Authors: Fynbo J. P. U., Burud I., Møller P.,

- A&A 358, 88-94 (2000)

- Abstract:

We present deep imaging in the U, B and I bands obtained under excellent seeing conditions of the double quasar Q0151+048A,B and of the Damped Ly-α (DLA) absorbing galaxy at z(abs) = 1.9342 named S4. We analyse the data employing two separate and independent methods. First we deconvolve the images using the MCS algorithm, secondly we decompose the images via an object based iteration process where we fit models to objects without any attempt to improve the resolution of the data. Our detailed analysis of the images reveals, somewhat surprisingly, that extended objects centred on the quasars themselves are much brighter continuum sources than the DLA galaxy. Due to the complexity caused by the many superimposed objects, we are unable to certify whether or not continuum emission from the DLA galaxy is detected. Continuum emission from the extended objects centred on the positions of the quasars is clearly seen, and the objects are tentatively identified as the ``host galaxies'' of the quasars. The flux of those host galaxies is of order 2-6% of the quasar flux, and the light profile of the brighter of the two is clearly best fit with a de Vaucouleurs profile. We discuss two alternative interpretations of the origin of the extended flux: i) the early stage of a massive elliptical galaxy in the process of forming the bulk of its stars, and ii) quasar light scattered by dust.

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- Authors: Sahu, K. C., Vreeswijk, P., Bakos, G., Menzies, J. W., Bragaglia A., Frontera F., Piro L., Albrow M. D., Bond I. A., Bower R., Caldwell J. A. R., Castro-Tirado A. J., Courbin F., Dominik M., Fynbo J. P. U., Galama T., Glazebrook K., Greenhill J., Gorosabel J., Hearnshaw J., Hill K., Hjorth J., Kane S., Kilmartin P. M., Kouveliotou C., Martin R., Masetti N., Maxted P., Minniti D., Møller P., Muraki Y., Nakamura T., Noda S., Ohnishi K., Palazzi E., van Paradijs J., Pian E., Pollard K. R., Rattenbury N. J., Reid M., Rol E., Saito T., Sackett P. D., Saizar P., Tinney C., Vermaak P., Watson R., Williams A., Yock P., Dar A.

- ApJ 540, 74 (2000)

- Abstract:

We present the discovery observations of the optical counterpart of the gamma-ray burst GRB 990712 taken 4.16 hr after the outburst and discuss its light curve observed in the V, R, and I bands during the first 35 days after the outburst. The observed light curves were fitted with a power-law decay for the optical transient (OT), plus an additional component that was treated in two different ways. First, the additional component was assumed to be an underlying galaxy of constant brightness. The resulting slope of the decay is 0.97+0.05-0.02, and the magnitudes of the underlying galaxy are V=22.3+-0.05, R=21.75+-0.05, and I=21.35+-0.05. Second, the additional component was assumed to be a galaxy plus an underlying supernova with a time-variable brightness identical to that of GRB 980425, appropriately scaled to the redshift of GRB 990712. The resulting slope of the decay is similar, but the goodness of fit is worse, which would imply that either this GRB is not associated with an underlying supernova or the underlying supernova is much fainter than the supernova associated with GRB 980425. The galaxy in this case is fainter: V=22.7+-0.05, R=22.25+-0.05, and I=22.15+-0.05, and the OT plus the underlying supernova at a given time is brighter. Measurements of the brightnesses of the OT and the galaxy by late-time Hubble Space Telescope observation and ground-based observations can thus assess the presence of an underlying supernova.

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- Authors: Jensen B.L., Fynbo J.U., Gorosabel J., Hjorth J., Holland S., Møller P., Thomsen B., Bjoernsson G., Pedersen H., Burud I., Henden A., Tanvir N.R., Davis C.J., Vreeswijk P., Rol E., Hurley K., Cline T., Trombka J., McClanahan T., Starr R., Goldsten J., Castro-Tirado A.J., Greiner J., Bailer-Jones C.A.L., Kummel M., Mundt R.

