The origin of dark bursts - i.e. that have no observed afterglows in X-ray,
optical/NIR and radio ranges - is unclear yet. Different possibilities -
instrumental biases, very high redshifts, extinction in the host galaxies - are
discussed and shown to be important. On the other hand, the dark bursts should
not form a new subgroup of long gamma-ray bursts themselves.Comment: published in Nuovo Ciment

Bagoly, Z.

Balazs, L. G.

Borgonovo, L.

Horvath, I.

Klose, S.

Larsson, S.

Meszaros, A.

Ryde, F.

English

arXiv

Mészáros, Attila

Bagoly, Zsolt

Klose, S

Ryde, Felix,

Larsson, Stefan,

Balázs, L. G.

Horváth, István

Borgonovo, Luis,

Swepub

On the origin of the dark gamma-ray bursts

The origin of dark bursts—i.e. that have no observed afterglows in X-ray, optical/NIR and radio ranges—is unclear yet. Different possibilities—instrumental biases, very high redshifts, extinction in the host galaxies—are discussed and shown to be important. On the other hand, the dark bursts should not form a new subgroup of long 

gamma-ray bursts themselves

Mészáros, A.

Horváth, I.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2005-10048-xIL NUOVO CIMENTO Vol. 28 C, N. 3 Maggio-Giugno 2005On the origin of the dark gamma-ray bursts(∗)A. Me´sza´ros(1), Z. Bagoly(2), S. Klose(3), F. Ryde(4), S. Larsson(4)L. G. Bala´zs(5), I. Horva´th(6) and L. Borgonovo(4)(1) Astronomical Institute of the Charles UniversityV Holesˇovicˇka´ch 2, CZ-180 00 Prague 8, Czech Republic(2) Laboratory for Information Technology, Eo¨tvo¨s UniversityH-1117, Pa´zma´ny P. s. 1./A, Hungary(3) Tautenburg Observatory - Sternwarte 5, D-07778 Tautenburg, Germany(4) Stockholm Observatory - AlbaNova, SE-106 91 Stockholm, Sweden(5) Konkoly Observatory - POB 67, H-1525 Budapest, Hungary(6) Department of Physics, Bolyai Military University - Box-12, H-1456 Budapest, Hungary(ricevuto il 23 Maggio 2005; pubblicato online il 19 Ottobre 2005)Summary. — The origin of dark bursts—i.e. that have no observed afterglowsin X-ray, optical/NIR and radio ranges—is unclear yet. Different possibilities—instrumental biases, very high redshifts, extinction in the host galaxies—are dis-cussed and shown to be important. On the other hand, the dark bursts should notform a new subgroup of long gamma-ray bursts themselves.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 01.30.Cc – Conference proceedings.1. – IntroductionIn Jochen Greiner’s list [1], up to June 30, 2004, there were collected 222 gamma-ray bursts (GRBs). Only 71 objects (i.e. roughly the one-third fraction) have observedafterglows (AGs) at lower energies. In the following, the remaining two-thirds are definedas the fraction of the dark bursts. (Note that there is no unambigous definition of thedark burst itself, and usually only that GRBs are dark, which have no optical AGs [2].For our purpose the best definition is to take a GRB dark when there is no observed AG(regardless in which band) and also no redshift is measured.)In accordance with [3] there can exist four possible explanations for this phenomenon:I. the observational biases play a role;II. a large fraction of bursts is at high redshifts;(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.c© Societa` Italiana di Fisica 311312 A. ME´SZA´ROS, Z. BAGOLY, S. KLOSE, ETC.