اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدA Survey for NV Absorption at z~z_GRB in GRB Afterglow Spectra: Clues to Gas Near the Progenitor Star
95
0
0.0
(
0
)
تأليف
Jason X. Prochaska
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We survey NV absorption in the afterglow spectra of long-duration gamma-ray bursts (GRBs) with the intent to study highly ionized gas in the galaxies hosting these events. We identify a high incidence (6/7) of spectra exhibiting NV gas with z~z_GRB and the majority show large column densities NV > 10^14 cm^-2. With one exception, the observed line-profiles are kinematically `cold, i.e. they are narrow and have small velocity offset (Dv < 20 km/s) from absorption lines associated with neutral gas. In addition, the NV absorption has similar velocity as the UV-pumped fine-structure lines indicating these high ions are located within ~1kpc of the GRB afterglow. These characteristics are unlike those for NV gas detected in the halo/disk of the Milky Way or along sightlines through high z damped Lya systems but resemble the narrow absorption line systems associated with quasars and some high z starbursts. We demonstrate that GRB afterglows photoionize nitrogen to NV at r~10pc. This process can produce NV absorption with characteristics resembling the majority of our sample and and we argue it is the principal mechanism for NV along GRB sightlines. Therefore, the observations provide a snapshot of the physical conditions at this distance. In this scenario, the observations imply the progenitors stellar wind is confined to r<10pc which suggests the GRB progenitors occur within dense (n > 10^3 cm^-3) environments, typical of molecular clouds. The observations, therefore, primarily constrain the physical conditions -- metallicity, density, velocity fields -- of the gas within the (former) molecular cloud region surrounding the GRB.
قيم البحث
اقرأ أيضاً
We report results from a comprehensive follow-up observing campaign of the afterglow of GRB 030226, including VLT spectroscopy, VLT polarimetry, and Chandra X-ray observations. In addition, we present BOOTES-1 wide-field observations at the time of t
he occurrence of the burst. First observations at ESO started 0.2 days after the event when the GRB afterglow was at a magnitude of R~19 and continued until the afterglow had faded below the detection threshold (R>26). No underlying host galaxy was found. The optical light curve shows a break around 0.8 days after the burst, which is achromatic within the observational errors, supporting the view that it was due to a jetted explosion. Close to the break time the degree of linear polarization of the afterglow light was less than 1.1%, which favors a uniform jet model rather than a structured one. VLT spectra show two absorption line systems at redshifts z=1.962+/-0.001 and at z=1.986+/-0.001, placing the lower limit for the redshift of the GRB close to 2. We emphasize that the kinematics and the composition of the absorbing clouds responsible for these line systems is very similar to those observed in the afterglow of GRB 021004. This corroborates the picture in which at least some GRBs are physically related to the explosion of a Wolf-Rayet star.
We present early WHT ISIS optical spectroscopy of the afterglow of gamma-ray burst GRB 050730. The spectrum shows a DLA system with the highest measured hydrogen column to date: N(HI) = 22.1 +/- 0.1 at the third-highest GRB redshift z = 3.968. Our an
alysis of the Swift XRT X-ray observations of the early afterglow show X-ray flares accompanied by decreasing X-ray absorption. From both the optical and the X-ray spectra we constrain the dust and gas properties of the host galaxy. We find the host to be a low metallicity galaxy, with low dust content. Much of the X-ray absorbing gas is situated close to the GRB, whilst the HI absorption causing the DLA is most likely located further out.