- A&A 370, 909 (2001)

- Abstract:

We present Ulysses and NEAR data from the detection of the short or intermediate duration (2 s) gamma-ray burst GRB000301C (2000 March 1.41 UT). The gamma-ray burst (GRB) was localised by the Inter Planetary Network (IPN) and RXTE to an area of 50 arcmin^2. A fading optical counterpart was subsequently discovered with the Nordic Optical Telescope (NOT) about 42h after the burst. The GRB lies at the border between the long-soft and the short-hard classes of GRBs. If GRB000301C belongs to the latter class, this would be the first detection of an afterglow to a short-hard burst. We present UBRI and JHK photometry from the time of the discovery until 11 days after the burst. Finally, we present spectroscopic observations of the optical afterglow obtained with the ESO VLT Antu telescope 4 and 5 days after the burst. The optical light curve is consistent with being achromatic from 2 to 11 days after the burst and exhibits a break. A broken power-law fit yields a shallow pre-break decay power-law slope of α_1=-0.72+-0.06, a break time of t_b=4.39+-0.26 days after the burst, and a post-break slope of α_2=-2.29+-0.17, which is best explained by a sideways expanding jet in an ambient medium of constant mean density. In the optical spectrum we find absorption features that are consistent with FeII, CIV, CII, SiII and Ly-α at a redshift of 2.0404+-0.0008. We find evidence for a curved shape of the spectral energy distribution of the observed afterglow. It is best fitted with a power-law spectral distribution with index β=-0.7 reddened by an SMC-like extinction law with A_V=0.1 mag. Based on the Ly-α absorption line we estimate the HI column density to be log(N(HI))=21.2+-0.5. This is the first direct indication of a connection between GRB host galaxies and Damped Ly-a Absorbers.

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- Authors: Smette A., Fruchter A. S., Gull T. R., Sahu K. C., Petro L., Ferguson H., Rhoads J., Lindler D. J., Gibbons R., Hogg D. W., Kouveliotou C., Livio M., Macchetto D., Metzger M. R., Pedersen H., Pian E., Thorsett S. E., Wijers R. A. M. J., Fynbo J. P. U., Gorosabel J., Hjorth J., Jensen B. L., Levine A., Smith D. A., Cline T., Hurley K., Trombka J.

- ApJ, 555, 70 (2001)

- Abstract:

We present HST/STIS observations of the optical counterpart (OT) of the gamma-ray burster GRB 000301C obtained on 2000 March 6, five days after the burst. CCD clear aperture imaging reveals a R = 21.50+/-0.15 source with no apparent host galaxy. An 8000 s, 1150 < lambda(AA) < 3300 NUV-MAMA prism spectrum shows a relatively flat continuum (in f_lambda) between 2800 and 3300 AA, with a mean flux 8.7 (+0.8,-1.6)+/- 2.6 10^(-18) ergs/s/cm^2/A, and a sharp break centered at 2797+/-25 A. We interpret it as HI Lyman break at z = 2.067+/-0.025 indicating the presence of a cloud with a HI column density log(HI) > 18 on the line-of-sight to the OT. This value is conservatively a lower limit to the GRB redshift. However, the facts that large N(HI) system are usually considered as progenitors of present day galaxies and that other OTs are found associated with star forming galaxies strongly suggest that it is the GRB redshift. In any case, this represents the largest direct redshift determination of a gamma-ray burster to date. Our data are compatible with an OT spectrum represented by a power-law with an intrinsic index α = 1.2 (f_nu proportinal to nu^-α) and no extinction in the host galaxy or with α = 0.5 and extinction by a SMC-like dust in the OT rest-frame with A_V = 0.15. The large N(HI) and the lack of detected host is similar to the situation for damped Ly-α absorbers at z > 2.

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- Authors: Fynbo J.U., Møller P., Thomsen B.

- A&A 368, 408-413 (2001)

- Abstract:

In a ongoing programme aimed at studying galaxy counterparts of Damped Ly-α Absorbers (DLAs) we have obtained high resolution deep I-band imaging data of the field around the z(em) = 0.457 BAL QSO Q2112+059. In the literature this QSO is listed to have a candidate DLA at z(abs) = 0.2039 along the line of sight. After subtraction of the QSO Point Spread Function (PSF) we detect a galaxy centred on the position of Q2112+059. To help answer whether this galaxy is the DLA or the QSO host galaxy we retrieved a GHRS spectrum of Q2112+059 from the HST-archive. This spectrum shows that there is no Ly-α absorption line at z(abs) = 0.2039. This fact in combination with the perfect alignment on the sky of the galaxy and Q2112+059 lead us to the conclusion that the galaxy must be the host galaxy of Q2112+059. The host galaxy of Q2112+059 is bright (M_I^obs = -23.6), and has a radial profile well fitted by a {\it modified Hubble + de Vaucouleurs} profile with R_c = 0.5 kpc and R_e = 3.6 kpc. Our results are well in line with the conclusion of earlier work done at lower redshifts, that bright low redshift QSOs preferentially reside in luminous, elliptical galaxies. The host of Q2112+059 is however, despite it's brightness, very compact when compared to early type galaxies at lower redshifts.