-0.5 0 0.5 1 1.5 2log τ [s]0510152025NFig. 1. – Distribution of the autocorrelation function widths τ (for more details see [7]) for 160GRBs. The redshifts were based on [8]. The logarithmic histograms of the widths are corrected(solid line) or not corrected (dashed line) for the cosmological time dilation. In both cases thedistributions seem to be unimodal, in contradiction with [7].III. a large fraction of bursts is obscured by the interstellar matter in host galaxies;IV. the darkness is intrinsic and some bursts really have no AGs.Obviously, if only the first three possibilities were occurring, then any GRB would stillhave AG; i.e. for any GRB the AG would exist, but simply would not be detected. Onthe other hand, if the fourth possibility is also occurring, then there would be a specialsubclass of long GRBs having no AG. The study of GRB 000330 [3] has shown thatall four eventualities were possible. References [4] and [5] give maximally a  (10–30)fraction for the fourth explanation.Our aim in this article is to search for more concrete conclusions concerning thesefour possibilities.It is well-known that there are short and long bursts; probably also intermediate ones(see [6] and the references therein). Here we discuss only long GRBs, because all GRBsin Greiner’s list should belong to this subclass. This also means that—if possibilityIV were occurring—then the long subclass would be separated and there would be—intotal—4 subclasses of GRBs. The most recent support for this separation of the longsubgroup itself comes from [7]. Nevertheless, repeating the same procedure with thegamma photometric redshifts of [8], we obtain no evident separation (fig. 1). In thisarticle we will give further arguments against the subclass separation of long GRBs.2. – The methodWe will discuss the possibilities I-III. Let us assume that all long GRBs have AGs,and the fourth possibility is incorrect. If this is the case, then biases, GRBs at highredshifts, and absorption at the hosts must be, together, so effective that the two-thirdfraction of GRBs must remain unseen due to these effects. We will show by statisticalarguments that really such great amount of AGs may remain unseen by these effects.Then, simply, the fourth possibility is not needed.ON THE ORIGIN OF THE DARK GAMMA-RAY BURSTS 313Table I. – The redshifts (z) and peak-fluxes (P256) of GRBs that have both BATSE triggers andmeasured spectroscopic redshifts. The values are compiled after [7] using the Current BATSECatalog [10].Burst BATSE z P256 Remarktrigger ph/(cm2s)970508 6225 0.835 1.17970828 6350 0.9587 – no P256 value971214 6533 3.4127 2.32980329 6665 3.97 13.28 z upper limit only980425 6707 0.0085 1.08 SN1998bw980703 6891 0.966 2.59990123 7343 1.60 16.63990506 7549 1.3066 22.16990510 7560 1.619 10.20991216 7906 1.020 15.17000131 7975 4.5 – no P256 valueThe observed dark–non-dark separation is in the ratio ∼ 2 : 1 at most. It is practicallysure that some GRBs will have AGs, but these AGs remain unseen. As is shown by [9],there is a selection effect leading to the result that bright GRBs have more effectivelyobserved AGs. This observational bias can be called a “brightness bias”. Hence, therecan be, in principle, three types of long GRBs: GRBs having observed AGs (non-darkGRBs) & GRBs having AGs, but they remain unseen due to this bias (“observationallydark” GRBs) & GRBs having no AGs (“intrinsically dark” GRBs).What is the ratio of the second type GRBs to the first type? It is sure that this ratiois non-zero, because both non-dark GRBs and observationally dark GRBs exist [9]. Toanswer this question consider the GRBs—observed by BATSE [10]—which have measuredredshifts (either concrete redshifts or at least upper limits) from the AG observations.In table I these GRBs are listed using [7]. It is immediately remarkable that the P256’sare unusually high. For example, 5 GRBs have P256 > 10 ph/(cm2s) among the 9 P256values. On the other hand, in the BATSE Catalog for T90 > 10 s only 9% of GRBs fulfilthe condition P256 > 10 ph/(cm2s). In [9] it is shown that this is not chance: the brighterGRBs in gamma-ray range can be observed more easily at lower energies.In the BATSE Catalog 9% of long GRBs are