arXiv subjects

J. P. U. Fynbo

Publications and source records attributed to J. P. U. Fynbo.

At least 271 records · Page 15Linked to original sources

arXiv2006

Star formation rates and stellar masses in z ~ 1 gamma ray burst hosts

We analyse 4.5, 8 and 24 um band Spitzer images of six gamma ray burst host galaxies at redshifts close to 1. We constrain their star formation rates (SFR) based on the entire available spectral energy distribution rather than the 24 um band only. Further, we estimate their stellar masses (M*) based on rest frame K band luminosities. Our sample spans a wide range of galaxy properties: derived SFRs range from less than 10 to a few hundred solar masses per year; values of M* range from 10^9 to 10^10 Mo with a median of 5.6 x 10^9 Mo. Comparing the specific star formation rate (PHI = SFR/M*) of our sample as a function of M* to other representative types of galaxies (distant red galaxies, Ly-alpha emitters, Lyman break galaxies, submillimeter galaxies and z ~ 2 galaxies from the Great Observatories Origins Deep Survey-North field), we find that gamma ray burst hosts are among those with the highest PHI.

astro-ph↗

arXiv2006

Fundamental Properties of GRB-Selected Galaxies: A Swift/VLT Legacy Survey

We present the motivation, aims and preliminary result from the Swift/VLT legacy survey on gamma-ray burst host galaxies. This survey will produce a homogeneous and well-understood host sample covering more than 95% of the lookback time to the Big Bang, and allow us to characterize their fundamental properties.

astro-ph↗

arXiv2006

A log N(HI) = 22.6 DLA in a dark gamma-ray burst: the environment of GRB 050401

The optical afterglow spectrum of GRB050401 (at z=2.8992+/-0.0004) shows the presence of a DLA, with log(nHI)=22.6+/-0.3. This is the highest column density ever observed in a DLA, and is about five times larger than the strongest DLA detected so far in any QSO spectrum. From the optical spectrum, we also find a very large Zn column density, allowing us to infer an abundance of [Zn/H]=-1.0+/-0.4. These large columns are supported by the X-ray spectrum from Swift-XRT which shows a column density (in excess of Galactic) of log(nH)=22.21^{+0.06}_{-0.08} assuming solar abundances (at z=2.9). The comparison of this X-ray column density, which is dominated by absorption due to alpha-chain elements, and the HI column density derived from the Ly-alpha absorption line, allows us to derive a metallicity for the absorbing matter of [alpha/H]=-0.4+/-0.3. The optical spectrum is reddened and can be well reproduced with a power-law with SMC extinction, where A_V=0.62+/-0.06. But the total optical extinction can also be constrained in a way which is independent of the shape of the extinction curve: from the optical-to-X-ray spectral energy distribution we find, 0.5<~A_V<~4.5. However, even this upper limit, independent of the shape of the extinction curve, is still well below the dust column that is inferred from the X-ray column density, i.e. A_V=9.1^{+1.4}_{-1.5}. This discrepancy might be explained by a small dust content with high metallicity (low dust-to-metals ratio). `Grey' extinction cannot explain the discrepancy since we are comparing the metallicity to a measurement of the total extinction (without reference to the reddening). Little dust with high metallicity may be produced by sublimation of dust grains or may naturally exist in systems younger than a few hundred Myr.

