Observations of GRB 020813

Summary.

Burst Facts.

On 2002, April 13th, at 2:44:19 UT, the HETE-2 spacecraft detected a bright flash of gamma-ray radiation, GRB 020813. For this event, a rapid (4 min) and precise (4 arcmin) localization was distributed, allowing the earliest detection of an optical afterglow (Bloom, Fox & Price, GCN 1470), just 1.9 hours after the trigger. The afterglow was very bright (R ~ 18) and slowly fading (Gladders & Hall, GCN 1472), but the decay soon steepened, indicating a break in the light curve some hour after trigger (e.g. Bloom, Fox & Hunt, GCN 1476). The redshift, z = 1.254, was measured using a spectrum taken with KECK 1-LRIS (Price et al., GCN 1465). Chandra observations, performed between 21.02 and 42.67 hours after trigger, detected a bright X-ray afterglow with an average flux F ~ 10^-12 erg cm^-2 s^-1 (Vanderspeck et al., GCN 1504). Radio detection (@ 8.46 GHz) was achieved by Frail & Berger (GCN 1490), 30 hours after the GRB.

Moreover, during the night of Aug 13th, it was possible to perform the first spectropolarimetric measurement, yielding a mean polarization level P = (2.9+-0.1)%, with only slight wavelength dependance (Barth et al., GCN 1477). The night after, another polarimetric observation (V-band) was performed with the ESO VLT-UT3 (Melipal), resulting in P = (1.17 +/- 0.16)% ((0.80 +/- 0.16)% after correcting for Galactic-induced polarization; Covino et al., GCN 1498). This is the first unambiguous highly-significant detection of polarization variability in a GRB afterglow.

GRB 020813 numbers.

Observations.

In the night between Aug 13 and Aug 14 UT, we observed the optical afterglow of GRB 020813 with the ESO 8.2m VLT UT3 (Melipal), in polarimetric mode. The subsequent nights, we activated the Italian 3.6m Telescopio Nazionale Galileo (TNG), beginning in the night of Aug 14th, to perform broad band photometry (ranging from U to K) and to sample the light curve. We also used the Campo Imperatore AZT24 1.1m telescope (near L'Aquila, Italy) to get additional near-infrared measurements. We also reanalyzed the publicly available data from Mike Gladders and Pat Hall (GCN 1519), taken with the Baade 6.5m (Magellan 1) Telescope during the night of Aug 13th and 14th. The observation log is reported here.

The picture shows the field (1'x1') of GRB 020813 afterglow (indicated by the arrow), as imaged by TNG on Aug 15.0 UT. This is a composite image from separate B, V and R exposures.

Data Analysis.

Collecting our multiband photometry (left panel in the figure), we could construct the light curve of the event for several bands, which shows a sharp achromatic break about 14 h after the GRB. Parametrizing the light curve as a broken power law (F(t) = kt^-d), we got asymptotical slopes d₁ = 0.78 and d₂ = 1.44. The break is very sharp, as it can be seen by analyzing early time data (t < 5 h; right panel), which do not show signs of curvature.

To further check if the break is indeed achromatic, we performed spectral analysis at different times, both before and after the break. We found that during all our observations the spectrum was well described by a powerlaw with spectral index a = 1.04 (once correcting for Galactic extinction). The figure on the left shows all our photometric spectra. Collecting measurements in other bands (GCNs 1487, 1490, 1497, 1504), we could also construct the spectral energy distribution of the afterglow from radio to X-rays (figure on the right). These data suggest that the optical light curve may suffer some extinction within the host galaxy.

Photometric data.

Here are reported all numbers from our measurements. Original data are available upon request (contact: Stefano Covino).

afterglow magnitudes;
calibration stars.

Polarization analysis.

With the VLT UT-3 (Melipal), we imaged the optical afterglow of GRB 020813, in the V-band. We found a positive level of polarization, with P = (0.80 +/- 0.16)% (after correcting for Galactic-induced polarization). This is one of the smallest values ever observed for a GRB afterglow, yet determined with good accuracy. Coupled with the measurement of Barth et al. (GCN 1477), this is also the first unambiguos higly-significant detection of polarization variability in a GRB afterglow. The figure shows the position of the afterglow (red point) and of some field stars (blue points) in the Stokes Q-U plane. The transient clears stands outside the region occupied by (slightly polarized) stars.

Links.

Literature about our data:

Covino S., Malesani D., Ghisellini G., et al.: Polarimetry of GRB 020813: evidence for variability (GCN 1498).
Malesani D., Covino S., Fugazza D., et al: GRB 020813: V-band decay slope (GCN 1500).
Covino S., Malesani D., Ghisellini G., et al.: GRB 020813: addendum to GCN 1498 (GCN 1502).
Covino S., Malesani D., Tavecchio F., et al.: Optical and NIR observations of the afterglow of GRB 020813; A&A, 404, L5 (2003), astro-ph/0304171.

External links:

Internal communication at the Brera Astronomical Observatory.
Magellan publicly available data.
KISO multiband optical observations of GRB 020813.
GCN circulars regarding GRB 020813.
Published papers about GRB 020813 (NASA ADS).
Preprints about GRB 020813 (astro-ph).
Chandra DDT observations of GRB 020813.
HETE-2 page about GRB 020813.
KAIT press release about GRB 020813.

Contacts:

Stefano Covino (covino@merate.mi.astro.it).
Daniele Malesani (malesani@dark-cosmology.dk).

Page maintained by Daniele Malesani (last update: 2003 Jun 15).