Engineering topics

Rol, E.

Publications and source records attributed to Rol, E..

13 records

NASA NTRS2008

Gamma-Ray Burst Afterglows as Probes of Environment and Blast Wave Physics. II. The Distribution of rho and Structure of the Circumburst Medium

We constrain blast wave parameters and the circumburst media ofa subsample of 10 BeppoSAX gamma-ray bursts (GRBs). For this sample we derive the values of the injected electron energy distribution index, p, and the density structure index of the circumburst medium, k, from simultaneous spectral fits to their X-ray, optical, and NIR afterglow data. The spectral fits have been done in count space and include the effects ofmetallicity, and are compared with the previously reported optical and X-ray temporal behavior. Using the blast wave model and some assumptions which include on-axis viewing and standard jet structure, constant blast wave energy, and no evolution of the microphysical parameters, we find a mean value ofp for the sample as a whole of 9.... oa -0.003.0" 2 a_ statistical analysis of the distribution demonstrates that the p-values in this sample are inconsistent with a single universal value forp at the 3 _ level or greater, which has significant implications for particle acceleration models. This approach provides us with a measured distribution ofcircumburst density structures rather than considering only the cases of k ----0 (homogeneous) and k - 2 (windlike). We find five GRBs for which k can be well constrained, and in four of these cases the circumburst medium is clearly windlike. The fifth source has a value of 0 < k < 1, consistent with a homogeneous circumburst medium.

Starling, R. L. C.↗

NASA NTRS2007

Gamma-Ray Burst Afterglows as Probes of Environment and Blastwave Physics II: The Distribution of p and Structure of the Circumburst Medium

We constrain blastwave parameters and the circumburst media of a subsample of BeppoSAX Gamma-Ray Bursts. For this sample we derive the values of the injected electron energy distribution index, p, and the density structure index of the circumburst medium, k, from simultaneous spectral fits to their X-ray, optical and nIR afterglow data. The spectral fits have been done in count space and include the effects of metallicity, and are compared with the previously reported optical and X-ray temporal behaviour. Assuming the fireball model, we can find a mean value of p for the sample as a whole of 2.035. A statistical analysis Of the distribution demonstrates that the p values in this sample are inconsistent with a single universal value for p at the 3sigma level or greater. This approach provides us with a measured distribution of circumburst density structures rather than considering only the cases of k = 0 (homogeneous) and k = 2 (wind-like). We find five GRBs for which k can be well constrained, and in four of these cases the circumburst medium is clearly wind-like. The fifth source has a value of 0 less than or equal to k less than or equal to 1, consistent with a homogeneous circumburst medium.

Starling, R. L. C.↗

NASA NTRS2006

Gamma-Ray Burst Afterglows as Probes of Environment and Blastwave Physics: Absorption by Host Galaxy Gas and Dust - 1

We use a new approach to obtain limits on the absorbing columns towards an initial sample of 10 long Gamma-Ray Bursts observed with BeppoSAX and selected on the basis of their good optical and nIR coverage, from simultaneous fits to nIR, optical and X-ray afterglow data, in count space and including the effects of metallicity. In no cases is a MIV-like ext,inction preferred, when testing MW, LMC and SMC extinction laws. The 2175A bump would in principle be detectable in all these afterglows, but is not present in the data. An SMC-like gas-to-dust ratio or lower value can be ruled out for 4 of the hosts analysed here (assuming Sh4C metallicity and extinction law) whilst the remainder of the sample have too large an error to discriminate. We provide a more accurate estimate of the line-of-sight extinction and improve upon the uncertainties for the majority of the extinction measurements made in previous studies of this sample. We discuss this method to determine extinction values in comparison with the most commonly employed existing methods.

Starling, R. L. C.↗

NASA NTRS2006

Gamma-ray Burst Afterglows as Probes of Environment and Blastwave Physics: Absorption by Host Galaxy Gas and Dust, Circumburst Media and the Distribution of P

We use a new approach to obtain limits on the absorbing columns towards a sample of 10 Gamma-ray Bursts observed by BeppoSAX from simultaneous fits to X-ray, optical and IR data, in counts space and including the effects of metallicity. For half the afterglows the best-fitting model to the SED includes SMC-like extinction (as opposed to LMC or MW) and in one LMC-like extinction, and in no cases is there a preference for MW-like extinction. Gas-to-dust ratios generally do not match those of the 3 standard and most well-known extinction models of SMC, LMC and MW, but tend to be higher. We compare the results from this method to those of previous works using other methods. We constrain the jet models for a subsample of the bursts by constraining the cooling break position and power law spectral slopes, allowing the injected electron energy index to be measured. We derive secure values of p from our spectral fits and comparison with the temporal optical and X-ray slopes for 4 afterglows. The mean of these single value, suggesting that either external factors such as circumburst medium play a strong role or that the microphysics is not identical for each GRB. For GRB 971214 we find that the circumburst medium has a wind-like density profile and the cooling frequency appears to be moving to higher frequencies.

