A Deep Upper Limit to RX J0822.0-4300 in Puppis A at 3.6 microns

ATel #4846; David L. Kaplan (University of Wisconsin-Milwaukee), Deepto Chakrabarty (MIT), Zhongxiang Wang (Shanghai Astronomical Observatory)
on 28 Feb 2013; 17:50 UT
Credential Certification: David L. Kaplan (kaplan@gravity.phys.uwm.edu)

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We report on an archival observation of RX J0822.0-4300, the compact central object in the Puppis A supernova remnant, with the IRAC instrument on the Spitzer Space Telescope using the 3.6 and 5.8 micron (um) channels. The observation began on 2006 Nov 29 at 0845 UT and consisted of 181 integrations of 96.8s duration each, for a total integration of 17,521s. Here we focus on results from the 3.6um channel, which was deeper and had a substantially lower background. We assembled the individual exposures into a mosaic with MOPEX (MOsaicker and Point source EXtractor), following the same procedure as described in Kaplan et al. (2009, ApJ, 700, 149). We detected no source at the position of RX J0822.0-4300 (as determined by Wang et al. 2007, ApJ, 655, 261), using an error circle of 1 arcsec in radius; this includes the roughly 0.6 arcsec uncertainty from the X-ray astrometry, and a roughly 0.4 arcsec uncertainty in the IRAC astrometry, which has been referenced to our ground-based K-band observations and to the 2MASS survey (Skrutskie et al. 2006, AJ, 131, 1163). In the region of RX J0822.0-4300, source-free areas have a rms flux density of 0.1-0.2 uJy/pixel. However, the location of RX J0822.0-4300 is in the wings of a nearby star (2.6 arcsec away, with Ks=18.0). We verified that there was no source at the position of RX J0822.0-4300 by subtracting the nearby star using APEX (Astronomical Point source EXtractor, as in Makovoz & Marleau 2005, PASP, 117, 1113) with the Spitzer-supplied point response function (PRF). The rms flux density after subtraction was 0.1 uJy, comparable to the uncertainty on detections of nearby faint stars (dominated by the background). Therefore, we quote a 3-sigma limit of 0.3 uJy, a factor of 3 fainter than the 4.5 um limit from Wang et al. (2007). Compared to the X-ray flux, our limit on nu*F_nu/F_x is ~6e-5, a factor of 2 less than Wang et al. (2007) and nearly the same as was detected from the anomalous X-ray pulsar 4U 0142+61 (Wang et al. 2006, Nature, 440, 772).