astro-ph↗

arXiv2006

HI column densities of z > 2 Swift gamma-ray bursts

Context: Before the launch of the Swift satellite, the majority of the gamma-ray burst (GRB) afterglows for which Ly-alpha was redshifted into the observable spectrum showed evidence for a damped Ly-alpha absorber. This small sample indicated that GRBs explode either in galaxies, or regions within them, having high neutral hydrogen column densities. Aims: To increase the spectroscopic sample of GRBs with z > 2 and hence establish the N(HI) distribution along GRB lines-of-sight. Methods: We have obtained six z > 2 GRB afterglow spectra and fitted the Ly-alpha absorption line in each case to determine N(HI). This has been complemented with 12 other Swift N(HI) values from the literature. Results: We show that the peak of the GRB N(HI) distribution is qualitatively consistent with a model where GRBs originate in Galactic-like molecular clouds. However, a systematic difference, in particular an excess of low column-density systems compared to the predictions, indicates that selection effects and conditions within the cloud (e.g. strong ionization) influence the observed N(HI) range. We also report the discovery of Ly-alpha emission from the GRB 060714 host, corresponding to a star-formation rate of approximately 0.8 M_Sun/year. Finally, we present accurate redshifts of the six bursts: z = 3.240 +/- 0.001 (GRB 050319), z = 2.198 +/- 0.002 (GRB 050922C), z = 3.221 +/- 0.001 (GRB 060526), z = 3.425 +/- 0.002 (GRB 060707), z = 2.711 +/- 0.001 (GRB 060714) and z = 3.686 +/- 0.002 (GRB 060906).

astro-ph↗

arXiv2006

Optical, Infrared, and Ultraviolet Observations of the X-Ray Flash GRB 050416A

We present ultraviolet, optical, and infrared photometry of the afterglow of the X-ray flash XRF 050416A taken between approximately 100 seconds and 36 days after the burst. We find an intrinsic spectral slope between 1930 and 22,200 Angstrom of beta = -1.14 +/- 0.20 and a decay rate of alpha = -0.86 +/- 0.15. There is no evidence for a change in the decay rate between approximately 0.7 and 4.7 days after the burst. Our data implies that there is no spectral break between the optical and X-ray bands between 0.7 and 4.7 days after the burst, and is consistent with the cooling break being redward of the K_s band (22,200 Angstrom) at 0.7 days. The combined ultraviolet/optical/infrared spectral energy distribution shows no evidence for a significant amount of extinction in the host galaxy along the line of sight to XRF 050416A. Our data suggest that the extragalactic extinction along the line of sight to the burst is only approximately A_V = 0.2 mag, which is significantly less than the extinction expected from the hydrogen column density inferred from $X$-ray observations of XRF 050416A assuming a dust-to-gas ratio similar to what is found for the Milky Way. The observed extinction, however, is consistent with the dust-to-gas ratio seen in the Small Magellanic Cloud. We suggest that XRF 050416A may have a two-component jet similar to what has been proposed for GRB 030329. If this is the case the lack of an observed jet break between 0.7 and 42 days is an illusion due to emission from the wide jet dominating the afterglow after approximately 1.5 days.

astro-ph↗

arXiv2006

Are short gamma-ray bursts collimated? GRB050709, a flare but no break

From the small sample of afterglow lightcurves of short duration gamma-ray bursts (GRBs), the decays are rapid, roughly following a power-law in time. It has been assumed that the afterglow emission in short GRBs is collimated in jets in the same way as in long GRBs. An achromatic break in a short GRB afterglow lightcurve would therefore be strong evidence in favour of collimation in short GRBs. We examine the optical lightcurve of the afterglow of the short GRB 050709, the only short GRB where a jet break has been claimed from optical data. We show that (1) the decay follows a single power-law from 1.4 to 19 days after the burst and has a decay index alpha = 1.73_{-0.04}^{+0.11}, (2) that an optical flare at ~10 days is required by the data, roughly contemporaneous with a flare in the X-ray data, and (3) that there is no evidence for a break in the lightcurve. This means that so far there is no direct evidence for collimation in the outflows of short GRBs. The available limits on the collimation angles in short GRBs now strongly suggest much wider opening angles than found in long GRBs.

astro-ph↗

arXiv2006

Velocity-Metallicity Correlation for high-z DLA Galaxies: Evidence for a Mass-Metallicity Relation?