Starling, R. L. C.↗

NASA NTRS2005

Low-resolution VLT Spectroscopy of GRBs 991216, 011211, 021211 and 030328

We present low-resolution VLT spectroscopy of the aftergiow position of the gamma ray bursts 991216, 011211, 021211 and 030328. The spectmm of GRB 991216 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, more uncertain, system may be detected at z = 0.77. HST imaging of the field obtained 4 months later, show two amorphous regions of emission, one at the projected OT position, the presumed host galaxy at z = 1.02, and the other 0"6 away. All significant lines in the spectrum of GRB 011211 are identified with lines originating in a single absorption system at z = 2.142 plus or minus 0.002, the redshift of the GRB 011211 host galaxy. We also detect Lya in the host, for which we fit a neutral hydrogen column density of log N(HI)=20.4 plus or minus 0.2, which indicates that it is a damped Lya system. For GRB021211, we detect a single emission line in a spectrum tens of days after the burst, which we identify as [OII] at z = 1.006. The galaxy l"5 away from the afterglow location has z = 0.800, and is therefore unrelated to the GRB. Finally, for GRB030328 at least two absorption systems are required to explain all significant lines: one at z = 1.522, the likely redshift of the GRB, and the other at z = 1.295. For the latter system we only detect two lines, and we consider the reality of this system to be uncertain.

Vreeswijk, P. M.↗

NASA NTRS2003

The Radio Afterglow of GRB030329 at Centimetre Wavelengths: Evidence for Multiple Jets or a Structured Jet

We present our centimetre wavelength (1.4, 2.3 and 4.9 GHz) light curves of the afterglow of GRB030329, which were obtained with the Westerbork Synthesis Radio Telescope. Modelling the data according to a collimated afterglow results in a jet-break time t(sub j) of 17 days. This is in contrast with earlier results obtained at higher frequencies, which indicate t(sub j) to be around 10 days. Furthermore, with respect to the afterglow model, some additional flux at the lower frequencies is present when these light curves reach their maximum. We subsequently show that the afterglow can be modelled with two or more components with progressively later jet breaks. From these results we infer that the jet is in fact a structured or a layered jet, where the ejecta with lower Lorentz factors produce additional flux which becomes visible at late times in the lowest frequency bands.

Rol, E.↗

NASA NTRS2001

First Detection of a Foreground Damped Ly-Alpha Absorber Along a GRB Line of Sight?

We present a VLT spectrum of the optical afterglow of GRB 991216, taken 1.5 days after the burst, and HST (Hubble Space Telescope) imaging of the host galaxy, obtained four months later. The spectrum contains three metal absorption-line systems with redshifts z = 1.024, z = 0.803, and z = 0.771, where the highest redshift most likely reflects the distance to the host galaxy. For the z = 1.024 and z = 0.803 systems we tentatively detect MgI which suggests a dense environment at these redshifts. This and the strength of the z = 0.803 Fe lines indicate that this system very likely is a damped Ly-alpha absorber (DLA), which would be the first foreground DLA to be detected along a GRB afterglow sight line. The HST images are consistent with these findings: they show two blobs of light, one underneath the projected OT position, the presumed host galaxy, and the other 0.6" away, which is probably responsible for the absorption lines at z = 0.803. The lowest redshift system can be explained by either one of the two galaxies that are located roughly 2" away from the transient. Including these newly found systems, the total number of DLAS and Lyman limit systems along GRB afterglow sight lines is consistent with the number expected from QSO (quasi-stellar object) absorption line studies. We expect early spectroscopy of GRB afterglows to significantly increase the number of detected foreground absorption systems, and we discuss some advantages over QSO lines of sight.