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- Authors: Holland S., Fynbo J.U., Hjorth J., Gorosabel J., Pedersen H., Andersen M.I., Dar A., Thomsen B., Møller P., Bjoernsson G., Jaunsen A.O., Natarajan P., Tanvir N.

- A&A 371, 52 (2001)

- Abstract:

We present deep HST/STIS and ground-based photometry of the host galaxy of the gamma-ray burst GRB 980703 taken 17, 551, 710, and 716 days after the burst. We find that the host is a blue, slightly over-luminous galaxy with V_gal = 23.00 +/- 0.10, (V-R)_gal = 0.43 +/- 0.13, and a centre that is approximately 0.2 mag bluer than the outer regions of the galaxy. The galaxy has a star-formation rate of 8-13 M_sun/yr, assuming no extinction in the host. We find that the galaxy is best fit by a Sersic R^(1/n) profile with n = 1.0 and a half-light radius of 0.13 arcsec (= 0.72/h_100 proper kpc). This corresponds to an exponential disk with a scale radius of 0.22 arcsec (=1.21/h_100 proper kpc). Subtracting a fit with elliptical isophotes leaves large residuals, which suggests that the host galaxy has a somewhat irregular morphology, but we are unable to connect the location of GRB 980703 with any special features in the host. The host galaxy appears to be a typical example of a compact star forming galaxy similar to those found in the Hubble Deep Field North. The R-band light curve of the optical afterglow associated with this gamma-ray burst is consistent with a single power-law decay having a slope of α = -1.37 +/- 0.14. Due to the bright underlying host galaxy the late time properties of the light-curve are very poorly constrained. The decay of the optical light curve is consistent with a contribution from an underlying Type Ic supernova like SN1998bw, or a dust echo, but such contributions cannot be securely established.

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- Authors: Fynbo J.U., Jensen B.L., Gorosabel J., Hjorth J., Pedersen H., Møller P., Abbott T., Castro-Tirado A.J., Delgado D., Greiner J., Henden A., Magazzu A., Masetti N., Merlino S., Masegosa J., Oestensen R., Palazzi E., Pian E., Schwarz H.E., Cline T., Guidorzi C., Goldsten J., Hurley K., Mazets E., McClanahan T., Montanari E., Starr R., Trombka J.,

- A&A 369, 373 (2001)

- Abstract:

We present the discovery of the optical transient of the long-duration gamma-ray burst GRB000630. The optical transient was detected with the Nordic Optical Telescope 21.1 hours after the burst. At the time of discovery the magnitude of the transient was R = 23.04+-0.08. The transient displayed a power-law decline characterized by a decay slope of α = -1.035+-0.097. A deep image obtained 25 days after the burst shows no indication of a contribution from a supernova or a host galaxy at the position of the transient. The closest detected galaxy is a R=24.68+-0.15 galaxy 2.0 arcsec north of the transient. The magnitudes of the optical afterglows of GRB980329, GRB980613 and GRB000630 were all R>=23 less than 24 hours from the burst epoch. We discuss the implications of this for our understanding of GRBs without detected optical transients. We conclude that i) based on the gamma-ray properties of the current sample we cannot conclude that GRBs with no detected OTs belong to another class of GRBs than GRBs with detected OTs and ii) the majority (>75%) of GRBs for which searches for optical afterglow have been unsuccessful are consistent with no detection if they were similar to bursts like GRB000630 at optical wavelengths.

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- Authors: Møller P., Fynbo J.U.

- A&A 372, L57 (2001)

- Abstract:

The filamentary structure of the early universe has until now only been seen in numerical simulations. Despite this lack of direct observational evidence, the prediction of early filamentary structure formation in a Cold Dark Matter dominated universe has become a paradigm for our understanding of galaxy assembly at high redshifts. Clearly observational confirmation is required. Lyman Break galaxies are too rare to be used as tracers of filaments and we argue that to map out filaments in the high z universe, one will need to identify classes of objects fainter than those currently accessible via the Lyman Break technique. Objects selected via their Ly-α emission, and/or DLA absorbers, populate the faintest accessible part of the high redshift galaxy luminosity function, and as such make up good candidates for objects which will map out high redshift filaments. Here we present the first observational confirmation of a z=3.04 filament mapped by those classes of objects. The observations are the deepest yet to have been done in Ly-α imaging at high redshift, and they reveal a string of proto-galaxies spanning about 5 Mpc (20 Mpc comoving). Expanding the cosmological test proposed by Alcock & Paczynski (1979), we outline how observartions of this type can be used to determine Omega_Lambda at z=3.

