Discovery of the Low‐Redshift Optical Afterglow of GRB 011121 and Its Progenitor Supernova SN 2001ke

Abstract

We present the discovery and follow-up observations of the afterglow of the Gamma-Ray Burst (GRB) 011121 and its associated supernova SN 2001ke. Images were obtained with the OGLE 1.3m telescope in BV RI passbands, starting 10.3 hours after the burst. The temporal analysis of our early data indicates a steep decay, independent of wavelength with F ν ∝ t −1.72±0.05 . There is no evidence for a break in the light curve earlier than 2.5 days after the burst. The spectral energy distribution determined from the early broad-band photometry is a power-law with F ν ∝ ν −0.66±0.13 after correcting for a large reddening. Spectra, obtained with the Magellan 6.5m Baade telescope, reveal narrow emission lines from the host galaxy which provide a redshift of z = 0.362 ± 0.001 to the GRB. We also present late R and J-band observations of the afterglow ∼ 7 − 17 days after the burst. The late-time photometry shows a large deviation from the initial decline and our data combined with Hubble Space Telescope photometry provide strong evidence for a supernova peaking about 12 rest-frame days after the GRB. The first spectrum ever obtained of a GRB supernova at cosmological distance revealed a blue continuum. SN 2001ke was more blue near maximum than SN 1998bw and faded more quickly which demonstrates that a range of properties are possible in supernovae which generate GRBs. The blue color is consistent with a supernova interacting with circumstellar gas and this progenitor wind is also evident in the optical afterglow. This is the best evidence to date that classical, long gamma-ray bursts are generated by core-collapse supernovae.Subject headings: gamma-rays: bursts -supernovae: general -supernovae: individual (SN 2001ke) 1 Based on data from the OGLE 1.3m and the Magellan 6.5m Baade telescopes and the Hubble Space Telescope.

GRB 011121: A Collimated Outflow into Wind‐blown Surroundings

“…Two days later, on 2001 November 24, 7:00 UT, we find ¼ 0:62 AE 0:05 based on our BVRJHK Note.-Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. data, which is consistent with the earlier result and is fully in agreement with the results obtained by Garnavich et al (2003) and Price et al (2002). These afterglow parameters are fully consistent with those expected from the simple versions of the fireball, as well as observations of previous GRB afterglows (e.g., van Paradijs, Kouveliotou, & Wijers 2000).…”

“…When we fit our R C -band data (Table 1) with a single power-law plus SN component, we get ¼ 1:98 AE 0:11, k ¼ 0:81 AE 0:11 with 2 =dof ¼ 1:49. Not only is the fit worse in comparison to the fit based on the Beuermann equation, but the value we now get for is notably larger (3 deviation) than the one deduced for the earlytime slope of the afterglow light curve (Price et al 2002;Garnavich et al 2003). This again points to a change in the fading behavior of the afterglow between about 0.4 and 2 days after the burst.…”

“…The exact time of the break is sensitive to the details of the fit (compare the results we obtain by fitting data prior to t ¼ 10 days vs. fitting all the data), but the existence of the break at t $ 1 day is clearly established. Previous studies of the afterglow light curve of GRB 011121 did not find this break (e.g., Price et al 2002; [See the electronic edition of the Journal for a color version of this figure.] Garnavich et al 2003), which can be attributed to sparse sampling of the afterglow at this particular time. It remains difficult to explain why the radio data of the afterglow do not agree with a jet model (Price et al 2002).…”

“…First, using the deduced spectral index of the early afterglow, we can estimate that at the time of the single AAO observations (Table 1), i.e., before the deduced break time, the R C -band magnitude of the optical transient was 20:12 AE 0:25. Second, we can include in our fit the R-band data from Garnavich et al (2003) for t < 0:6 days. In doing so, we get 1 ¼ 1:62 AE 0:39, 2 ¼ 2:44 AE 0:34, t b ¼ 1:20 AE 0:75 days, and 2 =dof ¼ 1:11.…”

Early re-brightenings in GRB afterglows as signatures of low-to-high density boundary

“…Many late-time bumps (e.g. GRBs 970228, 980326, and 011121) have been attributed to SN components because of the reddening of the observed spectra (Bloom, 2003;Bloom et al, 1999Bloom et al, , 2002Galama et al, 2000;Garnavich et al, 2003b). These late-time bumps are also argued to be due to dust echoes (Esin & Blandford, 2000).…”