Deep infrared observations of the puzzling central X-ray source in RCW103

Abstract

1E 161348-5055 (1E 1613) is a point-like, soft X-ray source originally identified as a radio-quiet, isolated neutron star, shining at the center of the 2000 yr old supernova remnant RCW103. 1E 1613 features a puzzling 6.67 hour periodicity as well as a dramatic variability over a time scale of few years. Such a temporal behavior, coupled to the young age and to the lack of an obvious optical counterpart, makes 1E 1613 a unique source among all compact objects associated to SNRs. It could either be the first low-mass X-ray binary system discovered inside a SNR, or a peculiar isolated magnetar with an extremely slow spin period. Analysis of archival IR observations, performed in 2001 with the VLT/ISAAC instrument, and in 2002 with the NICMOS camera onboard HST unveils a very crowded field. A few sources are positionally consistent with the refined X-ray error region that we derived from the analysis of 13 Chandra observations. To shed light on the nature of 1E 1613, we have performed deep IR observations of the field with the NACO instrument at the ESO/VLT, searching for variability. We find no compelling reasons to associate any of the candidates to 1E 1613. On one side, within the frame of the binary system model for the X-ray source, it is very unlikely that one of the candidates be a low-mass companion star to 1E 1613. On the other side, if the X-ray source is an isolated magnetar surrounded by a fallback disc, we cannot exclude that the IR counterpart be hidden among the candidates. If none of the potential counterparts is linked to the X-ray source, 1E 1613 would remain undetected in the IR down to Ks>22.1. Such an upper limit is consistent only with an extremely low-mass star (an M6-M8 dwarf) at the position of 1E 1613, and makes rather problematic the interpretation of 1E 1613 as an accreting binary system.