The Peculiar X-Ray Transient IGR J16358-4726

The new transient IGR J16358-4726 was discovered on 2003 March 19 with INTEGRAL. We detected the source serendipitously during our 2003 March 24 observation of SGR 1627-41 with the Chandra X-Ray Observatory at the 1.7×10^-10 ergs s^-1 cm^-2 flux level (2-10 keV) with a very high absorption column (N_H=3.3×10²³ cm^-2) and a hard power-law spectrum of index 0.5(1). We discovered a very strong flux modulation with a period of 5880(50) s and peak-to-peak pulse fraction of 70(6)% (2-10 keV), clearly visible in the X-ray data. The nature of IGR J16358-4726 remains unresolved. The only neutron star systems known with similar spin periods are low-luminosity persistent wind-fed pulsars; if this is a spin period, this transient is a new kind of object. If this is an orbital period, then the system could be a compact low-mass X-ray binary.