brighter than 10 ph/(cm2s) in the P256peak-flux. Hence, if there were no observational biases, and if all GRBs were followedby AGs, one would detect roughly 10 times more AGs for GRBs with peak-flux below10 ph/(cm2s) than above this level. Because we observe 5 AGs above the peak-fluxlevel, we should see a further ∼ 50 AGs below this P256 = 10ph/(cm2s) limit. Insteadof 50 GRBs, 4 ones have been detected. Instead of the 9 GRBs collected in table Iwe should have  55 GRBs in the same table. Because the brightness bias exists,[(55− 9)/55]× 100 = 84% of GRBs should have unseen AGs. Hence, the brightness biasalone is able to explain even a ratio 55 : 9  6 : 1 for the observationally dark–non-darkpopulation. This is much bigger than the observed  2 : 1 ratio.Of course, there is a great uncertainty concerning the obtained  6 : 1 ratio, becauseof the extrapolation of a property, obtained from a sample containing 9 objects to asample for 211 objects. Also the cut at P256 = 10ph/(cm2s) is ad hoc. In addition, thenewer instruments (BeppoSAX, HETE2, INTEGRAL) have different thresholds in the314 A. ME´SZA´ROS, Z. BAGOLY, S. KLOSE, ETC.gamma-ray band than that of BATSE. Nevertheless, keeping all this in mind, we stressthat the brightness bias is so strong that even alone it is able to explain the fact thatthe two-third fraction of long GRBs have existing but unseen AGs.3. – GRBs at very high redshifts and the extinctionSeveral papers ( [8], [11]– [16]) estimate that a few tens of % of GRBs are at 5 < z < 20,where z is the redshift. In [17] it is claimed that even the majority of GRBs are atz > 10. It is already an observational evidence (see [18] for more details and referencestherein) that some dark GRBs are dark due to the extinction at the hosts. Trivially, theexisting population of GRBs at very high redshifts together with the confirmed extinctionstrengthen the fraction of the observationally dark GRBs.4. – ConclusionThe three effects I-III together may well cause that GRBs, detected only in gamma-rayband but having existing unseen AGs, may have a six or more times bigger populationthan that of GRBs with seen AGs. Hence, dark GRBs in Greiner’s list may well beexplained by these three instrumental effects alone, and there is no need to introduceany intrinsically dark subgroup of long GRBs without AGs.∗ ∗ ∗Thanks are due to the valuable discussions with C.-I. Bjo¨rnsson, I. Csabai, C.Fransson, J. P. U. Fynbo, P. Me´sza´ros, G. Tusna´dy and R. Vavrek. The usefulremarks of the anonymous referee are kindly acknowledged. This research was supportedthrough OTKA grants T034549, T48870, and by a grant from the Wenner-Gren Foun-dations (A.M.).REFERENCES[1] Greiner J., http://www.mpe.mpg.de/∼jcg/grbgen.html.[2] Jakobsson P. et al., ApJ, 617 (2004) L21.[3] Fynbo J. U. et al., A&A, 369 (2001) 373.[4] Stratta G. et al., ApJ, 608 (2004) 846.[5] Lamb D. Q. et al., NewA.Rev., 48 (2004) 423.[6] Bala´zs L. G. et al., A&A, 401 (2003) 129.[7] Borgonovo L., A&A, 418 (2004) 487.[8] Bagoly Z. et al., A&A, 398 (2003) 919.[9] Me´sza´ros A. et al., in Third Rome Workshop on Gamma-Ray Bursts in the AfterglowEra, edited by Feroci M., Frontera F., Masetti N. and Piro L., ASP Conf. Ser.,Vol. 312 (ASP, San Francisco) 2004, p. 118.[10] Meegan C. A. et al., http://www.gammaray.msfc.nasa.gov/batse.[11] Me´sza´ros P. and Me´sza´ros A., ApJ, 449 (1995) 9.[12] Me´sza´ros A. and Me´sza´ros P., ApJ, 466 (1996) 29.[13] Horva´th I., Me´sza´ros P. and Me´sza´ros A., ApJ, 470 (1996) 56.[14] Reichart D. E. and Me´sza´ros P., ApJ, 483 (1997) 597.[15] Lamb D. Q. and Reichart D. E., ApJ, 536 (2000) 1.[16] Schmidt M., ApJ, 552 (2000) 36.[17] Lin J. R., Zhang S. N. and Li T. P., ApJ, 605 (2004) 579.[18] Klose S. et al., ApJ, 592 (2003) 1025.

