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PS1 MDS Discovery of an Extragalactic Nova near NGC 4258

ATel #2680; R. Chornock (Harvard/CfA), M. Huber (JHU), A. Rest, R. J. Foley, G. Narayan, E. Berger (Harvard/CfA), J. Tonry (IfA), C. Stubbs (Harvard/CfA), A. Riess (JHU), W. M. Wood-Vasey (Pitt), P. Challis, R. P. Kirshner, A. M. Soderberg (Harvard/CfA), P. A. Price, E. Magnier, K. Chambers, N. Kaiser, J. Morgan, W. Burgett, J. Heasley, W. Sweeney, C. Waters, and H. Flewelling (IfA)
on 16 Jun 2010; 12:53 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Ryan Chornock (chornock@astro.berkeley.edu)

Subjects: Optical, Nova, Transient

We report the discovery of an extragalactic classical nova in the field near NGC 4258 by the Pan-STARRS1 Medium Deep Survey. The transient (ID: PS1-1000254) was discovered at g=20.9 mag on 2010 June 3.30 UT at the position:

(J2000) 12:20:55.217 +46:49:55.52

The object did not have a clear association with a host galaxy and rapidly faded to g=22.4 mag by 2010 June 6.35. A pre-outburst observation on June 1.34 had a 5-sigma upper limit of i=23.1 mag.

Spectroscopic observations (range 4900-9150 Angstroms) obtained on 2010 June 11.30 with GMOS on the Gemini-North 8-m telescope (PI: Berger) revealed emission lines with boxy profiles from H-alpha and O I 7774 and 8446, consistent with a classical nova after maximum light. The mean radial velocity of H-alpha was about 650 km/sec (the asymmetry of the line profile may slightly bias this value), establishing the object as extragalactic.

The host galaxy of the nova is ambiguous. It is located 35 arcmin in projection from the center of NGC 4258 (73 kiloparsecs if they are at the same distance of 7.2 Mpc; Herrnstein et al. 1999, Nature, 400, 539) and the radial velocity is offset by about 200 km/sec from the NED value for the nucleus of NGC 4258. Some faint low-surface-brightness emission is present around the position of the nova in our Pan-STARRS1 images, which has been cataloged as the galaxy SDSS J122054.98+464950.4, with an integrated model magnitude of r=18.2 mag. If both the nova and the possible host galaxy are at the same distance as NGC 4258, then the absolute magnitudes are M_g=-8.4 mag for the nova (a typical value for a classical nova) and M_r=-11.1 mag for the galaxy. Therefore, we tentatively conclude that this object is a classical nova in an extremely-low-luminosity dwarf galaxy near NGC 4258, although the true host galaxy and distance are uncertain so other associations are possible.

We thank the PS1 and Gemini telescope staffs for their assistance with these observations. This discovery was enabled using the PS1 System operated by the PS1 Science Consortium (PS1SC) and its member institutions, and made possible through the PS1 Builders: http://www.ps1sc.org/PS1_System_ATel.shtml.