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UV/Optical Light Curves and Optical Spectrum of Transient ROTSE3 J115649.1+542726

ATEL # 1578; S. Gezari (JHU), D. Grupe (PSU), F. Yuan, C. Akerlof (U. Mich), B. Cenko, A. Rau, R. Quimby (Caltech), J.P. Halpern (Columbia U.), D. Terndrup (OSU), J.C. Wheeler (U. Texas)
on 13 Jun 2008; 22:51 UT
Password Certification: Jules Halpern (jules@astro.columbia.edu)

Subjects: Optical, Ultra-Violet, X-ray, AGN, Nova, Supernova, Transients

UV/Optical Light Curve and Optical Spectrum of Transient ROTSE3 J115649.1+542726

We report on the status of continuing observations of the optical transient ROTSE3 J115649.1+542726 discovered by Yuan et al. 2008 (ATel #1515). Monitoring by ROTSE and the Palomar 60" indicate a peak in the optical emission around 2008 May 13 UT, ~18 d after the first ROTSE detection on 2008 April 25 UT.

Since 2008 May 23 UT, the following exponential decays are observed:

V 0.024 mag/d (MDM 1.3m)
g 0.029 mag/d (Palomar 60")
r 0.021 mag/d (Palomar 60")
i 0.014 mag/d (Palomar 60")

We measure V=18.26 on June 12 UT.

Observations have been taken with the Swift satellite on six epochs from 2008 May 14 to June 7 UT. The first epoch Swift UVOT observation in all six filters measured a spectral energy distribution that peaked in the UV, as reported by Gezari et al. (ATel #1524). Since then, the transient has faded monotonically with the following slopes:

UVW2 0.074 mag/d
UVM2 0.071 mag/d
UVW1 0.061 mag/d
U 0.047 mag/d
B 0.027 mag/d
V 0.016 mag/d

The steeper decay of the UV bands than the optical bands is consistent with a cooling blackbody.

A total of 24,650 s exposure has been accumulated with the Swift XRT, which places a 3 sigma upper limit on the flux of F(0.5-10 keV) < 3.7e-14 erg/cm2/s for a power-law of photon index 2.

We obtained a follow-up spectrum on 2008 June 1 UT with the 9.2m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph). This spectrum shows significant evolution since the first set of spectra taken on 2008 May 1-8 UT (ATel #1515). The broad emission feature detected in the May spectra at 5650 A has evolved significantly. The June 1 spectrum shows several broad bumps that are roughly consistent with a Type II SN at z=0.08-0.09. We now favor this interpretation over the AGN or SMBH/tidal disruption scenario at z=1.02 because of the rapidly cooling temperature of the emission, and the emergence of SN-like broad bumps in the spectrum. A redshift of 0.08-0.09 would indicate a peak optical luminosity of the SN of M_V~-20. There is no host galaxy detected in the SDSS images down to r=23, suggesting possibly a dwarf galaxy host with M_r >-15. Similar, tentative conclusions were reached by Miller et al. (ATel #1576).


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