The magnetar candidate AX J1818.8-1559

Abstract

In October 2007, a hard X-ray burst was detected by the INTEGRAL satellite from a direction consistent with the position of AX J1818.8-1559, an X-ray source at low Galactic latitude discovered with the Advanced Satellite for Cosmology and Astrophysics (ASCA) satellite in 1996-1999. The short duration (0.8 s) and soft spectrum (power-law photon index of 3.0 ± 0.2) of the burst in the 20-100 keV range are typical of soft gamma-ray repeaters and anomalous X-ray pulsars. We report on the results of an observation of AX J1818.8-1559 obtained with the Suzaku satellite in October 2011. The source spectrum, a power-law with photon index ~1.5, and flux (~2 × 10^-12 erg cm^-2 s^-1, 2-10 keV) did not show significant variations from those derived from archival data of various satellites obtained from 1993 to 2011. We discuss possible interpretations of the data for AX J1818.8-1559 and, based on its association with the INTEGRAL burst, we propose it as a new member of the small class of magnetar candidates.