We find that although the absolute magnitude of the optical afterglow (OA) of GRB011121 in the late phase (t − T0 > 10 days) is approximately consistent with that of the SN 1998bw-type supernova SN 2001ke, previously found by Garnavich P. M. et al. (ApJ, 582 (2003) 924), a good match of their colors occurs only for t − T0 = 13–23 days in the observer frame of GRB011121. We are able to constrain the possible SN in the OA of GRB010222 to be of the peculiar SN 1998bw or SN 2002ap type, but not of SNe classified as Type Ic in the database of Poznanski D. et al. (PASP, 114 (2002) 833). Neither colors nor absolute magnitude of the OA of GRB970508 in the late phase are consistent with those of any of the above-mentioned (sub)types of SN. These results imply that the bumps in the late phase of the OAs can be caused by several mechanisms. Color indices thus appear to be a powerful tool in the analysis of these phenomena, even for archival data of OAs

Hudec, R.

Pizzichini, G.

Šimon, V.

English

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DOI 10.1393/ncc/i2005-10104-7IL NUOVO CIMENTO Vol. 28 C, N. 4-5 Luglio-Ottobre 2005Analysis of the late phase of three optical afterglows of GRBsusing color indices(∗)V. Sˇimon(1), R. Hudec(1) and G. Pizzichini(2)(∗∗)(1) Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 OndrˇejovCzech Republic(2) INAF/IASF, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy(ricevuto il 23 Maggio 2005; pubblicato online il 3 Ottobre 2005)Summary. — We find that although the absolute magnitude of the optical after-glow (OA) of GRB011121 in the late phase (t − T0 > 10 days) is approximatelyconsistent with that of the SN 1998bw-type supernova SN 2001ke, previously foundby Garnavich P. M. et al. (ApJ, 582 (2003) 924), a good match of their colors occursonly for t − T0 = 13–23 days in the observer frame of GRB011121. We are able toconstrain the possible SN in the OA of GRB010222 to be of the peculiar SN 1998bwor SN 2002ap type, but not of SNe classified as Type Ic in the database of Poz-nanski D. et al. (PASP, 114 (2002) 833). Neither colors nor absolute magnitudeof the OA of GRB970508 in the late phase are consistent with those of any of theabove-mentioned (sub)types of SN. These results imply that the bumps in the latephase of the OAs can be caused by several mechanisms. Color indices thus appearto be a powerful tool in the analysis of these phenomena, even for archival data ofOAs.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 97.60.Bw – Supernovae.PACS 95.30.Gv – Radiation mechanisms; polarization.PACS 95.85.Kr – Visible (390–750 nm).PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe method of the color indices of optical afterglows (OAs) of GRBs is a powerfuland straightforward approach to the study of such events. Although it has been appliedto other phenomena in astrophysics for many decades, it is only very rarely used in theanalysis of the OAs [12,13]. The main advantage of using color indices is that they enableto resolve small variations of the profile of the spectra of the OAs from the measurementsin the commonly used UBV RIJ filters. In addition, they enable us to resolve between(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.(∗∗) simon@asu.cas.cz, rhudec@asu.cas.cz, pizzichini@bo.iasf.cnr.it.c© Societa` Italiana di Fisica 549550 V. SˇIMON, R. HUDEC and G. PIZZICHINIFig. 1. – (a, b, c) Time evolution of the color indices of the OAs of GRB011121, GRB010222 andGRB970508. The horizontal solid line with the dashed error bars marks the mean color indicesof the ensemble of 25 OAs [13]. The synthetic colors of SN 1998bw, SN 2002ap and the group ofType-Ic SNe, with the passbands and t− T0 recalculated for z appropriate for a given OA, arealso plotted (GRB011121: z = 0.36 [4]; GRB010222: z = 1.477 [3]; GRB970508: z = 0.835 [6]).The colors of 25 OAs are found to be independent on the redshift. (d, e, f) Comparison of thelight curves of these OAs with those of SN 1998bw. The abscissa represents the rest-frame timeinterval but the labels display t−T0 in the observer frame for comparison. The passbands werechosen so that the rest-frame wavelengths of the individual objects in each panel were close toeach other. (g, h, i) Color-color diagrams. The points are connected by the line to show thetime evolution. The large cross denotes the centroid and standard deviations of the colors ofthe ensemble of 25 OAs for t− T0 < 10 days [13]. The numbers denote t− T0 in days, startingfrom the corresponding GRBs. Only (V −R)0 was available at t− T0 = 84 days in (i).LATE PHASE OF THREE OPTICAL AFTERGLOWS 551the individual mechanisms of radiation (e.g., synchrotron vs. supernova) [14]. Thismethod allows also investigation of faint OAs or OAs recovered from archival data.2. – Collection and analysis of the dataWe used the following sources of data for our analysis: GRB970508 [10, 11, 6];GRB010222 [2, 3]; GRB011121 [1, 4]. The magnitudes and color indices were correctedfor the Galactic reddening according to [9]. The contributions of the host galaxies ofGRB011121 and GRB010222 were eliminated because we used the data from HST forthe late phase. The magnitudes for the OA of GRB970508 were corrected for the lightcontribution of the host using its magnitudes from [11].Synthetic color indices of SN 1998bw, SN 2002ap and the group of Type-Ic SNe werecalculated using the code at http://wise-obs.tau.ac.il/∼dovip/typing [7]; both thewavelengths of the passbands and t−T0 were scaled to the redshift z of the correspondingGRB. Time interval between the start of explosion and maximum of brightness (z ≈ 0)was taken to be ∼ 16 days for SN 1998bw, and ∼ 10 days for SN 2002ap and the groupof Type-Ic SNe [5].3. – ResultsWe