Spitzer detections and pre-discovery archival limits for AT 2018akh in M81

ATel #11803; J. E. Jencson (Caltech), H. E. Bond (PSU), S. M. Adams (Caltech), M. M. Kasliwal (Caltech), on behalf of the SPIRITS collaboration
on 1 Jul 2018; 18:38 UT
Credential Certification: Jacob Jencson (jj@astro.caltech.edu)

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We report detections of an infrared counterpart to AT 2018akh in Spitzer/IRAC 3.6 and 4.5 micron images ([3.6] and [4.5]) of M81 taken as part of the Spitzer InfraRed Intensive Transient Survey (SPIRITS; Kasliwal et al. 2017). The transient, recovered by our automated transient detection pipeline and designated SPIRITS 18dq, was first reported as a possible nova by Z. Xu and X. Gao (see here), and subsequently by Kucakova et al. (ATEL #11465). Here, we list our photometry (Vega mag) of the source, and pre-outburst limits from non-detections:

 
UT Date       | MJD      | [3.6] (error) | [4.5] (error) 
2017-06-23.71 | 57927.71 | >18.6         | >18.3 
2018-02-04.26 | 58153.26 |   ...         | >18.3 
2018-06-19.84 | 58288.84 |  17.8 (0.1)   |  16.92 (0.06) 

Assuming a distance modulus for M81 of 27.8 mag (Tully et al. 2013), the absolute magnitude at [4.5] is -10.9 mag. The bright IR detections reported here (cf. the optical peak at R = 20.5 mag on 2018-03-16.88; ATEL #11465) suggest either a dust-forming nova or a luminous red nova, especially given the relatively late phase at 95 days post peak.

We examined the location of the transient in the archival Spitzer/IRAC channels 1-4 Super Mosaics. There is no source coincident with the transient position to [3.6] > 18.85 mag, [4.5] > 18.79 mag, [5.8] > 17.37 mag, [8.0] > 16.46 mag. We also examined the HST WFC3/UVIS (F336W, F438W, F555W, F625W, F814W) and IR (F105W, F125W, F160W) images taken in 2011-2012. To obtain a precise position, we registered the Spitzer/IRAC [4.5] image of the transient from 2018-06-19 with the F814W image. We find no candidate progenitor star at the location of SPIRITS 18dq (AT 2018akh) to F814W > 25.7 mag (-2.1 mag absolute). Similarly, there is no obvious progenitor candidate in any of the other HST/WFC3 frames covering the position.