AT2019krl, new transient in M74, on pre-discovery Spitzer/IRAC images

ATel #12934; T. Szalai (U. Szeged / Konkoly Obs., Hungary), O. D. Fox, T. Marston (STScI), J. Andrews (U. Arizona), R. Arendt, E. Dwek (NASA Goddard), G. Clayton, K. Krafton (LSU), A. Filippenko, J. Mauerhan (UC Berkeley), J. Johansson (U. Uppsala, Sweden), P. Kelly (U. Minnesota), S. Van Dyk (IPAC) on behalf of the LASTCHANCE collaboration, J. E. Jencson, M. M. Kasliwal (Caltech), N. Smith (U. Arizona) on behalf of the SPIRITS collaboration
on 12 Jul 2019; 20:07 UT
Credential Certification: Tamas Szalai (szaszi@titan.physx.u-szeged.hu)

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AT2019krl(= ZTF19abehwhj), a new transient appeared in the M74 galaxy, has been spectroscopically classified as a Type IIn supernova (SN) or LBV-outburst just after its discovery on July 6 (ATel #12913). We checked the Spitzer Heritage Archive (SHA) in order to look at the mid-IR pre-discovery evolution of the transient. M74 has been observed with Spitzer several times (PID 159, PI R. Kennicutt; PID 30494, PI B. Sugerman; PID 3248, PI P. Meikle; PID 40010, PI M. Meixner; PIDs 10139, 11053, 14098/LASTCHANCE, PI O. Fox), and has been also a target of Spitzer InfraRed Intensive Transient Survey (SPIRITS; Kasliwal et al. 2017).

At older archival images, there is an apparent quiescent source at the location of the transient (on both 3.6 or 4.5 micron images). The source seems to show a moderate brightening between December 2018 and April 2019, while the transient becomes a very bright mid-IR source at the last epoch (17 May 2019, ~50 days before the transient has been discovered by ZTF). A set of 4.5 micron images obtained between 2005 and 2019 can be seen here.

We determined the following mid-IR fluxes (in microJy) and Vega absolute magnitudes by applying simple aperture photometry on the Spitzer images (uncertainties are shown in parentheses). We adopted 9.0+/-0.5 Mpc and E(B-V)=0.06 mag as the distance of the host and the total interstellar reddening, respectively (Dhungana et al. 2016); we used JD 58670.5 MJD (2019 July 6) as the epoch of discovery.


UT_date   MJD   Epoch   F_3.6   F_4.5   Abs_m_3.6   Abs_m_4.5
2005-01-15 53385 -5285 7(5) 6(4) -10.8(0.8) -11.1(0.8)
2008-01-26 54491 -4179 7(5) 8(5) -10.9(0.7) -11.4(0.7)
2014-03-18 56734 -1936 10(5) 7(4) -11.2(0.6) -11.3(0.6)
2015-10-25 57320 -1350 6(4) 10(5) -10.6(0.8) -11.7(0.6)
2016-04-04 57482 -1188 12(6) 11(6) -11.4(0.5) -11.8(0.5)
2016-10-19 57680 -990 13(6) 15(6) -11.5(0.5) -12.1(0.5)
2017-10-28 58054 -616 13(6) 16(7) -11.6(0.5) -12.2(0.5)
2018-05-04 58242 -428 13(6) 16(7) -11.6(0.5) -12.1(0.5)
2018-11-05 58427 -243 15(6) 19(7) -11.6(0.5) -12.3(0.4)
2018-12-07 58459 -211 15(6) 19(7) -11.6(0.5) -12.4(0.4)
2019-04-21 58594 -76 22(8) 40(11) -12.0(0.4) -13.2(0.3)
2019-05-17 58620 -50 2700(87) 3848(99) -17.2(0.1) -18.1(0.1)

At the first two epochs, images were obtained also at 5.8 and 8.0 micron; at both channel, no source can be seen at the coordinates of the transient (upper limits are 5 and 15 microJy, respectively).

Figures showing the mid-IR pre-discovery light curves can be seen here and here (showing only the epochs from the last one year).

Based on mid-IR data, one or more smaller outbursts could have happened between December 2018 and April 2019 (earlier low-level flux changes are ambiguous to obviously connect to real physical changes), while the final explosion/outburst likely occurred between 21 April and 17 May. A more precise determination of the date of explosion based on archival ground-based data would be desirable, but it is challenging as M74 was behind the Sun during the April/May period. Mid-IR brightness measured at the last epoch support that AT2019krl is rather an SN than an LBV-outburst; at the same time, more information is necessary to reveal the true nature of the event.