show that large color variations occur in some OAs of GRBs in the late phases(t − T0 > 10 days) and we stress the great importance of multicolor photometry of theOAs in these epochs.The late bump in the OA of GRB011121, attributed to SN 2001ke by [4], impliesthat this SN was slightly more luminous than SN 1998bw in all rest-frame time intervals(t−T0)rest−fr if it exploded close to T0 of the GRB (fig. 1d). We find that a good matchof their colors occurs only for t− T0 = 13–23 days in the observer frame of GRB011121.The OA of GRB011121 remains very blue in R − I in the late phase and its colors areinconsistent with either SN 1998bw or SN 2002ap, as can be seen from the V − R vs.R−I diagram in fig. 1g. For the sake of completeness, we note that a transient reddeningwithin at most 28 days < t− T0 < 76 days might be missed due to a gap in the data.We find the color indices of the OA of GRB010222 inside t − T0 < 10 days to beconsistent with the ensemble of 25 OAs [12]. The available R − I curve remains almostconstant even for t − T0 > 20 days. Only a small transient reddening can be seennear t − T0 ≈ 43 days. This feature is defined only by a single point, with the otherneighbouring two points being consistent with the ensemble of the OAs (fig. 1b). Thelate colors are thus close to those of the synchrotron radiation of the ensemble of theOAs [13]. A comparison of the absolute magnitude of this event with that of SN 1998bwreveals that the maximum brightness of SN 1998bw which occurred at (t−T0)rest−fr ≈ 16days was close to that of the OA of GRB010222 at that (t−T0)rest−fr. It is interesting tonote that the transient reddening of R− I occurred right at this moment (fig. 1b) and iscompatible with the color of SN 1998bw or SN 2002ap, but not with the group of Type-IcSNe. We offer two possible interpretations for this OA. The light curve could graduallyand smoothly evolve from the synchrotron radiation to a SN without a prominent bump;the color is consistent with a SN exploding close to T0 of the GRB. The other possibilityis that the synchrotron radiation dominated the OA all the time and the underlying SNwas underluminous with respect to SN 1998bw.We find large color variations of the OA of GRB970508 in the red spectral region inits late phase (fig. 1c, h, i). The closest match of the absolute magnitude of this OA to552 V. SˇIMON, R. HUDEC and G. PIZZICHINIthat of SN 1998bw occurred at t − T0 ≈ 33 days. We argue that although the colors inB − V vs. V − R diagram are marginally consistent with SN 1998bw (fig. 1i), they arequite inconsistent with this SN in V −R vs. R− I (fig. 1h). The difference between theabsolute magnitude of this OA and SN 1998bw was increasing during the time intervalbetween 36 and 87 days for t − T0, with the maximum difference of ∼ 2.7mag. Sucha high absolute magnitude is hardly consistent with a SN (fig. 1f). The most extremevalues of the color indices occurred at t − T0 = 84 days, with the reddest R − I andthe bluest R− I; this is inconsistent with the color and luminosity of a SN 1998bw-typeSN and implies that a different explanation is necessary. An interaction of the blastwave with a pre-existing shell [8] may partly explain the observed color changes — areddening of the spectrum is expected in this model. Nevertheless, the observed colorvariations of the OA of GRB970508 suggest a more complicated spectral profile, with alarge reddening only in the red spectral region.Our findings also have implications for the reddening of GRBs and their associatedSNe inside the host galaxy. The clustering of the individual OAs in the color-colordiagrams implies their low reddening [12]: hence the underlying SNe are not reddenedeither. The luminosity differences of the individual SNe at a given (t − T0)rest−fr aretherefore real (fig. 1d, e, f).The increasing amplitude of the color variations toward longer wavelengths showsthe great importance of the deep observations of the OAs in the infrared region in lateepochs.The method of the color indices gives us a possibility to separate the contributions ofthe synchrotron radiation and SN, using the commonly available multiband photometry.This approach is important mainly for analysis of faint OAs for which obtaining spectrawith good signal to noise at late phases is often impossible. This approach is possibleeven for archival data of OAs. The color indices are thus a much better tool to resolvethe evolution of the supernova in the profile of an OA than the light curves themselves.∗ ∗ ∗This study was supported by the grant A3003206 provided by the Grant Agency of theAcademy of Sciences of the Czech Republic, and the project ESA PRODEX INTEGRAL14527. We also acknowledge the CNR-AVCˇR collaborative project Investigation of GRBs(2004/2005).REFERENCES[1] Bloom J. S. et al., ApJ, 572 (2002) L45.[2] Fruchter A. et al., GCN, 1087 (2001).[3] Garnavich P. M. et al., GCN, 965 (2001).[4] Garnavich P. M. et al., ApJ, 582 (2003) 924.[5] Mazzali P. A. et al., ApJ, 572 (2002) L61.[6] Metzger M. R. et al., Nature, 387 (1997) 879.[7] Poznanski D. et al., PASP, 114 (2002) 833.[8] Ramirez-Ruiz E. et al., MNRAS, 327 (2001) 829.[9] Schlegel D. J. et al., ApJ, 500 (1998) 525.[10] Sokolov V. V. et al., A&A, 334 (1998) 117.[11] Sokolov V. V. et al., A&A, 344 (1999) 43.[12] Sˇimon V. et al., A&A, 377 (2001) 450.[13] Sˇimon V. et al., Gamma-Ray Burst Symposium, AIP Conf. Proc., 727 (2004) 487.[14] Sˇimon V. et al., these proceedings (2005).

Analysis of the late phase of three optical afterglows of GRBs using color indices

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Analysis of the late phase of three optical afterglows of GRBs using color indices

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