The origin of dark bursts-i.e. that have no observed afterglows in X-ray, optical/NIR and radio ranges-is unclear yet. Different possibilities-instrumental biases, very high redshifts, extinction in the host galaxies-are discussed and shown to be important. On the other hand, the dark bursts should not form a new subgroup of long gamma-ray bursts themselves.</p

Ryde, Felix

The origin of dark bursts--i.e. that have no observed afterglows in X-ray, optical/NIR and radio ranges--is unclear yet. Different possibilities--instrumental biases, very high redshifts, extinction in the host galaxies--are discussed and shown to be important. On the other hand, the dark bursts should not form a new subgroup of long gamma-ray bursts themselves

Mészáros, A

Bagoly, Z

Balázs, L G

Ryde, F

Larsson, S

Horváth, I

Borgonovo, L

ELTE Digital Institutional Repository (EDIT)

arXiv:astro-ph/0701905v1  31 Jan 2007IL NUOVO CIMENTO Vol. ?, N. ? ?On the Origin of the Dark Gamma-Ray BurstsA. Mészáros(1), Z. Bagoly(2), S. Klose(3), F. Ryde(4), S. Larsson(4), L. G.Balázs(5), I. Horváth(6), L. Borgonovo(4)(1) Astronomical Institute of the Charles University, V Holešovičkách 2, CZ-180 00 Prague 8,Czech Republic(2) Laboratory for Information Technology, Eötvös University, H-1117 Budapest, Pázmány P.s. 1./A, Hungary(3) Tautenburg Observatory, D-07778 Tautenburg, Sternwarte 5, Germany(4) Stockholm Observatory, AlbaNova, SE-106 91 Stockholm, Sweden(5) Konkoly Observatory, H-1525 Budapest, POB 67, Hungary(6) Department of Physics, Bolyai Military University, Budapest, Box-12, H-1456, HungarySummary. — The origin of dark bursts - i.e. that have no observed afterglows inX-ray, optical/NIR and radio ranges - is unclear yet. Different possibilities - instru-mental biases, very high redshifts, extinction in the host galaxies - are discussed andshown to be important. On the other hand, the dark bursts should not form a newsubgroup of long gamma-ray bursts themselves.PACS 98.70 – 98.80.1. – IntroductionIn Jochen Greiner’s list [1], up to June 30, 2004, there were collected 222 gamma-ray bursts (GRBs). Only 71 objects (i.e. roughly the one-third fraction) have observedafterglows (AGs) at lower energies. In the following, the remaining two-thirds are definedas the fraction of the dark bursts. (Note that there is no unambigous definition of thedark burst itself, and usually only that GRBs are dark, which have no optical AGs [2].For our purpose the best definition is to take a GRB dark when there is no observed AG(regardless in which band) and also no redshift is measured.)In accordance with [3] there can exist four possible explanations for this phenomenon:I. the observational biases play a role;II. a large fraction of bursts is at high redshifts;III. a large fraction of bursts is obscured by the interstellar matter in host galaxies;IV. the darkness is intrinsic and some bursts really have no AGs.Obviously, if only the first three possibilities were occurring, then any GRB would stillhave AG; i.e. for any GRB the AG would exist, but simply would not be detected. Onc© Società Italiana di Fisica 12 A. MÉSZÁROS ETC.