Near-IR detection of the anomalous X-ray pulsar 1E 1048.1-5937 following a glitch

ATel #1044; Z. Wang, V. Kaspi (McGill), J. Bryant (U. of Sydney), R. Dib, C. Tam (McGill), F. Gavriil (NASA/GSFC)
on 4 Apr 2007; 15:27 UT
Credential Certification: Z. Wang (wangzx@physics.mcgill.ca)

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We observed the field of AXP 1E 1048.1-5937 on 2007 April 04, UT 00:37, at Ks band, using the near-IR imaging camera PANIC on the Magellan/Baade Telescope at Las Campanas Observatory in Chile. The exposure was 15 min and the seeing 0.6 arcsec. We detected the near-IR counterpart discovered by Wang & Chakrabarty (2002, ApJ, 579, L33), and found its Ks =19.9+/-0.2 mag (1-sigma uncertainty).

This Ks magnitude indicates that the AXP right now is comparably as bright as on 2002 April 8, when the counterpart was discovered, but is 1.3--1.5 mag brighter than in 2003 April/June (Durant & van Kerkwijk 2005, ApJ, 627, 376) and roughly 3 mag brighter than in 2006, during which 4 HST/NICMOS observations of the source were performed (Tam et al. 2007, in preparation). Given its extreme faintness in 2006 and its current brightness, the AXP's near-IR brightening is very likely to be associated with the glitch and flux enhancement event just reported (ATEL #1041, #1043). In addition, this suggests that the 2002 April near-IR brightening might also have been associated with the AXP's 2002--2004 long-term event in which the source showed enhanced spin-down and the contemporaneous X-ray flaring (Gavriil & Kaspi 2004, ApJ, 609, L67).

Comparing the unabsorbed Ks flux (assuming A_K =0.6 mag) to the Swift 1--10 keV X-ray flux (ATEL #1043), we find an X-ray/Ks flux ratio of approximately 3600, which is within the range of flux ratios previously obtained for the AXPs with known near-IR counterparts (Durant & van Kerkwijk 2005). This may suggest that in this event, the near-IR and X-ray emission are related, although previously, there was no evidence for correlated pulsed X-ray/near-IR emission (Durant & van Kerkwijk 2005). In order to understand the origin of its near-IR emission, close monitoring of the AXP in both the near-IR and X-rays is needed.