We used our database of VLT-UVES quasar spectra to build up a sample of 70 Damped Lyman-alpha (DLA) or strong sub-DLA systems with total neutral hydrogen column densities of log N(HI)>20 and redshifts in the range 1.7 2.43) and the lower (z_abs<2.43) redshift halves of our sample. However, the two populations of systems are statistically different. There is a strong redshift evolution in the sense that the mean metallicity and mean velocity width increase with decreasing redshift. We argue that the existence of a DLA velocity-metallicity correlation, over more than a factor of 100 spread in metallicity, is probably the consequence of an underlying mass-metallicity relation for the galaxies responsible for DLA absorption lines. Assuming a simple linear scaling of the galaxy luminosity with the mass of the dark-matter halo, we find that the slope of the DLA velocity-metallicity relation is consistent with that of the luminosity-metallicity relation derived for local galaxies. [...] abridged.

astro-ph↗

arXiv2006

Long gamma-ray bursts and core-collapse supernovae have different environments

When massive stars exhaust their fuel they collapse and often produce the extraordinarily bright explosions known as core-collapse supernovae. On occasion, this stellar collapse also powers an even more brilliant relativistic explosion known as a long-duration gamma-ray burst. One would then expect that long gamma-ray bursts and core-collapse supernovae should be found in similar galactic environments. Here we show that this expectation is wrong. We find that the long gamma-ray bursts are far more concentrated on the very brightest regions of their host galaxies than are the core-collapse supernovae. Furthermore, the host galaxies of the long gamma-ray bursts are significantly fainter and more irregular than the hosts of the core-collapse supernovae. Together these results suggest that long-duration gamma-ray bursts are associated with the most massive stars and may be restricted to galaxies of limited chemical evolution. Our results directly imply that long gamma-ray bursts are relatively rare in galaxies such as our own Milky Way.

astro-ph↗

arXiv2006

Supernova 2006aj and the associated X-Ray Flash 060218

We have studied the afterglow of the gamma-ray burst (GRB) of February 18, 2006. This is a nearby long GRB, with a very low peak energy, and is therefore classified as an X-ray Flash (XRF). XRF 060218 is clearly associated with a supernova -- dubbed SN 2006aj. We present early spectra for SN 2006aj as well as optical lightcurves reaching out to 50 days past explosion. Our optical lightcurves define the rise times, the lightcurve shapes and the absolute magnitudes in the U, V and R bands, and we compare these data with data for other relevant supernovae. SN 2006aj evolved quite fast, somewhat similarly to SN 2002ap, but not as fast as SN 1994I. Our spectra show the evolution of the supernova over the peak, when the U-band portion of the spectrum rapidly fades due to extensive line blanketing. We compare to similar spectra of very energetic Type Ic supernovae. Our first spectra are earlier than spectra for any other GRB-SN. The spectrum taken 12 days after burst in the rest frame is similar to somewhat later spectra of both SN 1998bw and SN 2003dh, implying a rapid early evolution. This is consistent with the fast lightcurve. From the narrow emission lines from the host galaxy we derive a redshift of z=0.0331+-0.0007. This makes XRF 060218 the second closest gamma-ray burst detected. The flux of these emission lines indicate a high-excitation state, and a modest metallicity and star formation rate of the host galaxy.

astro-ph↗

arXiv2006

Probing Cosmic Chemical Evolution with Gamma-Ray Bursts: GRB060206 at z=4.048

Aim: We present early optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 with the aim of determining the metallicity of the GRB absorber and the physical conditions in the circumburst medium. We also discuss how GRBs may be important complementary probes of cosmic chemical evolution. Method: Absorption line study of the GRB afterglow spectrum. Results: We determine the redshift of the GRB to be z=4.04795+/-0.00020. Based on the measurement of the neutral hydrogen column density from the damped Lyman-alpha line and the metal content from weak, unsaturated Sii lines we derive a metallicity of [S/H] =-0.84+/-0.10. This is one of the highest metallicities measured from absorption lines at z~4. From the very high column densities for the forbidden Siii*, Oi*, and Oi** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H$_2$ molecules with logN(H_2) ~ 17.0, translating into a molecular fraction of logf \~ -3.5 with f=2N(H_2)/(2N(H_2)+ N(Hi)). Even if GRBs are only formed by single massive stars with metallicities below ~0.3Z(solar), they could still be fairly unbiased tracers of the bulk of the star formation at z>2. Hence, metallicities as derived for GRB060206 here for a complete sample of GRB afterglows will directly show the distribution of metallicities for representative star-forming galaxies at these redshifts.

