The Keck optical spectrum of the GRB 020813 afterglow is the best ever
obtained for GRBs. Its large spectral range and very high S/N ratio allowed for
the first time the detection of a vast variety of absorption lines, associated
with the circumburst medium or interstellar medium of the host. The remarkable
similarity of the relative abundances of 8 elements with the dust depletion
pattern seen in the Galactic ISM suggests the presence of dust. The derived
visual dust extinction A_V=0.40+/-0.06 contradicts the featureless UV spectrum
of the afterglow, very well described by a unreddened power law. The
forthcoming Swift era will open exciting opportunities to explain similar
phenomena in other GRB afterglows.Comment: To be published in "Il Nuovo Cimento", Proceedings of the 4th Rome
  Workshop on Gamma-Ray Bursts in the Afterglow Era, eds. L. Piro, L. Amati, S.
  Covino, B. Gendr

Fall, S. M.

Savaglio, S.

English

arXiv

The Keck optical spectrum of the GRB 020813 afterglow is the best ever obtained for GRBs. Its large spectral range and very high S/N ratio allowed for the first time the detection of a vast variety of absorption lines, associated with the circumburst medium or interstellar medium of the host. The remarkable similarity of the relative abundances of 8 elements with the dust depletion pattern seen in

the Galactic ISM suggests the presence of dust. The derived visual dust extinction AV = 0.40±0.06 contradicts the featureless UV spectrum of the afterglow, very well

described by a unreddened power law. The forthcoming Swift era will open exciting opportunities to explain similar phenomena in other GRB afterglows

