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ROTSE3 J125642.7+273041 is probably a type Ia supernova of intergalactic origin

ATEL # 1617; Avishay Gal-Yam (Weizmann Institute) and Josh Simon (Caltech)
on 18 Jul 2008; 9:21 UT
Distributed as an Instant Email Notice (Supernova)
Password Certification: Avishay Gal-Yam (

Subjects: Optical, Nova, Supernova

We observed the recent supernova candidate ROTSE3 J125642.7+273041 (ATEL #1615) with the HIRES spectrograph mounted on the 10m Keck-I telescope on 2008 July 18.26 UT. A preliminary reduction reveals a blue spectrum similar to that of young type Ia supernovae. The minimum of the Si II "6100 Angstrom trough" is around 6250A, consistent with the redshift of the Coma cluster (Abell 1656, z=0.023). The lack of an apparent host galaxy in SDSS imaging for such a low redshift event, along with its location near the Coma cluster of galaxies (<50' projected; ATEL #1615), suggests it is a member of the population of intergalactic type Ia supernovae (Gal-Yam et al. 2003, AJ, 125, 1087). At the distance of the Coma cluster, the reported magnitude of this event (~16 mag) is consistent with that of a normal type Ia supernova near maximum light.

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