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Optical monitoring of the M31 field object iPTF14gnj

ATel #6705; K. Bognar, D. M. Russell (NYU Abu Dhabi), E. Breedt (University of Warwick), F. Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU), M. J. Mei, S. Dahal, F. Bernardini (NYU Abu Dhabi)
on 16 Nov 2014; 07:13 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Optical, Cataclysmic Variable, Nova, Transient

The recent transient source iPTF14gnj in the M31 field was discovered on Oct 11 at a magnitude of g = 16 (ATel #6567). A spectrum acquired on Oct 16 has ruled out that the source is a nova due to the absence of emission lines in its spectrum (ATel #6681).

We report optical photometry of iPTF14gnj with the Las Cumbres Observatory Global Telescope Network (LCOGT). We observed the source in g, r and i-band SDSS filters using the 1m LCOGT telescope at the McDonald Observatory, Texas on Oct 25, 26, 28 and Nov 9. The exposure times were 200s in each filter, except on Oct 28 when ten consecutive 60s exposures in g-band were acquired (the time resolution was 68s covering 11 minutes). Flux calibration was achieved using six SDSS stars in the field. The following magnitudes were measured: Oct 25.376 UT: g = 18.00 +- 0.03, r = 18.19 +- 0.12, i = 18.37 +- 0.07; Oct 26.328 UT: g = 18.07 +- 0.02, r = 18.19 +- 0.12, i = 18.34 +- 0.06; Oct 28.386 UT: g = 18.43 +- 0.09; Nov 9.045 UT: g > 19.79.

We do not detect significant variability in iPTF14gnj within the 10 consecutive images on Oct 28; the standard deviation of the 10 magnitudes is 0.06 mag, which is slightly smaller than the error on each magnitude. On these timescales, accretion activity could (but does not always) produce variations in amplitude of up to a few tenths of a mag in CVs.

The nearest source in the SDSS catalogue is 3.76 arcsec away, SDSS J005227.31+402703.2 at a magnitude of g = 24.11 +- 0.46. Even if this is the quiescent counterpart to iPTF14gnj, the amplitude of the outburst must have been ~ 8 mag or more in g-band, ruling out the possibility that this is a flare star or an AGN. The light curve can be described by an exponential decay of the flux since its peak, fading by 0.142 mag/day, whereas at the start of the outburst the rapid rise was > 4 mag in one day. The decay rate is consistent with a dwarf nova or a fast classical nova. If the transient is in M31 (at ~750 kpc), its absolute magnitude at peak was M_v ~ -8.4, which would imply a classical nova. If it were a type Ia supernova, it would have to lie at a distance of ~100 Mpc; furthermore, as no host galaxy is seen in DSS images, a supernova origin appears very unlikely. The g - i colour on Oct 25 and 26 is blue (also when de-reddened for Galactic extinction), and is similar to CVs and low-mass X-ray binaries in outburst. iPTF14gnj lies ~2 degrees from the centre of M31. If it were a Galactic/halo CV or X-ray binary (at <10 kpc), the peak absolute magnitude would be M_v ~>1, which is consistent with CVs and some faint X-ray transients. However, the lack of an X-ray counterpart (in MAXI data) makes the X-ray transient scenario unlikely. The lack of emission lines reported in ATel #6681 implies that the disc was still in a bright, optically thick state at the time the spectrum was taken.

In conclusion, the amplitude, decay rate and optical colours are consistent with either a fast classical nova at the distance of M31, or a Galactic CV (dwarf nova). A Galactic faint X-ray transient cannot be ruled out. This work makes use of observations from the LCOGT network.