astro-ph↗

arXiv2006

Physics of the GRB 030328 afterglow and its environment

We report on the photometric, spectroscopic and polarimetric, monitoring of the optical afterglow of Gamma-Ray Burst (GRB) 030328 detected by HETE-2. Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices alpha_1 = 0.76 +/- 0.03, alpha_2 = 1.50 +/- 0.07 and a break at t_b = 0.48 +/- 0.03 days after the GRB, provides the best fit of the optical afterglow decline. This shape is interpreted as due to collimated emission, for which we determine a jet opening angle theta_{jet} of about 3.2 degrees. An achromatic bump starting around 0.2 d after the GRB is possibly marginally detected in the optical light curves. Optical spectroscopy shows the presence of two rest-frame ultraviolet metal absorption systems at z = 1.5216 +/- 0.0006 and at z = 1.295 +/- 0.001, the former likely associated with the GRB host galaxy. Analysis of the absorption lines at z = 1.5216 suggests that the host of this GRB may be a Damped Lyman-alpha Absorber. The optical V-band afterglow appears polarized, with P= (2.4 +/- 0.6) % and theta = (170 +/- 7) degrees, suggesting an asymmetric blastwave expansion. An X-ray-to-optical spectral flux distribution of the GRB 030328 afterglow was obtained at 0.78 days after the GRB and fitted using a broken powerlaw, with an optical spectral slope beta_{opt} = 0.47 +/- 0.15, and an X-ray slope beta_{X} = 1.0 +/- 0.2. The discussion of these results in the context of the "fireball model" shows that the preferred scenario for this afterglow is collimated structured jet with fixed opening angle in a homogeneous medium.

astro-ph↗

arXiv2006

GRB 050814 at z = 5.3 and the Redshift Distribution of Swift GRBs

We report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that z = 5.3 +/- 0.3. We next present a carefully selected sample of 19 Swift GRBs, intended to estimate in an unbiased way the GRB redshift distribution, including the mean redshift (z_mean) as well as constraints on the fraction of high-redshift bursts. We find that z_mean = 2.7 and that at least 5% of the GRBs originate at z > 5. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. The high mean redshift of this GRB sample and the wide redshift range clearly demonstrates the suitability of GRBs as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe.

astro-ph↗

arXiv2006

Evidence for a Supernova Associated with the X-ray Flash 020903

We present ground-based and Hubble Space Telescope optical observations of the X-ray flash (XRF) 020903, covering 300 days. The afterglow showed a very rapid rise in the first day, followed by a relatively slow decay in the next few days. There was a clear bump in the light curve after ~25 days, accompanied by a drastic change in the spectral energy distribution. The light curve and the spectral energy distribution are naturally interpreted as the emergence -- and subsequent decay -- of a supernova (SN), similar to SN 1998bw. At peak luminosity, the SN is estimated to be 0.8 +/- 0.1 mag fainter than SN1998bw. This argues in favor of the existence of a supernova associated with this X-ray flash. A spectrum obtained 35 days after the burst shows emission lines from the host galaxy. We use this spectrum to put an upper limit on the oxygen abundance of the host at [O/H] < -0.6 dex. We also discuss a possible trend between the softness of several bursts and the early behavior of the optical afterglow, in the sense that XRFs and X-ray rich GRBs seem to have a plateau phase or even a rising light curve. This can be naturally explained in models where XRFs are similar to GRBs but seen off the jet axis.

astro-ph↗

arXiv2005

Outshining the quasars at reionisation: The X-ray spectrum and lightcurve of the redshift 6.29 Gamma-Ray Burst GRB050904

Gamma-ray burst (GRB) 050904 is the most distant X-ray source known, at z=6.295, comparable to the farthest AGN and galaxies. Its X-ray flux decays, but not as a power-law; it is dominated by large variability from a few minutes to at least half a day. The spectra soften from a power-law with photon index Gamma=1.2 to 1.9, and are well-fit by an absorbed power-law with possible evidence of large intrinsic absorption. There is no evidence for discrete features, in spite of the high signal-to-noise ratio. In the days after the burst, GRB 050904 was by far the brightest known X-ray source at z>4. In the first minutes after the burst, the flux was >10^{-9} erg cm^-2 s^-1 in the 0.2-10keV band, corresponding to an apparent luminosity >10^5 times larger than the brightest AGN at these distances. More photons were acquired in a few minutes with Swift-XRT than XMM-Newton and Chandra obtained in ~300 ks of pointed observations of z>5 AGN. This observation is a clear demonstration of concept for efficient X-ray studies of the high-z IGM with large area, high-resolution X-ray detectors, and shows that early-phase GRBs are the only backlighting bright enough for X-ray absorption studies of the IGM at high redshift.