We study the chemical abundances of the interstellar medium surrounding high z gamma-ray bursts (GRBs) through analysis of the damped Lya systems (DLAs) identified in afterglow spectra. These GRB-DLAs are characterized by large HI column densities N(
HI) and metallicities [M/H] spanning 1/100 to nearly solar, with median [M/H]>-1. The majority of GRB-DLAs have [M/H] values exceeding the cosmic mean metallicity of atomic gas at z>2, i.e. if anything, the GRB-DLAs are biased to larger metallicity. We also observe (i) large [Zn/Fe] values (>+0.6) and sub-solar Ti/Fe ratios which imply substantial differential depletion, (ii) large a/Fe ratios suggesting nucleosynthetic enrichment by massive stars, and (iii) low C^0/C^+ ratios (<10^{-4}). Quantitatively, the observed depletion levels and C^0/C^+ ratios of the gas are not characteristic of cold, dense HI clouds in the Galactic ISM. We argue that the GRB-DLAs represent the ISM near the GRB but not gas directly local to the GRB (e.g. its molecular cloud or circumstellar material). We compare these observations with DLAs intervening background quasars (QSO-DLAs). The GRB-DLAs exhibit larger N(HI) values, higher a/Fe and Zn/Fe ratios, and have higher metallicity than the QSO-DLAs. We argue that the differences primarily result from galactocentric radius-dependent differences in the ISM: GRB-DLAs preferentially probe denser, more depleted, higher metallicity gaslocated in the inner few kpc whereas QSO-DLAs are more likely to intersect the less dense, less enriched, outer regions of the galaxy. Finally, we investigate whether dust obscuration may exclude GRB-DLA sightlines from QSO-DLA samples; we find that the majority of GRB-DLAs would be recovered which implies little observational bias against large N(HI) systems.
We present a survey for neutral atomic-carbon (CI) along gamma-ray burst (GRB) sightlines, which probes the shielded neutral gas-phase in the interstellar medium (ISM) of GRB host galaxies at high redshift. We compile a sample of 29 medium- to high-r
esolution GRB optical afterglow spectra spanning a redshift range through most of cosmic time from $1 < z < 6$. We find that seven ($approx 25%$) of the GRBs entering our statistical sample have CI detected in absorption. It is evident that there is a strong excess of cold gas in GRB hosts compared to absorbers in quasar sightlines. We investigate the dust properties of the GRB CI absorbers and find that the amount of neutral carbon is positively correlated with the visual extinction, $A_V$, and the strength of the 2175 AA dust extinction feature, $A_{mathrm{bump}}$. GRBs with CI detected in absorption are all observed above a certain threshold of $log N$(HI)$/mathrm{cm}^{-2}$ + [X/H] > 20.7 and a dust-phase iron column density of $log N$(Fe)$_{mathrm{dust}}/mathrm{cm}^{-2}$ > 16.2. In contrast to the SED-derived dust properties, the strength of the CI absorption does not correlate with the depletion-derived dust properties. This indicates that the GRB CI absorbers trace dusty systems where the dust composition is dominated by carbon-rich dust grains. The observed higher metal and dust column densities of the GRB CI absorbers compared to H$_2$- and CI-bearing quasar absorbers is mainly a consequence of how the two absorber populations are selected, but is also required in the presence of intense UV radiation fields in actively star-forming galaxies.
Excited Si^+ and Fe^+ species are routinely observed in the host environment of gamma-ray burst (GRB) afterglows, but are not commonly seen in other extragalactic locations. Their presence signals unusual properties in the gaseous environment of thes
e GRB hosts that arise either as a result of the intense ionizing radiation of the afterglow or through collision excitation in a dense cloud. In particular, the photon pumping scenario has explicit expectations for temporal variation in the strength of the excited lines, owing to the decline in the ionizing flux of the GRB afterglow. We analyze afterglow spectra of GRB 020813 obtained in two epochs of ~16 hours apart, and examine transitions from the first excited state of Fe^+ at J=7/2 in these two sets of data. We report a significant decline by at least a factor of five in the equivalent width of the FeII 2396 transition, the strongest from the J=7/2 state. We perform a Monte-Carlo analysis and determine that this temporal variation is present at more than 3sigma level of significance. This observation represents the first detection in the temporal variation of the excited Fe+ states in the GRB host ISM, a direct influence of the burst itself on its environment. We further estimate that the Fe^+ gas resides in 50-100 pc from the afterglow, based on the afterglow lightcurve and the presence and absence of the excited FeII 2396 in the two-epoch observations.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
