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Late-time, Mid-IR Photometry of SN 2011fe

ATel #4986; P. Garnavich, C. McClelland (Notre Dame), and P. Milne (U. Arizona)
on 16 Apr 2013; 20:41 UT
Credential Certification: Peter Garnavich (pgarnavi@gmail.com)

Subjects: Infra-Red, Supernovae

We imaged the nearby, type Ia supernova 2011fe (aka PTF11kly) with the IRAC camera on board the Spitzer Space Telescope on 2013 February 25 (UT), 534 days after maximum light. The supernova is not detected in CH1 (3.6 micron) and we estimate a flux limit of <7 microJy. However, the supernova remains visible in CH2 (4.5 micron) with a flux of 16+/-4 microJy.

Extrapolating the decay measured by McClelland et al. (2013, ApJ, 767, 119) of 1.48 mag/100 days for CH1, we expected a flux of 3 microJy at the time of the observation, consistent with our measured limit.

For CH2, which has a more shallow decline, we expected a flux of 8 microJy, but observe about twice that value. We conclude the decay rate at 4.5 micron has slowed since 2012 August and is now less than 0.4 mag/100 days.

No other type Ia supernova has been detected at such late phases in the mid-IR, so it is hard to say if this behavior is typical. Several type Ia events have shown slow fading or constant flux in the J and H bands at late phases (e.g. SN2000cx; Sollerman et al. 2004, A&A, 428, 555). The CH2 photometric behavior appears to confirm the suggestion by McClelland et al. (2013) that the 4.5 micron band is dominated by FeII emission.

However, the slow fading beyond 4 microns could be caused by an IR light echo where circumstellar dust is heated by the supernova explosion.

Images and photometry can be found at http://www.nd.edu/~pgarnavi/sn2011fe