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 (mukai@milkyway.gsfc.nasa.gov)

Tweet

We have identified Swift/BAT detected source as a new hard X-ray bright polar.

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 encouraged.