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Optical-NIR SED and spectroscopy of ROTSE3 J115649.1+542726

ATel #1576; A. A. Miller, J. S. Bloom, R. Chornock, N. R. Butler, W. Li, D. A. Perley, M. Modjaz, J. M. Silverman, J. H. Shiode, A. V. Filippenko, R. J. Foley, L. Strubbe, E. Quataert, R. Mottsmith, D. A. Starr (UC Berkeley)
on 13 Jun 2008; 18:36 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Weidong Li (weidong@astron.berkeley.edu)

Subjects: Infra-Red, Optical, Request for Observations, AGN, Supernovae, Transient

Referred to by ATel #: 1578, 1593, 1644

ROTSE3 J115649.1+542726 (Yuan et al. 2008; ATel #1515) is a blue optical/UV transient claimed as an AGN, and later proposed by Gezari & Halpern 2008 (ATel #1524) as a tidal disruption (TDF) event at z~1. Here we report on extensive follow-up coverage and present evidence that does not support the z~1 TDF and AGN hypotheses. Instead, we suggest a plausible hypothesis is that ROTSE3 J115649.1+542726 is a peculiar, bright SN at z~0.07; other possibilities, such as a transient at z=0 or a TDF at z~0.07 are not excluded. Indeed, several spectroscopic lines remain unidentified, and there remains considerable uncertainty about the nature of this intriguing event. We observed ROTSE3 J115649.1+542726 starting at 2008 Jun 02 06:54 UT with PFCam on the 3-m Shane telescope at Lick Observatory. The transient was detected in five filters, with observed magnitudes: U = 17.72 ± 0.07 B = 18.20 ± 0.07 V = 17.90 ± 0.05 R = 17.82 ± 0.05 I = 17.68 ± 0.06 We observed the transient in the NIR starting at 2008 Jun 02 03:37 UT with the 1.3-m Peters Automated Infrared Imaging Telescope (PAIRITEL). We detected the transient in J and H band, with a non-detection in Ks band: J = 17.78 ± 0.10 H = 17.52 ± 0.10 Ks = [18.2 (3-sigma upper limit) Our nearly coincident measurements from PFCam and PAIRITEL cover 350 nm - 2.2 micron. The spectral energy distribution (see link below) no longer is well-fit by a ~25,000 K blackbody (solid black line on the SED) in the rest frame at z=1.02 as suggested in ATel #1524. Since the observations reported in ATel #1524, taken on 2008 May 14 UT, the transient has become redder with the flux at ~700 nm remaining nearly constant since the reported ATel #1515 observations on 2008 May 08 UT. We also note a slow evolution in the NIR, with J and H band measurements fading by <~10% between 2008 May 17 and 2008 Jun 02 UT. We obtained a spectrum of the transient with the Kast Dual Channel Spectrograph on the 3-m Shane telescope at Lick Observatory on 2008 May 16 UT covering a spectral range of 330-900 nm. The spectrum was blue and featureless. On 2008 Jun 07 UT we obtained a spectrum of the transient with LRIS on the 10-m Keck I Telescope covering a spectral range of 320-920 nm. We do not detect an emission feature at 5650 A as seen in ATel#1515. A few weak, broad absorption features (some with an apparent P-Cygni profile and characteristic velocity greater than 5000 km/sec) have developed. Three of these lines are consistent with arising from redshifted Hydrogen Balmer Halpha, Hbeta, and Hgamma at z=~0.07--0.09. The lack of emission at 5650 A leads us to disfavor z=1.02 for this transient which would seemingly disfavor the possibility of an AGN (ATel #1515) or a TDF at z~1 (ATel #1524). Preliminary photometry from eight V, R, and I images taken with the Katzman Automatic Imaging Telescope (KAIT) from 2008 May 30 to Jun 12 UT indicates a steady decline at a rate of 0.017 mag per day, steeper than the decay rate of 56^Co but still consistent with some type II SN. However, other lines in the spectrum do not agree with a SN II, and several expected lines from a SN II are not present. Though a TDF is expected to fade as a power-law (instead of exponentially as seen) and the lines seen are not specifically predicted by any theory published to-date, we feel that a TDF at z~0.07 cannot be ruled out at this time. While noting that the peak absolute brightness (M_R ~ -20) at z~0.07 would be consistent with nominal expectations of a TDF _or_ a bright Type II supernova, the precise classification of ROTSE3 J115649.1+542726 remains uncertain. (Offering some pause to even the extragalactic hypothesis is that any host galaxy at the location of the event would necessarily be fainter than M_r ~ -14.5 mag.) Since it appears the answer may be interesting regardless, we strongly encourage future observations.

Spectral Energy Distribution