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- Authors: Fynbo J.U., Gorosabel J., Dall T.H., Hjorth J., Pedersen H., Andersen M.I., Møller P., Holland S., Smail I., Kobayashi N., Rol E., Vreeswijk P., Burud I., Jensen B.L., Thomsen B., Henden A., Vrba F., Canzian B., Castro Ceron J.M., Castro-Tirado A.J., cline T., Goto M., Greiner J., Hanski M.T., Hurley K., Lund N., Pursimo T., Oestensen R., Solheim J., Tanvir N., Terada H.

- A&A 373, 796 (2001)

- Abstract:

We present the discovery of the Optical Transient (OT) of the long-duration gamma-ray burst GRB000926. The optical transient was detected independently with the Nordic Optical Telescope and at Calar Alto 22.2 hours after the burst. At this time the magnitude of the transient was R = 19.36. The transient faded with a decay slope of about 1.7 during the first two days after which the slope increased abruptly (within a few hours) to about 2.4. The light-curve started to flatten off after about a week indicating the presence of an underlying extended object. This object was detected in a deep image obtained one month after the GRB at R=23.87+-0.15 and consists of several compact knots within about 5 arcsec. One of the knots is spatially coincident with the position of the OT and hence most likely belongs to the host galaxy. Higher resolution imaging is needed to resolve whether all the compact knots belong to the host galaxy or to several independent objects. In a separate paper we present a discussion of the optical spectrum of the OT, and its inferred redshift (Møller et al. in prep.).

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- Authors: Fynbo J.U., Møller P., Thomsen B.

- A&A, 374, 443 (2001)

- Abstract:

We present spectroscopic observations obtained with the ESO Very Large Telecope (VLT) of seven candidate Ly-α emitting galaxies in the field of the radio quiet Q1205-30 at z=3.04 previously detected with deep narrow band imaging. Based on equivalent widths and limits on line ratios we confirm that all seven objects are Ly-α emitting galaxies. Deep images also obtained with the VLT in the B and I bands show that five of the seven galaxies have very faint continuum fluxes (I(AB) approx. 26.8 and B(AB) approx. 27.3). The star formation rates of these seven galaxies estimated from the rest-frame UV continuum around 2000AA, as probed by the I-band detections, as well as from the Ly-α luminosities, are 1-4 M_sun yr^{-1}. This is 1-3 orders of magnitude lower than for other known populations of star-forming galaxies at similar redshifts (the Lyman-Break galaxies and the sub-mm selected sources). The inferred density of the objects is high, 16+-4 per arcmin^2 per unit redshift. This is consistent with the integrated luminosity function for Lyman-Break galaxies down to R=27 if the fraction of Ly-α emitting galaxies is approx. 70% at the faint end of the luminosity function. However, if this fraction is 20% as reported for the bright end of the luminosity function then the space density in this field is significantly larger (by a factor of 3.5) than expected from the luminosity function for Lyman-Break galaxies in the HDF-North. This would be an indication that at least some radio quiet QSOs at high redshift reside in overdense environments or that the faint end slope of the high redshift luminosity function has been underestimated. These observations show that Ly-α emission is an efficient method by which to probe the faint end of the luminosity function at high redshifts.

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- Authors: Gorosabel J., Fynbo J.U., Hjorth J., Wolf C., Andersen M.I., Pedersen H., Christensen L., Jensen B.L., Møller P., Afonso J., Treyer M.A., Mallen-Ornelas G., Castro-Tirado A.J., Fruchter A., Greiner J., Pian E., Vreeswijk P., Frontera F., Kaper L., Klose S., Kouveliotou C., Masetti N., Palazzi E., Rol. E., Salamanca I., Tanvir N., Wijers R.A.M.J., van den Heuvel E.

- A&A, 384, 11-23 (2002)

- Abstract:

We report the discovery of the optical and near-infrared counterparts to GRB001011. The GRB001011 error box determined by Beppo-SAX was simultaneously imaged in the near-infrared by the 3.58-m New Technology Telescope and in the optical by the 1.54-m Danish Telescope 8 hr after the gamma-ray event. Here we implement the colour-colour discrimination technique proposed by Rhoads (2000) and extend it using near-IR data as well. We present the results provided by an automatic colour-colour discrimination pipe-line developed to discern the different populations of objects present in the GRB001011 error box. Our software revealed three candidates based on single-epoch images. Second-epoch observations carried out 3.2 days after the burst revealed that the most likely candidate had faded, thus identifying it with the counterpart to the GRB. In deep R-band images obtained 7 months after the burst a faint R=25.38+-0.25 elongated object, presumably the host galaxy of GRB001011, was detected at the position of the afterglow. The GRB001011 afterglow is the first discovered with the assistance of colour-colour diagram techniques. We discuss the advantages of using this method and its application to error boxes determined by future missions.

