The Type-Ic Supernova Associated with GRB 021211.

GRB 021211.

On 2002 Dec, at 11:18:34 UT, the HETE-2 satellite was triggered by a new gamma-ray burst, GRB 021211 (Crew et al., GCN 1734). The observing conditions were luckily suitable, so that several automatic telescopes succeded in imaging the early afterglow of this event: RAPTOR, KAIT, and Super-LOTIS detected the afterglow 65 s (Wozniak et al., GCN 1757), 105 s (Li et al., GCN 1737), and 143 s (Park et al., GCN 1736) respectively after the GRB trigger. The first observers reporting the discovery of the optical counterpart were Derek Fox and Paul Price (GCN 1731), just 53 minutes after the trigger. Observations went on for the subsequent days, allowing the determination of the redshift (z=1.006: Vreeswijk et al., GCN 1785) and the detection of the compact host galaxy (Fruchter et al., GCN 1781).
This burst was interesting in at least three respects:

The optical flash. GRB 021211 was the second case (after GRB 990123) for which an optical fIash could be detected. Thanks to the automatic telescopes, the early light curve could be closely sampled (see e.g. Li et al. 2003, ApJ, 586, L9; astro-ph/0302136).
The faintness. GRB 021211 showed one of the faintest optical afterglows ever detected. Compared with other events at similar redshift, it was about 3 magnitudes dimmer at comparable epochs. The colors demonstrated little or no extinction, thereby suggesting that many of the so-called dark bursts could be simply dim (see e.g. Fox et al. 2003, ApJ, 686, L5; astro-ph/0301377).
The associated supernova. Being so dim, if indeed the GRB was associated with a supernova, the supernova bump should have very prominent, allowing a clear detection and a detailed study of this component. The first hints of the supernova were reported by Fruchter et al. (GCN 1781).

See here the table with the burst numbers.

Supernova Search.

We started the follow-up of the late afterglow in order to search for a supernova rebrightening. In order to sample the light curve, photometry was performed with the Italian Telescopio Nazionale Galileo (TNG) and with the ESO Very Large Telescope (VLT-UT2). To find conclusive evidence of a possible supernova, moreover, a VLT spectrum was acquired on 2003 Jan 8, with the FORS2 instrument. 4 hours of integration were needed to get the spectrum for such a faint object, but the task was eventually successful.

Photometric Data.

Fig. 1 shows the light curve of the afterglow. Data (red points) are from Li et al. (2003), Fox et al. (2003), Sagar et al. (2003, A&A, 408, L21; astro-ph/0304481), and Fruchter et al. (2002). Blue diamonds represent our late-time values, measured with the VLT. The measurements of January clearly show a rebrightening. At that time, the afterglow had already vanished (contributing at most 5% of the total flux). The bump was therefore an independent component of the emission, and not directly linked to the afterglow emission itself.

Spectroscopic Data.

To asses the nature of the bump, spectra are the most effective tool. Fig. 2 shows the VLT-FORS2 spectrum of the bump (red line). A significant deep is seen at ~3770 Å (rest frame), its width being ~130 Å. Such broadness is indicative of large outflow velocities, of the order of 15000 km/s. The only astrophysical objects with this characteristic are supernovae. In Fig. 2 it is also shown the spectrum of the Type-Ic SN 1994I (blue line). The resemblance is intriguing. The broad absorption can be interpreted as due to calcium (K & H edges); this is a common feature in SN spectra.

Conclusions.

GRB 021211 is the first GRB for which a firm association with a supernova was estabilished. This indicates that such association is not limited to the peculiar case of GRB 980425 (found coincident with SN 1998bw). However, the spectrum of the GRB 021211 bump was similar to the one of a `normal' type-Ic supernova (SN 1994I), rather than to that of a hypernova, like SN 1998bw. This association was till now unexplored.
By fitting the light curve of the afterglow with a 1994I-like component yields the solid line shown in Fig. 1. In this plot a null time delay between the GRB and the SN is assumed. Our data limit this delay to be no more than ~2 comoving days, assuming that the SN lightcurve was similar to the one of SN 1994I.

Fig. 1: GRB 021211 light curve. The dotted line shows the afterglow contribution, and the dot-dashed line shows the host galaxy contribution. The green line is the lightcurve of SN 1994I at z=1 (from Lee et al. 1995, JKAS, 28, 31). The solid line is the sum of all contributions.

Fig. 2: GRB 021211 spectrum on 2003 Jan 8.27 UT (red line). Note the broad absorption feature due to Calcium II (H+K). The blue line shows the spectrum of SN 1994I (10 days after its maximum light; from Filippenko et al. 1995, ApJ, 450, L11).

References.

Our work is described in detail in our paper:
Massimo Della Valle, Daniele Malesani, Stefano Benetti, et al. 2003 (A&A, 406, L33; astro-ph/0306298):
Evidence for Supernova Signatures in the Spectrum of the Late-time Bump of the Optical Afterglow of GRB 021211.

Preliminary analysis of our data can be found in GCN 1809 (Della Valle et al.) and in GCN 1821 (Testa et al.). Please note that, due to a revised calibration of the field, the photometric values reported in the GCNs are not very accurate. Refer to our paper for the correct numbers.

External Links.

Italian Press Releases.

Highlight at Brera Astronomical Observatory.
Comunicato Stampa INAF and figures (in Italian).
News from Galileo (in Italian)
News in Urania - Notiziario di astronomia (in Italian).
Comunicato Ansa (in Italian).
News in La Stampa (in Italian).
News in CremonaOnLine (in Italian).
Paper on Il Piccolo, the Trieste local journal (in Italian); really overemphasized!

This page is maintained by Daniele Malesani and was last updated on 2003 Oct 9.