Volume 572, December 2014
|Number of page(s)||12|
|Published online||27 November 2014|
In the last years, the detection of soft tails in short GRBs (e.g., GRB 050724, Barthelmy et al. 2005b) and of peculiar long events like GRB 060614 (Gehrels et al. 2006), have shown the limitations of the standard short/long classification based on duration only (e.g., Zhang et al. 2009). Thus, to test the nature of GRB 050219A and whether it is a real LGRB, we investigate whether this burst satisfies the Ep,i − Eiso correlation (Amati 2006). We based our analysis on the spectral fits, light curves and fluences reported in Tagliaferri et al. (2005) and on our personal analysis of the Swift/BAT spectrum. They show that the ~24 s pulse observed by Swift/BAT was followed by softer emission observed with the XRT, and thus the central engine was active for at least ~120 s. However, this later softer phase accounts only for <20% of the total prompt emission and therefore the following conclusions do not depend on the softer emission. Luckily, the spectral peak falls within the BAT band and it is Ep,i = 90 ± 9 keV. We find that assuming a redshift z = 0.211 this burst is consistent with the Ep,i − Eiso correlation within 2σ. Any value between 0.2 ≲ z ≲ 2 is good within 2σ. The GRB would lie in the region occupied by short GRBs only for z ≲ 0.1 (see Fig. A.1).
We also checked the lag-luminosity relation, following the method described in Ukwatta et al. (2010). We assumed the same redshift as above, and we measure a peak luminosity of (6.7 ± 0.8) × 1049ergs-1. We find a lag of 3.4 ± 1.2 s between BAT channels 3 (50–100 keV) and 1 (15–25 keV) and of 1.4 ± 0.6 s between BAT channels 4 (100–200 keV) and 2 (25–50 keV). Figure A.2 shows that GRB 050219A has one of the largest lags measured for a LGRB and it is in very good agreement with the lag-luminosity relation for LGRBs presented in Ukwatta et al. (2010).
Therefore, even though a redshift of about 0.5 − 1 would place it in the regions of the Ep,i − Eiso and lag – luminosity planes most populated by typical LGRBs (see Figs. A.1 and A.2), the energetics, luminosity, spectrum and timing properties of GRB 050219A are consistent with the hypothesis that it is a LGRB at the redshift of the putative host galaxy.
LGRBs (black) and short GRBs (red) in the Ep,i − Eiso plane. The blue dot indicates the position of GRB 050219A assuming z = 0.211. The dashed vertical lines indicate the position of the GRB at different redshifts. The green line shows how this position changes together with the redshift. The solid and dotted lines show the 2σ and 3σ regions respectively.
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Isotropic peak luminosity as a function of spectral lag between BAT channels 3 (50–100 keV) and 1 (15–25 keV). The image is adapted from Ukwatta et al. (2010) and GRBs are marked in red. The dotted lines indicate the estimated 1σ confidence level. We marked in blue the position of GRB 050219A at different redshifts, including the redshift of the bright galaxy at z = 0.211 (bigger blue dot).
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Within the standard afterglow theory (Sari et al. 1998) the X-ray afterglow should have an X-ray spectral slope βX ≳ 0.5, and βX ≳ 1 in case of a break below the X-ray frequency which is usually the case (Greiner et al. 2011). Jakobsson et al. (2004) quantify the optical dimness by testing whether the optical to X-ray spectral slope βOX is lower than 0.5. We downloaded and analysed the XRT spectrum (both pc and wt modes) and we found 0.6 <βX< 1.2, consistent with the values in the Swift/XRT on-line repository, but not good enough to distinguish between the two scenarios. However, without a break between optical and X-rays (i.e., βOX ≳ 1), the afterglow would have had R ≳ 18 at the time of the MOA observations, surely dominating over the galaxy light and well-detected, but we do not see this in
our re-analysis of the data. In the case of a break between optical and X-rays, lying close to the X-ray band, the afterglow should have been R ~ 20 or brighter. This is similar to the MOA upper limit, but because of the bad seeing it could be easily confused with the host with R ~ 20. Thus, we conclude that the burst had a βOX ≲ 0.5 and therefore it can be considered a marginal member of the dark GRB population according to the Jakobsson et al. (2004) criterion.
Observation log for the GRB 050219A field.
Finding chart of the field of GRB 050219A (GROND r′-band). The X-ray afterglow position and the secondary photometric standards used (Table B.2) are indicated.
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© ESO, 2014
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