The frontier of darkness: the cases of GRB 040223, GRB 040422, GRB 040624

0.16 MB·English

by  P. D'Avanzo

The frontier of darkness: the cases of
GRB 040223, GRB 040422, GRB 040624
P. D’Avanzo∗,†, P. Filliatre∗∗,‡, P. Goldoni∗∗,‡, L. A. Antonelli§, S.
Campana∗, G. Chincarini¶, S. Covino∗, A. Cucchiarak, M. Della Valle††, A.
De Luca‡‡, S. Foley§§, D. Fugazza∗, N. Gehrels¶¶, D. Götz‡‡, L. Hanlon§§,
6
0 G. L. Israel§, D. Malesani∗∗∗, B. McBreen§§, S. McBreen†††, S. McGlynn§§,
0 S. Mereghetti‡‡, L. Moran‡‡‡, J. A. Nousekk, R. Perna§§§, L. Stella¶¶¶ and G.
2
Tagliaferri∗
n
a
J ∗INAF,OsservatorioAstronomicodiBrera,viaE.Bianchi46,I-23807Merate(LC),Italy
5 †UniversitàInsubria,DipartimentodiFisicaeMatematica,viaValleggio11,I-22100Como,Italy
2 ∗∗LaboratoireAstroparticuleetCosmologie,UMR7164,11placeMarcelinBerthelot,F-75231
ParisCedex05,France

