Optical "Vanishing" of the 2015 M31 Stellar Merger
ATel #7537; D. Bersier (LJMU), C. S. Kochanek (OSU), R. M. Wagner (LBTO), S. Adams (OSU), Subo Dong (KIAA-PKU)
on 20 May 2015; 02:01 UT
Credential Certification: R. Mark Wagner (rmw@as.arizona.edu)
Subjects: Optical, Transient
In a 15 minute SDSS i-band observation on May 15.21 with the IO:O camera on the Liverpool telescope, we failed to detect the 2015 M31 stellar merger ("Nova" M31N 2015-01a = MASTER J004207.99+405501.1, Shumkov et al. ATEL #6911) to a limiting magnitude of I>21 mag (S/N=5) after calibrating the image using the I-band magnitudes of nearby sources from Massey et al. (2006) and no color corrections for the filter difference. Figure 1 compares the new i-band image convolved with a Gaussian (right) to the 15 second unfiltered MDM acquisition image from Wagner et al. (ATEL #7208), with a 5" radius circle marking the source location.
Spitzer observations on April 23.69 (Adams et al. ATEL #7485) found that the source had [3.6]=12.99+/-0.08 and [4.5]=13.15+/-0.10 mag, corresponding to a luminosity of log(L/Lsun)=5.34 assuming a cool stellar photosphere (a T=3500K Castelli & Kurucz (2004) model) for an M31 distance of 0.8 Mpc (Stanek & Garnavich 1998) and (only) Galactic extinction of E(B-V)=0.05 mag. This fit is shown in Figure 2 along with optical/near-IR SEDs from March (Pessev et al. ATEL #7272; Williams et al. 2015) The new limit, also shown in Figure 2, corresponds to a limit on the band luminosity of νL_ν < 800Lsun or log(L/Lsun)<3.1 for T=3500K.
Either the luminosity has collapsed by a factor of ~100 or a significant optical depth of dust (A_V~8 mag) has formed in the last three weeks. Ground based near-IR observations are required to determine which of these hypothesis is correct because the Spitzer observation window has closed and no new Spitzer data will be obtained until the fall.