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2005-10102-9IL NUOVO CIMENTO Vol. 28 C, N. 4-5 Luglio-Ottobre 2005The dust depletion and extinction of the GRB 020813 afterglow(∗)S. Savaglio(1) and S. M. Fall(2)(1) Johns Hopkins University - Baltimore, USA(2) Space Telescope Science Institute - Baltimore, USA(ricevuto il 23 Maggio 2005; pubblicato online il 19 Settembre 2005)Summary. — The Keck optical spectrum of the GRB 020813 afterglow is the bestever obtained for GRBs. Its large spectral range and very high S/N ratio allowed forthe first time the detection of a vast variety of absorption lines, associated with thecircumburst medium or interstellar medium of the host. The remarkable similarityof the relative abundances of 8 elements with the dust depletion pattern seen inthe Galactic ISM suggests the presence of dust. The derived visual dust extinctionAV = 0.40±0.06 contradicts the featureless UV spectrum of the afterglow, very welldescribed by a unreddened power law. The forthcoming Swift era will open excitingopportunities to explain similar phenomena in other GRB afterglows.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe optical afterglow of GRB 020813 (z = 1.25) was observed a few hours after theburst with Keck at a reasonable spectral resolution [1] when the burst was still relativelybright. The unprecedented(1) good quality of rest-frame UV spectrum, and the largewavelength range covered, allowed the detection of a tremendous number of absorptionlines, including very weak transitions never detected before in GRB spectra.The GRB rest-frame UV spectrum shows another notable feature: the continuumemission in the interval 7.4 × 1014–2.1 × 1015Hz is very well represented by a perfectpower law of the form fν ∝ ν−0.918±0.001 [3]. Such a nearly perfect power law is not onlya very rare event, but it also strongly suggests (by the standard view of extinction laws)no reddening, or equivalently, little dust along the sight-line.Here we discuss the dust properties in the interstellar or circumburst medium in frontof GRB 020813 by using two different and completely independent means: i) studyingthe relative abundances of several elements with very different refractory properties (i.e.,(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.(1) Much higher resolution spectra have been obtained for GRB 020813 and GRB 021004 [2],however the information available is somehow limited by the low S/N in large parts of thespectrum.c© Societa` Italiana di Fisica 541542 S. SAVAGLIO and S. M. FALLFig. 1. – Heavy element column densities in GRB 020813. The dashed line shows where pointswould be in the case of solar relative abundances and no dust. The solid line shows the expectedcolumn densities in the case that elements are partly locked in dust grains, as in the gas of thewarm disk+halo of the Milky Way. Other depletion patterns in the MW have been considered,with slightly worse fit (higher χ2).how easily they are locked in dust grains); ii) trying to reconcile the observed straightUV continuum with the presence of several extinction laws.2. – Metal abundances and visual extinctionWe derived column densities of FeII, SiII, SiII∗, ZnII, CrII, NiII, MgI, CI, CaII, CIV,MnII, MgII, AlII, AlIII, and TiII in the GRB 020813 afterglow, using independently linefitting, curve of growth and apparent optical depth analysis [3]. High column densities oflow ionization species (e.g., FeII), relative to higher-ionization species (CIV and AlIII), isa good indication that the gas is nearly neutral (the column density of hydrogen is mostlyin HI form) and the column density of each element can be approximated by the columndensity of the ion with ionization potential below 13.6 eV (e.g., Fe ∼ FeII). As the Lymanseries lines are not in the observed spectral range, we cannot estimate the HI columndensity. However, we can derive relative metal abundances, compare them with solarvalues and with relative abundances expected if the various elements are partly locked inthe dust grains as in the Milky Way ISM (dust depletion pattern). Figure 1 shows withno ambiguity that relative abundances can be explained if the gas is polluted by dust.The dust extinction of the GRB afterglow as a function of wavelength depends basi-cally on the dust column density and the grain-size distribution. While there is no obviousway we can determine the latter, the former can be derived by assuming that the elementpattern observed in fig. 1 is a dust depletion pattern, with no need to know the metalabundance relative to hydrogen. The main assumption is that the total (dust + gas)element relative abundance in the GRB ISM is as in the solar neighborhood, and whatis missing in the gas form is locked into dust grains.To estimate the visual extinction for GRB 020813, we also assume that the rate ofvisual extinction per unit column of dust is as in the solar neighborhood [4], or half aTHE DUST DEPLETION AND EXTINCTION OF THE GRB 020813 AFTERGLOW 543Fig. 2. – GRB 020813 spectrum before and after a dust extinction screen is applied to an intrinsicGRB emission. The MW, LMC, SMC and power law extinctions are used. For the first three,AV is the maximum value allowed by the observed spectrum. For the power law extinction,AV = 0.4 is assumed, while the power index γ = 0.845 is the maximum value allowed by theobserved GRB spectrum.magnitude for a total column density of Fe (dust and gas) NFeII = 1016.5 cm−2. Fromfig. 1, we estimated a total Fe column density NFeII = 1016.46±0.08, and translated into avisual extinction AV = 0.40± 0.06.3. – The dust extinction lawA possible extinction law for the ISM of the GRB has to be shallow enough and/ora weak function of the wavelength in order to make the GRB UV emission consistentwith the observed undistorted power law. To constrain the extinction law, we onlyassumed that the intrinsic GRB emission is itself described by a (unconstrained) powerlaw. We used the MW, LMC, SMC extinctions, and a power law extinction of the form∝ λ−γ . Starting from all possible intrinsic GRB emissions, we constrained AV for allfour different extinctions. Figure 2 shows for MW, LMC and SMC extinctions, themaximum allowed AV still compatible with the observed GRB spectrum (2 σ), togetherwith the associated intrinsic emission. For the MW and LMC extinctions, AV < 0.08 andAV < 0.18, respectively, mainly due to the absence in the observed GRB spectrum of the2200 A˚ feature. For the power law extinction, if AV = 0.4, then only extinctions withγ < 0.85 make the GRB spectrum consistent with the observed one. This last resultis summarized in fig. 3, where the allowed regions of AV (from the depletion patternanalysis) and γ (from the extinction law analysis) are displayed together.544 S. SAVAGLIO and S. M. FALLFig. 3. – The shaded regions indicate the values of visual extinction AV and of slope of thepower law extinction γ not allowed by the analysis of the dust properties in the GRB 020813afterglow.4. – SummaryThe relative metal abundances of 8 elements are derived with unprecedented accuracyfor a GRB afterglow. These are remarkably similar to the metal pattern observed inGalactic ISM clouds, polluted by dust. If dust is indeed responsible of the metal patternshown in fig. 1, then the visual extinction is AV  0.4. Such a relatively large extinctionis compatible with the observed spectrum (well represented by a power law of the formfν ∝ ν−0.92) only assuming a shallow extinction law. If we use the known extinctionlaws (MW, LMC and SMC) the extincted spectrum is still compatible with the observedspectrum if AV < 0.2. The observed GRB spectral shape and the AV > 0.3 (95% CL)derived from the depletion pattern, can only be explained for a flatter extinction. Forinstance, if a power law is assumed and AV = 0.4, then the slope has to be shallowerthan γ  0.85.GRB UV spectra will show their overwhelming potentiality for the understanding ofthe cosmic chemical enrichment and physical state of the high-z interstellar medium whenthe Swift satellite will be fully operational. From the low/medium resolution spectraobserved so far, it is already very clear that GRBs have a very different (i.e., muchstronger absorption) heavy element pattern than other high redshift absorbers [3, 5, 2].Among many other GRB fundamental parameters, the Swift era will reveal whetherthe strong absorption is typical of the ISM in high-z star forming galaxies, or it is apeculiarity of the GRB circumburst medium only.REFERENCES[1] Barth A. J. et al., ApJ, 584 (2003) L47.[2] Fiore F. et al., ApJ, 624 (2005) 853.[3] Savaglio S. and Fall S. M., ApJ, 614 (2004) 293.[4] Bohlin R. C., Savage B. D. and Drake J. F., ApJ, 224 (1978) 132.[5] Savaglio S., Fall S. M. and Fiore F., ApJ, 585 (2002) 638.

The dust depletion and extinction of the GRB 020813 afterglow

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The Dust Depletion and Extinction of the GRB 020813 Afterglow

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