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- Authors: Gorosabel J., Andersen M.I., Hjorth J., Pedersen H., Jensen B.L., Fynbo J.U., Lehto H.J., Katajainen S., Hurley K., Lund N.

- A&A, 383, 112 (2002)

- Abstract:

We report optical observations of the short/hard burst GRB 010119 error box, one of the smallest error boxes reported to date for short/hard GRBs. Limits of R>22.3 and I>21.2 are imposed by observations carried out 20.31 and 20.58 hours after the gamma-ray event, respectively. They represent the most constraining limits imposed to date on the optical emission from a short/hard gamma-ray burst afterglow.

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- Authors: Møller P., Warren S.J., Fall S.M., Fynbo J.U., Jakobsen P.

- ApJ 574, 51-58 (2002)

- Abstract:

We use HST STIS and NICMOS images of three spectroscopically confirmed galaxy counterparts of high-redshift damped Ly-α absorbers (one of which is a new discovery) to test the hypothesis that high-redshift DLA galaxies are Lyman-break galaxies. If the hypothesis is correct the emission properties of DLA galaxies must lie within the range of emission properties measured for Lyman0break galaxies of similar absolute magnitude. This will be true regardless of selection bias in the sample of detected DLA galaxies. We test this prediction using several emission properties: half-light radius, radial profile (Sersic n parameter), optical-to-near-infared colour, morphology, Ly-α emission equivalent width, and Ly-α velocity structure. In all cases the measured values for the DLA galaxies lie within the range measured for the population of Lyman-break galaxies. None of the measurements is in conflict with the prediction. We conclude that the measured emission properties of the three DLA galaxies studied here are consistent with the conjecture that high-redshift DLA galaxies are Lyman-break galaxies. We show that this result does not conflict with the observation that the few high-redshift DLA galaxies discovered are mostly fainter than spectroscopically confirmed L* Lyman-break galaxies.

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- Authors: Fynbo J.P.U., Møller P., Thomsen B., Hjorth J., Gorosabel J., Andersen M.I., Egholm M.P., Holland S., Jensen B.L., Pedersen H., Weidinger M.

- A&A, 388, 425-438 (2002)


- Abstract:

We report on the results of deep narrow-band Ly-α and broad-band U and I imaging of the fields of two Gamma-Ray bursts at redshift z=2.04 (GRB 000301C and GRB 000926). We find that the host galaxy of GRB 000926 is an extended (more than 2 arcsec), strong Ly-α emitter with a rest-frame equivalent width of 71+20-15 AA. The galaxy consists of two main components and several fainter knots. GRB 000926 occurred in the western component, whereas most of the Ly-α luminosity (about 65%) originates in the eastern component. Using archival HST images of the host galaxy we measure the spectral slopes (f_lambda proportional to lambda^β) of the two components to β = -2.4+-0.3 (east) and -1.4+-0.2 (west). This implies that both components contain at most small amounts of dust, consistent with the observed strong Ly-α emission. The western component has a slightly redder V-I colour than the eastern component, suggesting the presence of at least some dust. We do not detect the host galaxy of GRB 000301C in neither Ly-α emission nor in U and I broad-band images. The strongest limit comes from combining the narrow and U-band imaging where we infer a limit of U(AB)>27.7 (2-sigma limit per arcsec^2). The upper limits on the Ly-α flux implies a Ly-α equivalent width upper limit of 150 AA. We find eleven and eight other galaxies with excess emission in the narrow filter in the fields of GRB 000301C and GRB 000926 respectively. These galaxies are candidate Ly-α emitting galaxies in the environment of the host galaxies. Based on these detections we conclude that GRB 000926 occurred in one of the strongest centres of star formation within several Mpc, whereas GRB 000301C occurred in an intrinsically very faint galaxy far from being the strongest centre of star formation in its galactic environment. Under the hypothesis that GRBs trace star formation, the wide range of GRB host galaxy luminosities implies a very steep faint end slope of the high redshift galaxy luminosity function.

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- Authors: Piro L., Frail D.A., Gorosabel J., Carmire G., Soffitta P., Amati L., Andersen M.I., Antonelli L.A., Berger E., Frontera F., Fynbo J.U., Gandolfi G., Garcia M.R., Hjorth J., in't Zand J., Jensen B.L., Masetti N., Møller P., Perdersen H., Pian E., Wieringa M.H,

- ApJ, 577, 680-690 (2002)

- Abstract:

The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet but it did not have a detected optical afterglow. It is therefore one of the events recently classified as dark GRBs or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear. Chandra observations allowed us to localize this GRB within 1" and a radio transient was detected with the VLA. We identify the likely (P=0.01) host galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the optical flux of this object is consistent with the above HI column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a Giant Molecular Cloud. We therefore conclude that either the gas is local to the GRB, but is condensed in small-scale high-density (n>10**9 cm-3) clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy. Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be substantially different from that observed in GRBs with optical afterglows.

