اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدSoft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM
103
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Two-thirds of long duration gamma-ray bursts (GRBs) show soft X-ray absorption in excess of the Milky Way. The column densities of metals inferred from UV and optical spectra differ from those derived from soft X-ray spectra, at times by an order of magnitude, with the latter being higher. The origin of the soft X-ray absorption excess observed in GRB X-ray afterglow spectra remains a heavily debated issue, which has resulted in numerous investigations on the effect of hot material both internal and external to the GRB host galaxy on our X-ray afterglow observations. Nevertheless, all models proposed so far have either only been able to account for a subset of our observations (i.e. at z > 2), or they have required fairly extreme conditions to be present within the absorbing material. In this paper, we investigate the absorption of the GRB afterglow by a collisionally ionised and turbulent interstellar medium (ISM). We find that a dense (3 per cubic centimeters) collisionally ionised ISM could produce UV/optical and soft X-ray absorbing column densities that differ by a factor of 10, however the UV/optical and soft X-ray absorbing column densities for such sightlines and are 2-3 orders of magnitude lower in comparison to the GRB afterglow spectra. For those GRBs with a larger soft X-ray excess of up to an order of magnitude, the contribution in absorption from a turbulent ISM as considered here would ease the required conditions of additional absorbing components, such as the GRB circumburst medium and intergalactic medium.
قيم البحث
اقرأ أيضاً
We present a study of the C K-edge using high-resolution LETGS Chandra spectra of four novae during their super-soft-source (SSS) phase. We identified absorption lines due to C~ii K$alpha$, C~iii K$alpha$ and C~ iii K$beta$ resonances. We used these
astronomical observations to perform a benchmarking of the atomic data, which involves wavelength shifts of the resonances and photoionization cross-sections. We used improved atomic data to estimate the C~ii and C~iii column densities. The absence of physical shifts for the absorption lines, the consistence of the column densities between multiple observations and the high temperature required for the SSS nova atmosphere modeling support our conclusion about an ISM origin of the respective absorption lines. Assuming a collisional ionization equilibrium plasma the maximum temperature derived from the ratio of C~ii/C~iii column densities of the absorbers correspond to $T_{max}< 3.05times10^{4}$ K.
We present a detailed analysis of the gaseous component of the Si K edge using high-resolution Chandra spectra of low-mass X-ray binaries. We fit the spectra with amodified version of the ISMabs model, including new photoabsorption cross sectionscomp
uted for all Si ionic species. We estimate column densities for Si i, Si ii, Si iii, Si xii and Si xiii, which trace the warm, intermediate temperature and hot phases of the Galactic interstellar medium. We find that the ionic fractions of the first two phases are similar. This may be due to the physical state of the plasma determined by the temperature or to the presence of absorber material in the close vicinity of the sources. Our findings highlight the need for accurate modeling of the gaseous component before attempting to address the solid component.
X-ray absorption of $gamma$-ray burst (GRB) afterglows is prevalent yet poorly understood. X-ray derived neutral hydrogen column densities ($N_{rm H}$) of GRB X-ray afterglows show an increase with redshift, which might give a clue for the origin of
this absorption. We use more than 350 X-ray afterglows with spectroscopic redshift ($z$) from the Swift XRT repository as well as over 100 Ly,$alpha$ absorption measurements in $z>1.6$ sources. The observed trend of the average optical depth $tau$ at 0.5 keV is consistent with both a sharp increase of host $N_{rm H}(z)$, and an absorbing diffuse intergalactic medium, along with decreasing host contribution to $tau$. We analyze a sub-sample of high-$z$ GRBs with $N_{rm H}$ derived both from the X-ray afterglow and the Ly,$alpha$ line. The increase of X-ray derived $N_{rm H}(z)$ is contrasted by no such increase in the Ly,$alpha$ derived column density. We argue that this discrepancy implies a lack of association between the X-ray and Ly,$alpha$ absorbers at high-$z$. This points towards the X-ray absorption at high $z$ being dominated by an intervening absorber, which lends credibility to an absorbing intergalactic medium contribution.
We propose to study cosmic reionization using absorption line spectra of high-redshift Gamma Ray Burst (GRB) afterglows. We show that the statistics of the dark portions (gaps) in GRB absorption spectra represent exquisite tools to discriminate among
different reionization models. We then compute the probability to find the largest gap in a given width range [Wmax, Wmax + dW] at a flux threshold Fth for burst afterglows at redshifts 6.3 < z < 6.7. We show that different reionization scenarios populate the (Wmax, Fth) plane in a very different way, allowing to distinguish among different reionization histories. We provide here useful plots that allow a very simple and direct comparison between observations and model results. Finally, we apply our methods to GRB 050904 detected at z = 6.29. We show that the observation of this burst strongly favors reionization models which predict a highly ionized intergalactic medium at z~6, with an estimated mean neutral hydrogen fraction xHI = 6.4 pm 0.3 times 10^-5 along the line of sight towards GRB 050904.
Interstellar abundance determinations from fits to X-ray absorption edges often rely on the incorrect assumption that scattering is insignificant and can be ignored. We show instead that scattering contributes significantly to the attenuation of X-ra
ys for realistic dust grain size distributions and substantially modifies the spectrum near absorption edges of elements present in grains. The dust attenuation modules used in major X-ray spectral fitting programs do not take this into account. We show that the consequences of neglecting scattering on the determination of interstellar elemental abundances are modest; however, scattering (along with uncertainties in the grain size distribution) must be taken into account when near-edge extinction fine structure is used to infer dust mineralogy. We advertise the benefits and accuracy of anomalous diffraction theory for both X-ray halo analysis and near edge absorption studies. An open source Fortran suite, General Geometry Anomalous Diffraction Theory (GGADT), is presented that calculates X-ray absorption, scattering, and differential scattering cross sections for grains of arbitrary geometry and composition.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
