We report the serendipitous discovery with XMM-Newton that 3XMM J185246.6+003317 is an 11.56 s X-ray pulsar located 1away from the southern boundary of supernova remnant Kes 79.The spin-down rate of 3XMM J185246.6+003317 is <1.1 × 10-13 S S-1, which, together with the long period P =11.5587126(4) s, indicates a dipolar surface magnetic field of <3.6 × 1013 G, a characteristic age of > 1.7 Myr, and a spin-down luminosity of <2.8 × 1030 erg s-1.Its X-ray spectrum is best-fitted with a resonant Compton scattering model and also can be adequately described by a blackbody model.The observations covering a seven-month span from 2008 to 2009 show variations in the spectral properties of the source, with the luminosity decreasing from 2.7 × 1034 erg s-1 to 4.6 × 1033 erg S-1, along with a decrease of the blackbody temperature from kT ≈ 0.8 keV to ≈0.6 keV.The X-ray luminosity of the source is higher than its spin-down luminosity, ruling out rotation as a power source.The combined timing and spectral properties, the non-detection of any optical or infrared counterpart, together with the lack of detection of the source in archival X-ray data prior to the 2008 XMM-Newton observation, point to 3XMM J185246.6+003317 being a newly discovered transient low-B magnetar undergoing an outburst decay during the XMM-Newton observations.The non-detection by Chandra in 2001 sets an upper limit of 4 × 1032 erg s-1 to the quiescent luminosity of 3XMM J 185246.6+003317.Its period is the longest among currently known transient magnetars.The foreground absorption toward 3XMM J185246.6+003317 is similar to that of Kes 79, suggesting a similar distance of ~7.1 kpc.
PING ZHOU YANG CHEN XIANG-DONG LI SAMAR SAFI-HARB MARIANO MENDEZ YUKIKATSU TERADA WEI SUN MING-YU GE
Department of Astronomy, Nanjing University, Nanjing 210093, China;Department of Physics and Astrono Department of Astronomy, Nanjing University, Nanjing 210093, China;Key Laboratory of Modem Astronomy Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB R3T 2N2, Canada Kapteyn Astronomical Institute, University of Groningen, P.O.Box 800, 9700 AV Groningen, The Netherl Graduate School of Science and Engineering, Saitama University, 255 Simo-Ohkubo, Sakura-ku, Saitama Department of Astronomy, Nanjing University, Nanjing 210093, China Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Scien