TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1554 SUBJECT: XRF 020903: Supernova DATE: 02/09/29 05:12:28 GMT FROM: Alicia Soderberg at Caltech <email@example.com> A.M. Soderberg, P.A. Price, D.W. Fox, S.R. Kulkarni, S.G. Djorgovski, E. Berger and F. Harrison, S. Yost (Caltech); M. Hamuy and S. Shectman (OCIW); and N. Mirabal and J. Halpern (Columbia) report: We imaged the entire error-box of XRF 020903 with the Palomar 200-inch telescope + Large Format Camera (LFC) on Sep 4.3 UT (epoch 1) and Sep 10.3 UT (epoch 2) in Steidel R-band. Visual comparison of these images with the Digitised Sky Survey and with each other did not reveal any obvious transient. Recently, we undertook a more detailed analysis, namely PSF-matched image subtraction of these two epochs, and found an optical transient (OT) located at coordinates: RA: 22:48:42.34 Dec: -20:46:09.3 J2000 with a bright elliptical galaxy 4 arcsec SE (hereafter G2). The source is also present in images from the MSO 50-inch telescope (GCN #1533) when the second epoch LFC image is subtracted. Upon discovery of the OT, we immediately pursued further observations, thereby obtaining a third epoch of imaging with the MDM 1.3-meter telescope on Sep 28.3 UT in the R-band. Initial photometry demonstrates that the object has re-brightened by ~ 0.3-0.4 mag between epochs 2 and 3. We propose that this optical rebrightening is from an associated supernova, peaking between ~7-24 days after the initial XRF trigger. Spectroscopic observations by M. Hamuy and S. Shectman for the source were also obtained with the Magellan 6.5m telescopes (Baade+LDSS2, Clay+B&C) on Sep 28.1 UT. We detect narrow emission lines from an underlying host galaxy (hereafter G1) are (Halpha, Hbeta, Hgamma, [O III], [Ne III], [O II]) at a mean redshift of z = 0.25 +/- 0.01. Subtraction of the emission lines reveals a continuum that is consistent with a SN-like transient near maximum. In particular, the spectrum reveals deficit of emission at rest wavelengths < 4000A, as would be expected from a SN. Spectra of G2 (R ~ 17 mag) reveal it to be an elliptical galaxy at z = 0.23 and therefore is not associated with the optical transient. If the OT is indeed associated with XRF020903, it is the first known optical afterglow of an XRF, and may be the most nearby cosmological high energy transient known yet. Parenthetically we note that microlensing from G2 may result in sharp variations of the light curve (see Garnavich, Loeb and Stanek, 2000). We encourage further monitoring of this OT/SN. A web page summarizing the discovery and subsequent observations can be found at: http://vsnet.astro.caltech.edu/~ams/XrF.html This message may be cited.