The Peculiar X - Ray Transient Igr 16358 − 4726

The new transient IGR 16358−4726 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 ergs s cm flux level (2−10 keV) with a very high absorption column (NH = 3.3 × 10 23 cm) 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 16358− 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 (LMXB). Subject headings: pulsars: individual (IGR 16358− 4726)