astro-ph↗

arXiv2005

A mean redshift of 2.8 for Swift gamma-ray bursts

The exceptionally high luminosities of gamma-ray bursts (GRBs), gradually emerging as extremely useful probes of star formation, make them promising tools for exploration of the high-redshift Universe. Here we present a carefully selected sample of Swift GRBs, intended to estimate in an unbiased way the GRB mean redshift (z_mean), constraints on the fraction of high-redshift bursts and an upper limit on the fraction of heavily obscured afterglows. We find that z_mean = 2.8 and that at least 7% of GRBs originate at z > 5. In addition, consistent with pre-Swift observations, at most 20% of afterglows can be heavily obscured. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. We also report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that z = 5.3 +/- 0.3. The high mean redshift of GRBs and their wide redshift range clearly demonstrates their suitability as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe.

astro-ph↗

arXiv2005

The galaxies in the field of the nearby GRB980425/SN1998bw

We present spectroscopic observations of ESO 184-G82, the host galaxy of GRB980425/SN1998bw, and six galaxies in its field. A host redshift of z=0.0087+/-0.0006 is derived, consistent with that measured by Tinney et al. (1998). Redshifts are obtained for the six surrounding galaxies observed. Three of these galaxies lie within 11 Mpc of each other, confirming the suggestion that some of these galaxies form a group. However, all of the field galaxies observed lie at significantly greater distances than ESO 184-G82 and are therefore not associated with it. The host galaxy of GRB980425/SN1998bw thus appears to be an isolated dwarf galaxy and interactions with other galaxies do not seem to be responsible for its star formation.

astro-ph↗

arXiv2005

Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211

We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at z=0.80 and z=1.02, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at z=0.77. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0.6" away. The spectrum shows a depression in flux in between 4000 A and 5500 A. This could be the result of a 2175 A-type extinction feature in the host of GRB991216, but at a rather red wavelength of 2360 A. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB011211 are identified with lines originating in a single absorption system at z=2.142+/-0.002, the redshift of the GRB011211 host galaxy. We also detect the Lyman alpha absorption line in the host, to which we fit a neutral hydrogen column density of log N(HI)=20.4+/-0.2, which indicates that it is a damped Lyman alpha system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB011211 redshift. For GRB021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [OII] 3727 at z=1.006. The corresponding unobscured [OII] star-formation rate is 1.4 Msun/yr.

astro-ph↗

arXiv2005

The host galaxy cluster of the short gamma-ray burst GRB 050509B

The first arcsecond localization of a short gamma-ray burst, GRB 050509B, has enabled detailed studies of a short burst environment. We here report on studies of the environment of GRB 050509B using the Swift X-ray Telescope (XRT). The XRT error circle of the burst overlaps with an elliptical galaxy in the cluster of galaxies ZwCl 1234.0+02916. Based on the measured X-ray flux of the cluster we estimate that the probability for a chance superposition of GRB 050509B and a cluster at least as X-ray bright as this cluster is $< 2\times 10^{-3}$, presenting the first strong case of a short burst located in a cluster of galaxies. We also consider the case for GRB 050509B being located behind ZwCl 1234.0+02916 and gravitationally lensed. From the velocity dispersion of the elliptical galaxy and the temperature of hot intracluster gas, we model the mass distribution in the elliptical galaxy and the cluster, and calculate the gravitational lensing magnification within the XRT error circle. We find that, if GRB050509B would be positioned significantly behind the cluster, it is most likely magnified by a factor less than two, but that the burst could be strongly lensed if it is positioned within 2 arcsec of the center of the bright elliptical galaxy. Further mapping of arcsecond size short burst error boxes is a new promising route to determine the spatial distribution of old stars throughout the Universe.

astro-ph↗