-0.5 0 0.5 1 1.5 2log τ [s]0510152025NFig. 1. – Distribution of the autocorrelation function widths τ (for more details see [7]) for 160GRBs. The redshifts were based on [8]. The logarithmic histograms of the widths are corrected(solid-line) or not corrected (dashed-line) for the cosmological time dilation. In both cases thedistributions seem to be unimodal, in contradiction with [7].the other hand, if the fourth possibility is also occurring, then there would be a specialsubclass of long GRBs having no AG. The study of GRB 000330 [3] has shown that allfour eventualities were possible. [4] and [5] give maximally a ≃ (10− 30) fraction for theIV-th explanation.Our aim in this article is to search for more concrete conclusions concerning thesefour possibilities.It is well-known that there are short and long bursts; probably also intermediate ones(see [6] and the references therein). Here we discuss only long GRBs, because all GRBsin Greiner’s list should belong to this subclass. This also means that - if possibilityIV were occurring - then the long subclass would be separated and there would be - intotal - 4 subclasses of GRBs. The most recent support for this separation of the longsubgroup itself comes from [7]. Nevertheless, repeating the same procedure with thegamma photometric redshifts of [8], we obtain no evident separation (Fig. 1). In thisarticle we will give further arguments against the subclass separation of long GRBs.2. – The MethodWe will discuss the possibilities I-III. Let us assume that all long GRBs have AGs, andthe IV-th possibility is incorrect. If this is the case, then biases, GRBs at high redshifts,and absorption at the hosts must be, together, so effective that the two-third fraction ofGRBs must remain unseen due to these effects. We will show by statistical argumentsthat really such great amount of AGs may remain unseen by these effects. Then, simply,ON THE ORIGIN OF THE DARK GAMMA-RAY BURSTS 3Table I. – The redshifts (z) and peak-fluxes (P256) of GRBs that have both BATSE triggers andmeasured spectroscopic redshifts. The values are compiled after [7] using the Current BATSECatalog [10].Burst BATSE z P256 Remarktrigger ph/(cm2s)970508 6225 0.835 1.17970828 6350 0.9587 − noP256 value971214 6533 3.4127 2.32980329 6665 3.97 13.28 z upper limit only980425 6707 0.0085 1.08 SN1998bw980703 6891 0.966 2.59990123 7343 1.60 16.63990506 7549 1.3066 22.16990510 7560 1.619 10.20991216 7906 1.020 15.17000131 7975 4.5 − noP256 valuethe fourth possibility is not needed.The observed dark–non-dark separation is in the ratio ∼ 2 : 1 at most. It is practicallysure that some GRBs will have AGs, but these AGs remain unseen. As is shown by [9],there is a selection effect leading to the result that bright GRBs have more effectivelyobserved AGs. This observational bias can be called a ”brightness bias”. Hence, therecan be, in principle, three types of long GRBs: GRBs having observed AGs (non-darkGRBs) & GRBs having AGs, but they remain unseen due to this bias (”observationallydark” GRBs) & GRBs having no AGs (”intrinsically dark” GRBs).What is the ratio of the second type GRBs to the first type? It is sure that this ratiois non-zero, because both non-dark GRBs and observationally dark GRBs exist [9]. Toanswer this question consider the GRBs - observed by BATSE [10] - which have measuredredshifts (either concrete redshifts or at least upper limits) from the AG observations.In Table 1 these GRBs are listed using [7]. It is immediately remarkable that the P256’sare unusually high. For example, 5 GRBs have P256 > 10 ph/(cm2s) among the 9 P256values. On the other hand, in the BATSE Catalog for T90 > 10 s only 9% of GRBsfulfil the condition P256 > 10 ph/(cm2s). In [9] it is shown that this is not chance: thebrighter GRBs in gamma-ray range can be observed more easily at lower energies.In the BATSE Catalog 9% of long GRBs are brighter than 10 ph/(cm2s) in the P256peak-flux. Hence, if there were no observational biases, and if all GRBs were followed byAGs, one would detect roughly 10 times more AGs for GRBs with peak-flux below 10ph/(cm2s) than above this level. Because we observe 5 AGs above the peak-flux level, weshould see