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- Authors: Hjorth J., Thomsen B., Nielsen S.R., Andersen M.I., Holland S.T., Fynbo J.P.U., Pedersen H., Jaunsen A.O., Halpern J.P., Fesen R., Gorosabel J., Castro-Tirado A.J., McMahon R.G., Roenig M.D., Bjoernsson G., Amati L., Tanvir N.R., Natarajan P.

- ApJ, 576, 113-119 (2002)

- Abstract:

We report the identification of the optical afterglow of GRB 980613 in R- and I-band images obtained between 16 and 48 hours after the gamma-ray burst. Early near-infrared (NIR) H and K' observations are also reported. The afterglow was optically faint (R ~ 23) at discovery but did not exhibit an unusually rapid decay (power-law decay slope α < 1.8 at 2 sigma). The optical/NIR spectral index (β_RH < 1.1) was consistent with the optical-to-X-ray spectral index (β_RX ~ 0.6), indicating a maximal reddening of the afterglow of ~0.45 mag in R. Hence the dimness of the optical afterglow was mainly due to the fairly flat spectral shape rather than internal reddening in the host galaxy. We also present late-time HST/STIS images of the field in which GRB 980613 occurred, obtained 799 days after the burst. These images show that GRB 980613 was located close to a very compact, blue V = 26.1 object inside a complex region consisting of star-forming knots and/or interacting galaxy fragments. Therefore, GRB 980613 constitutes a strong case for the association of cosmological gamma-ray bursts with star-forming regions.

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- Authors: Weidinger M., Møller P., Fynbo J.P.U., Thomsen B., Egholm M.P.

- A&A, 391, 13-20 (2002)

- Abstract:

In this paper we present a scale free method to determine the cosmological parameters Omega_m,Omega_Lambda. The method is based on the requirement of isotropy of the distribution of orientations of cosmological filaments. The current structure formation paradigm predicts that the first structures to form are voids and filaments, causing a web-like structure of the matter distribution at high redshifts. Recent observational evidence suggests that the threads, or filaments, of the cosmic web most easily are mapped in Ly-α emission. We describe how such a 3D map can be used to constrain the cosmological parameters in a way which, contrary to most other cosmological tests, does not require the use of a standard rod or a standard candle. We perform detailed simulations in order to define the optimal survey parameters for the definition of an observing programme aimed to address this test, and to investigate how statistical and observational errors will influence the results. We conclude that observations should target filaments of comoving size 15-50 Mpc in the redshift range 2-4, and that each filament must be defined by at least four Ly-α emitters. Detection of 20 filaments will be sufficient to obtain a result, while 50 filaments will make it possible to place significant new constraints on the values of Omega_m and Omega_Lambda permitted by the current supernova observations. In a future paper we study how robust these conclusions are to systematic velocities in the survey box.

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- Authors: Jaunsen A.O., Andersen M.I., Hjorth J., Fynbo J.P.U., Holland S.T., Thomsen B., Gorosabel J., Schaefer B.E., Bjoernsson G., Natarajan P., Tanvir N.R.

- A&A 402, 125 (2003)

- Abstract:

As part of the HST/STIS GRB host survey program we present the detection of three faint gamma-ray burst (GRB) host galaxies based on an accurate localisation using ground-based data of the optical afterglows (OAs). A common property of these three hosts is their extreme faintness. The location at which GRBs occur with respect to their host galaxies and surrounding environments are robust indicators of the nature of GRB progenitors. The bursts studied here are among the four most extreme outliers, in terms of relative distance from the host center, in the recent comprehensive study of Bloom et al. 2002. We obtain a revised and much higher probability that the galaxies identified as hosts indeed are related to the GRBs (P(n_chance)=0.78, following Bloom et al. 2002), thereby strengthening the conclusion that GRBs are preferentially located in star-forming regions in their hosts. Apart from being faint, the three hosts consist of multiple structures, indicative of merging and active star-formation. For one of the hosts, GRB 980329, we estimate a photometric redshift of z~3.5.

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- Authors: Castro Ceron J.M., Castro-Tirado A.J., Gorosabel J., Hjorth J., Fynbo J.P.U., Jensen B.L. et al.

