keV) with a 15-50 keV flux of 1.05 x 10-11
ergs cm-2s-1. This is not a spectrum typical of a BAT Seyfert galaxy.
Pointed Swift/XRT observations were performed for 2 ksec in 2007 May and
8 ksec in 2007 June. The strongest source in the field is located at
23 19 30.3, +26 15 16.5 (J2000), with an error circle radius of 3.6
arcsec (90% confidence). This source is 3.8 arcmin from the BAT position
and can be identified with the ROSAT All-sky Survey source
1RXS J231930.9+261525. A second XRT source is detected 9.8 arcmin from the
BAT position, is five times weaker, and can be identified with Mrk 322.
A third source SWIFT J231951.19+261545.7 can be identified with the
galaxy UGC 12515, which is 5.9 arcmin from the BAT position and is 6 times
weaker than the new source. We therefore identify 1RXS J231930.9+261525 as
the soft X-ray counterpart of Swift J2319.4+2619, although Mrk 322 and
UGC 12515 may also contribute to the BAT counts. The XRT spectrum of
Swift J2319.4+2619 consists of a hard power-law like component (photon
index ~ 1.13), with a soft, blackbody-like excess below 0.5 keV. It shows a
factor of 2 variability between the two XRT exposures.
The Swift/XRT as well as ROSAT positions allow us to identify USNO B1
1162-0585089 at 23 19 30.43, +26 15 19.1 (3.0 arcsec from the XRT
position) as the probable optical counterpart. We have therefore
obtained a spectrum of this object with the Double Spectrograph
mounted on the 200-inch Palomar Hale telescope on the night of 13
November 2007 UT. A dichroic at 5500 A was employed, yielding
coverage from ~ 3800 - 5500 A on the blue arm and 5500 - 9000 A on
the red arm. The spectrum has a weak continuum, a strong Balmer jump
in emission, and strong emission lines of hydrogen and helium. The H-
beta and HeII 4686 lines both have equivalent widths of ~ 34 A. This
is a textbook example of a polar type magnetic cataclysmic variable
(CV) in a high state, which also fits with the X-ray spectrum.
Synthetic photometry yields measurements of B ~ 17.0 mag, R ~ 15.6
mag at the time of our spectrum; however these values are subject to
significant systematic uncertainties due to slit losses, errors in
photometric calibration, etc.
Zickgraf et al. 2003 (A&A 406, 535) observed a star at 23 19 30.4,
+26 15 17 (J2000). They note a blue continuum and a faint point-like
image at B=21.6, B-R=1.0, and classify it as a possible AGN. We
speculate that Zickgraf et al. observed the polar in a low state,
dominated by the photosphere of the white dwarf.
Although magnetic CVs are a significant group of hard X-ray emitting
objects, they are mostly of the intermediate polar sub-type (Barlow et al.
2006, MNRAS 372, 224). Since only a few polars are known to be strong
hard X-ray sources, follow-up observations of Swift J2319.4+2619 are