a further ∼ 50 AGs below this P256 = 10ph/(cm2s) limit. Instead of 50 GRBs,4 ones have been detected. Instead of the 9 GRBs collected in Table 1 we should have≃ 55GRBs in the same table. Because the brightness bias exists, [(55−9)/55]×100 = 84% ofGRBs should have unseen AGs. Hence, the brightness bias alone is able to explain evena ratio 55 : 9 ≃ 6 : 1 for the observationally dark–non-dark population. This is muchbigger than the observed ≃ 2 : 1 ratio.Of course, there is a great uncertainty concerning the obtained ≃ 6 : 1 ratio, becauseof the extrapolation of a property, obtained from a sample containing 9 objects to asample for 211 objects. Also the cut at P256 = 10ph/(cm2s) is ad hoc. In addition, the4 A. MÉSZÁROS ETC.newer instruments (BeppoSAX, HETE2, INTEGRAL) have different thresholds in thegamma-ray band than that of BATSE. Nevertheless, keeping all this in mind, we stressthat the brightness bias is so strong that even alone it is able to explain the fact thatthe two-third fraction of long GRBs have existing but unseen AGs.3. – GRBs at very high redshifts and the extinctionSeveral papers ([11], [12], [13], [14], [15], [16], [8]) estimate that a few tens of % ofGRBs are at 5 < z < 20, where z is the redshift. In [17] it is claimed that even themajority of GRBs are at z > 10. It is already an observational evidence (see [18] formore details and references therein) that some dark GRBs are dark due to the extinctionat the hosts. Trivially, the existing population of GRBs at very high redshifts togetherwith the confirmed extinction strengthen the fraction of the observationally dark GRBs.4. – ConclusionThe three effects I-III together may well cause that GRBs, detected only in gamma-ray band but having existing unseen AGs, may have a six or more times bigger populationthan that of GRBs with seen AGs. Hence, dark GRBs in Greiner’s list may wellbe explained by these three instrumental effects alone, and there is no needto introduce any intrinsically dark subgroup of long GRBs without AGs.∗ ∗ ∗Thanks are due to the valuable discussions with Claes-Ingvar Björnsson, István Csa-bai, Claes Fransson, Johan Peter Uldall Fynbo, Peter Mészáros, Gábor Tusnády, andRoland Vavrek. The useful remarks of the anonymous referee are kindly acknowledged.This research was supported through OTKA grants T034549, T48870, and by a grantfrom the Wenner-Gren Foundations (A.M.).REFERENCES[1] Greiner J., http://www.mpe.mpg.de/∼ jcg/grbgen.html.[2] Jakobsson P. et al., ApJ, 617 (2004) L21.[3] Fynbo J.U. et al., A&A, 369 (2001) 373.[4] Stratta G. et al., ApJ, 608 (2004) 846.[5] Lamb D.Q. et al., NewA.Rev., 48 (2004) 423.[6] Balázs L.G. et al., A&A, 401 (2003) 129.[7] Borgonovo L., A&A, 418 (2004) 487.[8] Bagoly Z. et al., A&A, 398 (2003) 919.[9] Mészáros A. et al., in Third Rome Workshop on Gamma-Ray Bursts in the AfterglowEra, edited by Feroci M., Frontera F., Masetti N. and Piro L. (ASP, San Francisco)2004, 118.[10] Meegan C.A. et al., http://www.gammaray.msfc.nasa.gov/batse.[11] Mészáros P. and Mészáros A., ApJ, 449 (1995) 9.[12] Mészáros A. and Mészáros P., ApJ, 466 (1996) 29.[13] Horváth I., Mészáros, P. and Mészáros A., ApJ, 470 (1996) 56.[14] Reichart D.E. and Mészáros P., ApJ, 483 (1997) 597.[15] Lamb D.Q. and Reichart D.E., ApJ, 536 (2000) 1.[16] Schmidt M., ApJ, 552 (2000) 36.ON THE ORIGIN OF THE DARK GAMMA-RAY BURSTS 5[17] Lin J.R., Zhang S.N. and Li T.P., ApJ, 605 (2004) 579.[18] Klose S. et al., ApJ, 592 (2003) 1025.

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On the Origin of the Dark Gamma-Ray Bursts

On the Origin of the Dark Gamma-Ray Bursts

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