Probable M31 Stellar Merger - NOT ALFOSC I-band Photometry
ATel #7572; P. Pessev, S. Geier (ULL, IAC, GRANTECAN), A. Kurtenkov (University of Sofia, Bulgaria), L. D. Nielsen, D. Slumstrup (NOT), T. Tomov (Uniwersytet Mikolaja Kopernika, Torun, Poland)
on 27 May 2015; 22:24 UT
Credential Certification: Toma Tomov (tomtom@astri.uni.torun.pl)
We report a series of optical observations aiming to recover the MASTER OT J004207.99+405501.1 / M31N 2015-01a / M31LRN 2015 transient (ATel #7150, #6911; see also Williams et al. 2015) after M31 became observable again. All observations were carried out with the 2.5m Nordic Optical Telescope (NOT) at the Observatorio Roque de Los Muchachos (ORM), La Palma, utilizing the ALFOSC instrument. The observations were executed at the beginning of the astronomical twilight/end of the night at high airmass. To mitigate the effects of the atmospheric extinction we utilized the approach outlined in ATel #7272. The photometric data points are listed below. We were able to recover the object only on the I-band images.
UT Band Mag Err AM Seeing
2015-05-01.23 I 21.1 0.3 2.80 0.8"
2015-05-01.24 R >21.0 --- 2.65 1.0"
2015-05-10.20 I 21.6 0.2 3.11 1.4"
Note that astrometry of earlier epoch (when the transient was brighter) GTC and BTA images using the Massey et al. (2006) catalog yields RA=00:42:08.065 Dec=+40:55:01.33 with a <0.1" uncertainty (Kurtenkov et al. 2015, submitted to A&A), see also Figure 1 . Massey et al. list an I~21.1 source at ~0.4" from this location (see also ATel #7173). This other source could be affecting our photometry as we have used r>0.6" apertures.
We conclude that on May 01st 2015 the transient had become more than a 100 times less luminous in I band, compared to March 14th 2015. The magnitude difference is 5.4 +/- 0.3 over a time period of ~ 47 days). Comparing with the March 11th and 14th 2015 results, the rate of decline of the I-band luminosity had increased significantly (magnitude difference of only 0.18 +/- 0.03 over a time period of ~ 3 days in March). On the other hand, only a marginal decline of 0.5 +/- 0.4 is present in the May data.
Our data are in a good agreement with the results reported in ATel #7537. A combination of the May 23rd H-band measurements (ATel #7555) and our I-band results implies I-H > 5.6 +/- 0.3 , which means that the optical-NIR colors are shifted to the red. Meanwhile the NIR H-band magnitude had declined by only 2.6 +/- 0.2 between March 6th and May 23rd (ATel #7236 and #7555), suggesting that the maximum of the emission of the object is shifting to the NIR.
Further NIR and optical observations are needed as the Andromeda galaxy is now observable again. Considering the optical photometric evolution of the transient, it will be still observable in near future, but with a significant investment of resources under good observing conditions. Coordinated efforts are encouraged to provide optimal follow-up of this rare transient event.