- A&A, 393, 445 (2002)

- Abstract:

We present optical follow up observations of the long GRB 001007 between 6.14 hr and 468 days after the event. An unusually bright optical afterglow (OA) was seen to decline following a steep achromatic power law with index alpha = -2.05+/-0.11, possibly indicating a break in the lightcurve at t-t0 < 3.5 days, as found in other bursts. Upper limits imposed by the LOTIS alerting system 6.14 hours after the gamma-ray event also suggests the presence of a knee in the optical lightcurve. The spectral index beta of the OA yields -1.24 +/- 0.57. These values may be explained by several fireball jet models and by the cannonball model. Fireball spherical expansion models are not favoured. Late epoch deep imaging revealed the presence of a complex host galaxy system, composed at least by two objects located 1.2" and 1.9" from the afteglow position.

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- Authors: Bjørnsson G., Hjorth J., K. Pedersen, Fynbo J.P.U.

- ApJL, 579, L59 (2002)

- Abstract:

The optical lightcurve of GRB 010222 exhibited one of the slowest decays of any gamma-ray burst to date. Its broadband properties have been difficult to explain with conventional afterglow models, as they require the power law index of the underlying electron energy distribution to be low, p < 2, or that the outflow is quasi-spherical thus reviving the energy problem. We argue that the slow decay of GRB 010222 and a linear polarization of 1.36+-0.64%, is naturally explained by a jet model with continuous energy injection. The electron eneergy distribution than has p=2.49+-0.05, fully consistent with the expectation from detailed modelling of acceleration in relativistic shocks, p > 2, thus alleviating the "p-problem".

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- Authors: Møller P., Fynbo J.P.U., Hjorth J., Thomsen B., Egholm M.P., Andersen M.I., Gorosabel J., Holland S.T., Jakobsson P., Jensen B.L., Pedersen H., Pedersen K., Weidinger M.

- A&AL, 396, L21 (2002)

- Abstract:

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- Authors: Holland S.T.,, Weidinger M., Fynbo J.P.U., Gorosabel J., Hjorth J., Pedersen K., Andersen M.I., Augusteijn T., Castro Ceron J.M., Castro-Tirado A., Dahle H., Egholm M.P., Jakobsson P., Jensen B.L., Levan A., Mendez J., Møller P., Pedersen H., Pursimo T., Ruiz-Lapeunte P., Thomsen B.

- AJ, 125, 2291 (2003)

- Abstract:

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- Authors: Jakobsson P., Hjorth J., Fynbo J.P.U., Gorosabel J., Pedersen K., Burud I., Levan A., Fruchter A., Rhoads J., Grav T., Hansen M.W., Michelsen R., Andersen M.I., Jensen B.L., Pedersen H., Thomsen B., Weidinger M., Bhargavi S.G., Cowsik R., Pandey S.B.

- A&A, 408, 941 (2003)

- Abstract:

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- Authors: Gorosabel J., Christensen L., Hjorth J., Fynbo J.P.U., Pedersen H., Jensen B.L., Andersen M.I., Lund N., Jaunsen A.O., Castro Ceron J.M., Castro-Tirado A., Fruchter A., Greiner J., Pian E., Vreeswijk P.M., Burud I., Frontera F., Kaper L., Klose S., Kouveliotou C., Masetti N., Palazzi E., Rhoads J., Rol E., Salamanca I., Tanvir N., Wijers R.A.M.J., van den Heuvel E.

- A&A, 400, 127 (2003)

- Abstract:

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- Authors: Masetti N., Palazzi E., Pian E., Simoncelli A., Hunt L.K., Maiorano E., Levan A., Christensen L., Rol E., Savaglio S., Falomo R., Castro-Tirado A.J., Hjorth J., Delsanti A., Pannella M., Mohan V., Pandey S.B., Sagar R., Amati L., Burud I., Castro Ceron J.M., Frontera F., Fruchter A.S., Fynbo J.P.U., Gorosabel J., Greiner J., Kaper L., Klose S., Kouveliotou C., Nicastro L., Pedersen H., Rhoads J., Salamanca I., Tanvir N., Vreeswijk P.M., Wijers R.A.M.J., van den Heuvel E.P.J.

- Accepted for publication in A&A (2003)

- Abstract:

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- Authors: Greiner J., Klose S., Salvato M., Zeh A., Schwarz R., Hartmann D.H., Masetti N., Stecklum B., Lamer G., Lodieu N., Scholz R.D., Sterken C., Gorosabel J., Burud I., Rhoads J., Mitrofanov I., Litvak M., Sanin A., Grinkov V., Andersen M.I., Castro Ceron J.M., Castro-Tirado A.J., Fruchter A., Fynbo J.P.U., Hjorth J., Kaper L., Kouveliotou C., Palazzi E., Pian E., Rol E., Salamanca I., Tanvir N.R., Vreeswijk P., Wijers R.A.M.J., van den Heuvel E.

