XRF 020903

TITLE: GCN GRB OBSERVATION REPORT

NUMBER: 1554

SUBJECT: XRF 020903: Supernova

DATE: 02/09/29 05:12:28 GMT

FROM: Alicia Soderberg at Caltech

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, J. Halpern, E. Armstrong, C. Espaillat (Columbia) and J. Kemp (Joint Astronomy Center and Columb7) 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.

Coordinates:
OT 22:48:42.341 -20:46:09.28
Star1 22:48:36.057 -20:44:35.23 R > 16 mag
Star2 22:48:53.474 -20:43:23.68 R > 16 mag

Offsets:
Star1 -> OT 88.135" E 94.057" S
Star2 -> OT 156.143" W 165.622" S

Use position angle 162.62 degrees to get both OT and galaxy.

---

The Optical Transient: The image stamps below are taken from Palomar 200" LFC data and are 36 arcsec on each side:

Epoch1 R=19.23 mag @ T=0.9 days

Epoch2 R=20.60 mag @ T=6.9 days

Subtract Decay Index=-1.

---

The Spectroscopy: The OT Spectrum (including underlying host):
(observers=M. Hamuy & S. Shectman)

---

The Interpretation and Discussion:


FIGURE 1:
Histogram of peak radio luminosity for GRB afterglows and Type Ib/c SNe. There appears to be a continuum of luminosities bridged by SN1998bw. GRB 020903 appears to be at the low end of the distribution of afterglow luminosities, though definitely not unusually faint.


FIGURE 2:
Top: histogram of isotropic-equivalent gamma-ray energies for several GRBs, including GRB 980425 (SN1998bw) and GRB 020903. Clearly, GRB 020903 appears to be a transition object between typical cosmological GRBs and the near-by GRB980425/SN1998bw. Bottom: gamma-ray energies corrected for beaming (gaussian curve), as well as the energies in the radio band for Type Ib/c SNe. Both are indicators of relativistic, or mildly relativistic ejecta. Again, it is clear that GRB 020903 has a lower energy than typical cosmological GRBs, and may represent a transition between GRBs and hypernovae such as SN1998bw.


FIGURE 3:
X-ray (bottom) and radio 8.5 GHz (top) lightcurves of SN1998bw. The ratio of X-ray to radio flux about 40 days after GRB 980425 is 0.1 (in the units used in the plot). Using the same ratio, we predict an X-ray flux of about 1e-14 erg/cm^2/sec from GRB 020903.

---

page by A. M. Soderberg
Send comments to: ams@astro.caltech.edu