1 ‡Serviced’Astrophysique,CEA/DSM/DAPNIA/SAp,CE-Saclay,OrmedesMerisiers,Bât.709,
v F-91191Gif-sur-YvetteCedex,France
7 §INAF,OsservatorioAstronomicodiRoma,viaFrascati33,MonteporzioCatone,I-00040Roma,
6
Italy
5
¶UniversitàdeglistudidiMilano-Bicocca,DipartimentodiFisica,PiazzadelleScienze3,I-20126
1
Milano,Italy
0
6 kDepartmentofAstronomyandAstrophysics,PennsylvaniaStateUniversity,525Davey
0 Laboratory,UniversityPark,PA16802
/ ††INAF,OsservatorioAstrofisicodiArcetri,largoE.Fermi5,I-50125Firenze,Italy
h
‡‡INAF-IstitutodiAstrofisicaSpazialeeFisicaCosmicadiMilano,viaE.Bassini15,I-20133
p
Milano,Italy
-
o §§DepartmentofExperimentalPhysics,UniversityCollegeDublin,Dublin4,Ireland
r ¶¶NASAGoddardSpaceFlightCenter,Code661,Greenbelt,MD20771
t
s ∗∗∗Internationalschoolforadvancedstudies(SISSA/ISAS),viaBeirut2-4,I-34014Trieste,Italy
a †††AstrophysicsMissionsDivision,ResearchScientificSupportDepartmentofESA,ESTEC,
:
v Noordwijk,TheNetherlands
i ‡‡‡DepartmentofPhysics&Astronomy,UniversityofSouthampton,Southampton,SO171BJ,
X
UnitedKingdom
r §§§DepartmentofAstrophysicalandPlanetarySciences,UniversityofColoradoatBoulder,440
a
UCB,Boulder,CO,80309,USA
¶¶¶INAF,OsservatorioAstronomicodiRoma,viaFrascati33,MonteporzioCatone,00040Rome,
Italy
Abstract. Understanding the reasons for the faintness of the optical/near-infrared afterglows of
the so-called dark bursts is essential to assess whether they form a subclass of GRBs, and hence
for the use of GRBs in cosmology. With VLT and other ground-based telescopes, we searched
for the afterglows of the INTEGRAL bursts GRB040223, GRB040422 and GRB040624 in the
first hours after the triggers. A detection of a faint afterglow and of the host galaxy in the K
band was achieved for GRB040422,while only upper limits were obtained for GRB040223and
GRB040624,although in the former case the X-ray afterglow was observed.A comparison with
the magnitudes of a sample of afterglows clearly shows the faintness of these bursts, which are
goodexamplesofapopulationthatanincreasingusageoflargediametertelescopesisbeginningto
unveil.
Keywords: gamma-raysources;gamma-rayburst
PACS: 98.70.Rz
INTRODUCTION
The study of GRB afterglows is a promising tool for cosmology, since their absorption
spectra convey information on the distance and the chemical composition of a new
set of galaxies (e. g. [4]), with the possibility of exploration up to the reionization
epoch [9]. However, a debated fraction of GRBs – from less than 10% [10] to 60%
[11] – did not show any detectable afterglow in the optical band. Popular and non-
mutually exclusive explanations are: these bursts have intrinsically faint afterglows in
the optical band (e. g. [5]; [11]); their decay is very fast [1]; the optical afterglow
is extinguished by dust in the vicinity of the GRB or in the star-forming region in
which the GRB occurs (e. g. [8]; [15]); their redshift is above 6, so that the Lyman-
a absorption by neutral hydrogen in the host galaxy and along the line of sight damps
the optical radiation of the afterglow [8]. To these physical explanations, one must add
the possibility that the search techniques are neither accurate nor quick enough [10].
The possibility that some afterglows are intrinsically faint has of course a big impact
onthemodellingoftheGRBs themselves,as well as on theirapplicationin cosmology.
On the other hand, if one or several of the other explanations are correct, a substantial
reduction of the fraction of dark bursts can be achieved by quick observations in the
infrared. In any case, a fast and multiwavelength follow-up campaign of observations
is mandatory for the study of GRB afterglows. To this aim, the ESA’s International
Gamma-Ray Astrophysics Laboratory INTEGRAL [17], launched in October 2002, has
a burst alert system called IBAS (INTEGRAL Burst Alert System, [12]). IBAS carries
outrapid localizationsfor GRBs incident on theIBIS detectorwith aprecision ofa few
arcminutes [13]. The public distribution of these coordinates enables multi-wavelength
searches for afterglows at lower energies. INTEGRAL data on the prompt emission in
combination with the early multi-wavelength studies, such as those presented in this
work for GRB040223, GRB040422 and GRB040624, can probe these high energy
phenomena.
THE AFTERGLOW OF GRB040422
GRB040422 was detected by the INTEGRAL satellite at an angle of only 3 degrees
from the Galactic plane. We observed the afterglow of GRB040422 with the ISAAC
and FORS2 instruments at the VLT less than 2 hours after the burst. Such a prompt
reaction, together with careful inspection of the crowded field of this GRB, led to
the discovery of its near-infrared afterglow, for which we obtained the astrometry and
photometry. We measured for the afterglow a magnitude K = 18.0±0.1 (1.9 hours
after the burst), a valuewhich is below the limitof the 2MASS catalogue. No detection
could be obtained in the R and I bands, partly due to the large extinction in the Milky
Way. We imaged the position of the afterglow again two months later in the K band,
and detected a likely bright (K ∼ 20) host galaxy (Fig. 1, for more details see [2]).
We compare the magnitude of the afterglow with those of a compilation of promptly
FIGURE 1. The regionof the afterglowof GRB040422.From top to bottom:2004Apr 22.37,2004
May05.33,2004Jun26.11UT.
observed counterparts of previous GRBs, and show that the afterglow of GRB040422
lies at the very faint end of the distribution, after accounting for Milky Way extinction
(Fig. 2). This observation suggests that the proportion of dark GRBs can be lowered
significantlybyamoresystematicuseof8-mclasstelescopesintheinfraredinthevery
early hoursaftertheburst.
THE DARK GRB040223 AND GRB040624
GRB040223wasdetectedbyINTEGRALclosetotheGalacticplanewhileGRB040624
was at high Galactic latitude. The two GRBs have long durations, slow pulses and are
weak. Theg -ray spectraofbothburstsarebest fitwith steeppower-laws,implyingthey
areX-rayrich.GRB040223isamongtheweakestandlongestofINTEGRALGRBs.The
X-ray afterglow of this burst was detected 10 hours after the prompt event by XMM-
Newton. The measured spectral properties are consistent with a column density much
higher than that expected from the Galaxy, indicating strong intrinsic absorption. We
carriedoutnear-infraredobservations17hoursaftertheburstwiththeESO-NTT,which
yielded upper limits. Given the intrinsic absorption, we find that these limits are com-
patiblewith a simpleextrapolationof the X-ray afterglow properties. For GRB040624,
we carried out optical observations 13 hours after the burst with FORS1 and 2 at the
VLT, and DOLoRes at the TNG, again obtaining upper limits. As for GRB040422, we
compare these limits with the magnitudes of a compilation of promptly observed coun-
terparts of previous GRBs and find again that they lie at the very faint end of the distri-
bution (Fig. 2, for a more detailed analysis see [3]). Together with GRB040422, these
twoburstsaregoodexamplesofapopulationofburstswithdarkorfaintafterglowsthat
arebeingunveiledthroughtheincreasingusageoflargediametertelescopesengagedin
comprehensiveobservationalprogrammes.
FIGURE 2. Light curves of a set of 39 afterglow data, observations of GRB040422 and limits of
GRB040223andGRB040624.Forclarity,differentlinestylesareusedtoindicatethetemporalpower-
law decay extrapolations,which only cover our observation epochs. (Left) Magnitudes extrapolatedto
the R band (when necessary) after correction for Galactic absorption. The cross is placed at the time
of observation. The diamond and the square indicate the magnitude limits for GRB040223 with and
withoutcorrectionforGalacticextinction,respectively,at2-s ([6]).Thecrossesindicatethemagnitude
limits for GRB040624.The first two come from the GCNs ([14, 7]). The last three points are the 3-s
limits reportedin [3]. (Right)Magnitudesextrapolatedto the K band (whennecessary) after correction
for Galactic absorption. The cross is placed at the time of observation. The diamonds and the squares
indicateour3-s magnitudelimitswithandwithoutcorrectionfromGalacticextinction,respectively.The
KmagnitudeforGRB040422([2])andthelimitforGRB051001([16])arealsoreported.
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