- ApJ, 599, 1223 (2003)

- Abstract:

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- Authors: Rol E., Wijers R. A. M. J., Fynbo J.P.U., Hjorth J., Gorosabel J., Egholm M. P., Castro Ceron J. M., Castro-Tirado A. J., Kaper L., Masetti N., Palazzi E., Pian E., Tanvir N., Vreeswijl P., Kouveliotou C., Møller P., Pedersen H., Fruchter A. S., Rhoads J., Burud I., Salamanca I., van den Heuvel E. P. J.

- A&AL, 405, L23-L27 (2003)

- Abstract:

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- Authors: Fynbo J.P.U., Ledoux C., Møller P., Thomsen B., Burud I.

- A&A, 407, 147-157 (2003)

- Abstract:

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- Authors: Hjorth J., Møller P., Gorosabel J., Fynbo J.P.U., Toft S., Jaunsen A.O., Kaas A.A., Pursimo T., Torii K., Kato T., Yamaoka H., Yoshida A., Thomsen B., Andersen M.I., Burud I., Castro Ceron J.M., Castro-Tirado A.J., Fruchter A.S., Kaper L., Kouveliotou C., Masetti N., Palazzi E., Pedersen H., Pian E., Rhoads J., Rol E., Tanvir N.R., Vreeswijk P.M., Wijers R.A.M.J., van den Heuvel E.P.J.

- ApJ, 597, 699-705 (2003)

- Abstract:

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- Authors: Vreeswijk P., Møller P., Fynbo J.P.U.

- A&AL, 409, L5-L8 (2003)

- Abstract:

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- Authors: Fynbo J.P.U., Jakobsson P., Møller P., Hjorth J., Thomsen B., Andersen M.I., Fruchter A.S., Gorosabel J., Holland S.T., Ledoux C., Pedersen H., Rhoads J., Weidinger M., Wijers R.A.M.J.

- A&A Letters, 406, L63-L66 (2003)

- Abstract:

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- Authors: Gorosabel J., Klose S., Christensen L., Fynbo J.P.U., Hjorth J., Greiner J., Tanvir N., Jensen B.L., Pedersen H., Holland S.T., Lund N., Jaunsen A.O., Castro Ceron J.M., Castro-Tirado A.J., Fruchter A., Pian E., Vreeswijk P., Burud I., Frontera F., Kaper L., Kouveliotou C., Masetti N., Palazzi E., Rhoads J., Rol E., Salamanca I., Wijers R.A.M.J.,, van den Heuvel E.

- A&A, 409, 123-133 (2003)

- Abstract:

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- Authors: Vreeswijk P.M., Ellison S.L., Ledoux C., Wijers R.A.M.J., Møller P., Henden A., Hjorth J., Masi G., Rol E., Tanvir N., Levan A., Fynbo J.P.U., Castro Ceron J.M., Gorosabel J., Castro-Tirado A.J., Fruchter A.S., Kouveliotou C., Burud I., Rhoads J., Masetti N., Palazzi E., Pian E., Pedersen H., Andersen M.I., Kaper L., Gilmore A., Kilmartin P., Buckle J.V., Seigar M.S., van den Heuvel E.P.J.

- A&A, 419, 927-940 (2004)

- Abstract:

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- Authors: Castro-Tirado A.J. , Gorosabel J., Guziy S. et al.

- A&A, 411, L315-L319 (2003)

- Abstract:

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- Authors: Jakobsson P., Hjorth J., Ramirez-Ruiz E., Kouveliotou C., Pedersen K., Fynbo J.P.U., Gorosabel J., Watson D., Jensen B.L., Grav T., Hansen M.W., Michelsen R., Andersen M.I., Pedersen H., Weidinger M.

- New Astronomy, 9, 435 (2004)

- Abstract:

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- Authors: Castro Ceron, J.M., Gorosabel, J., Castro-Tirado, A.J. et al.,

- A&A, 424, 833 (2004)

- Abstract:

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- Authors: Jakobsson, P., Hjorth, J., Fynbo, J.P.U. et al.,

- Submitted to A&A (2004)

- Abstract:

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- Authors: Fynbo, J.P.U. et al.,

- Submitted to ApJ (2004)

- Abstract:

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- Authors: Jakobsson P. et al.,

- Submitted to ApJ (2005)

- Abstract:

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- Authors: Jakobsson et al.,

- published in ApJL (2005)

- Abstract:

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- Authors: Monaco P et al.

- A&A in press (2005)

- Abstract:

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