Vreeswijk, P. M.↗

NASA NTRS2000

GRB970228: A Supernova Gamma-Ray Burst

We present B-, V-, R(sub c)-, I(sub c)-, J-, H-, and K-band observations of the optical transient (OT) associated with GRB970228 based on a reevaluation of published data. and present yet unpublished data. In order to minimize small calibration differences we collected and analyzed most of the photometry and determined the magnitude of the OT relative to a set of secondary field stars. We confirm that the early decay of the light curves (before March 6. 1997) was faster than that at later times (between March 6 and April 7. 1997). The early-time observations of GRB 970228 are consistent with relativistic blast wave models but the late-time observations are hard to understand in this framework. The observations are well explained by an initial power law decay with alpha = -1.46 +/- 0.33 modified at later times by a type-I(sub c) supernova light curve. together with the evidence for GRB980326 and GRB 980425 this is further evidence that at least some GRBs are associated with an unusual class of core-collapse supernovae.

Galama, T. J.↗

NASA NTRS2000

Spectral Evolution in GRB 990510

We present time-resolved spectroscopy of the afterglow of GRB 990510. Through the identification of several absorption lines in the first epoch spectrum, we determine the redshift for this burst to be z >= 1.6190 +/- 0.0016. No clear emission lines are detected. From the absence of the Ly.alpha drop, we can put an upper limit to the redshift of z <= 2.3. We study the time evolution of the MgII absorption line in our spectra taken 0.8 and 3.9 days after the burst, whose equivalent width (E.W.) is expected to change in case the burst resides in a dense compact medium (Perna & Loeb 1998). We measure an E.W. of 2.5 /- 0.2 and 2.3 +/- 0.6 in the spectra 0.8 and 3.8 days after the burst, respectively. Our results suggest that the atoms responsible for the absorption are not in the vicinity of the site of the burst.

Vreeswijk, P. M.↗

NASA NTRS2000

GRB 990712: First Detection of Polarization Variability in a Gamma-ray Burst Afterglow

We report the detection of significant polarization in the afterglow of GRB 990712 on three instances 0.44-1.45 days after the gamma-ray burst. This polarization is intrinsic to the afterglow. The degree of polarization is not constant, and smallest at the second measurement. The polarization angle does not vary significantly during these observations. We find that none of the existing models predict such polarization variations constant polarization angle, and discuss ways in which these models might be modified to accommodate the observed behavior of this afterglow.

Rol, E.↗

NASA NTRS1999

The Effect of Magnetic Fields on Gamma-Ray Bursts Inferred from Multi-Wavelength Observations of the Bursts of 23 January 1999

Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature at which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of gamma-ray, optical, infrared, submillimeter, millimeter and radio observations of the burst ORB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.

Galama, T. J.↗

NASA NTRS1999

Spectral Energy Distributions and Light Curves of GRB 990123 and its Afterglow

Investigations of the 'fireball' model currently believed to explain the prompt gamma-ray and afterglow emissions of gamma-ray bursts. On January 23 a gamma-ray burst (GRB) occurred for which for the first time prompt optical emission was detected. We here report the results of gamma-ray, optical/infrared, sub-mm, mm and radio observations of this burst and its afterglow, which indicate that the prompt and afterglow emissions from GRB 990123 are associated with three distinct regions in the fireball. The afterglow synchrotron spectrum one day after the burst has a much lower peak frequency than those of previous bursts; this explains the short-lived nature of the radio emission, which is not expected to reappear. We suggest that such differences reflect variations in the magnetic-field strengths in the afterglow emitting regions.

Galama, T. J.↗

NASA NTRS1999

Spectral Energy Distributions and Light Curves of GRB 990123 and Its Afterglow

Gamma-ray bursts (GRBs) are thought to result from the interaction of an extremely relativistic outflow interacting with a small amount of material surrounding the site of the explosion. Multi-wavelength observations covering the gamma-ray to radio wavebands allow investigations of this "fireball" model. On 23 January 1999 optical emission was detected while the gamma-ray burst was still underway. Here we report the results of gamma-ray, optical/infra-red, sub-mm, mm and radio observations of this burst and its afterflow, which indicate that the prompt and afterflow emissions from GRB 990123 are associated with three distinct regions in the fireball. The afterglow one day after the burst has a much lower peak frequency than those of previous bursts; this explains the short-lived nature of the radio emission, which is not expected to reapear. We suggest that such differences reflect variations in the magnetic-field strengths in the afterglow emitting regions.

Galama, T. J.↗