552 research outputs found
The peak luminosity - peak energy correlation in GRBs
We derive the peak luminosity - peak energy (L_iso - E_peak) correlation
using 22 long Gamma-Ray Bursts (GRBs) with firm redshift measurements. We find
that its slope is similar to the correlation between the time integrated
isotropic emitted energy E_iso and E_peak (Amati et al. 2002). For the 15 GRBs
in our sample with estimated jet opening angle we compute the collimation
corrected peak luminosity L_gamma, and find that it correlates with E_peak.
This has, however, a scatter larger than the correlation between E_peak and
E_gamma (the time integrated emitted energy, corrected for collimation;
Ghirlanda et al. 2004), which we ascribe to the fact that the opening angle is
estimated through the global energetics. We have then selected a large sample
of 442 GRBs with pseudo--redshifts, derived through the lag-luminosity
relation, to test the existence of the L_iso-E_peak correlation. With this
sample we also explore the possibility of a correlation between time resolved
quantities, namely L_iso,p and the peak energy at the peak of emission
E_peak,p.Comment: 5 pages, 5 figures, 2 tables - MNRAS Letters submitte
GRB 990413: Insight into the thermal phase evolution
GRB 990413 shows a very hard spectrum (with a low energy spectral component
F(E) \propto E^{2.49}) which is well represented by a black body model with
characteristic temperature ~70 keV. It thus belongs to the subset of GRBs which
might be revealing a thermal phase. We find that the temperature/luminosity
evolution is consistent with that found in the other ``thermal'' GRBs. The time
resolved spectral analysis indicates the presence of a second non--thermal
component contributing (for about 1 s) up to 30 per cent of the total flux.
Differently from the other thermal GRBs, GRB 990413 shows significantly high
level of variability and the evolution of the thermal/non--thermal spectral
components is strongly correlated with the flux variations. This GRB thus
offers the unique opportunity to test the standard fireball photospheric and
internal shock phases and their reciprocal influence. GRB 990413 was not
selected on the basis of its spectrum and thus hints to the possibility that
this early behavior might be more common than currently known.Comment: 6 pages, 5 figures, 2 tables. Accepted for publication in MNRAS
Lette
Behind the dust curtain: the spectacular case of GRB 160623A
We report on the X-ray dust-scattering features observed around the afterglow
of the gamma ray burst GRB 160623A. With an XMM-Newton observation carried out
~2 days after the burst, we found evidence of at least six rings, with angular
size expanding between ~2 and 9 arcmin, as expected for X-ray scattering of the
prompt GRB emission by dust clouds in our Galaxy. From the expansion rate of
the rings, we measured the distances of the dust layers with extraordinary
precision: 528.1 +\- 1.2 pc, 679.2 +\- 1.9 pc, 789.0 +\- 2.8 pc, 952 +\- 5 pc,
1539 +\- 20 pc and 5079 +\- 64 pc. A spectral analysis of the ring spectra,
based on an appropriate dust-scattering model (BARE-GR-B from Zubko et al.
2004}) and the estimated burst fluence, allowed us to derive the column density
of the individual dust layers, which are in the range 7x10^20-1.5x10^22 cm^-2.
The farthest dust-layer (i.e. the one responsible for the smallest ring) is
also the one with the lowest column density and it is possibly very extended,
indicating a diffuse dust region. The properties derived for the six
dust-layers (distance, thickness, and optical depth) are generally in good
agreement with independent information on the reddening along this line of
sight and on the distribution of molecular and atomic gas.Comment: 9 pages, 10 figures, 1 table; accepted for publication in MNRA
Cosmological implications of Compton tails in long duration GRB
The recent suggestion of the possible presence of a significant amount of material (Thomson optical depth ∼ 1) at rest and at a typical distance of ∼ 1015 cm with respect to the GRB is presented. The relevance of such interpretation for GRB energetics and its cosmological implications is outlined
Long Gamma-Ray Bursts and Their Host Galaxies at High Redshift
Motivated by the recent observational and theoretical evidence that long
Gamma-Ray Bursts (GRBs) are likely associated with low metallicity, rapidly
rotating massive stars, we examine the cosmological star formation rate (SFR)
below a critical metallicity Z_crit Z_sun/10 - Z_sun/5, to estimate the event
rate of high-redshift long GRB progenitors. To this purpose, we exploit a
galaxy formation scenario already successfully tested on a wealth of
observational data on (proto)spheroids, Lyman break galaxies, Lyman alpha
emitters, submm galaxies, quasars, and local early-type galaxies. We find that
the predicted rate of long GRBs amounts to about 300 events/yr/sr, of which
about 30 per cent occur at z>~6. Correspondingly, the GRB number counts well
agree with the bright SWIFT data, without the need for an intrinsic luminosity
evolution. Moreover, the above framework enables us to predict properties of
the GRB host galaxies. Most GRBs are associated with low mass galaxy halos
M_H<~10^11 M_sun, and effectively trace the formation of small galaxies in such
halos. The hosts are young, with age smaller than 5*10^7 yr, gas rich, but
poorly extincted (A_V<~0.1) because of their chemical immaturity; this also
implies high specific SFR and quite extreme alpha-enhancement. Only the
minority of hosts residing in large halos with M_H>~10^12 M_sun have larger
extinction (A_V~0.7-1), SFRs exceeding 100 M_sun/yr and can be detected at
submm wavelengths. Most of the hosts have UV magnitudes in the range -20
<~M_1350<~ -16, and Lyman alpha luminosity in the range 2*10^40
<~L_Lya<~2*10^42 erg/s. GRB hosts are thus tracing the faint end of the
luminosity function of Lyman break galaxies and Lyman alpha emitters.Comment: 11 pages, 8 figures, uses mn2e.cls. Minor changes. In press on MNRA
A Strong Lexical Matching Method for the Machine Comprehension Test
Machine comprehension of text is theoverarching goal of a great deal of re-search in natural language processing. TheMachine Comprehension Test (Richard-son et al., 2013) was recently proposed toassess methods on an open-domain, exten-sible, and easy-to-evaluate task consistingof two datasets. In this paper we developa lexical matching method that takes intoaccount multiple context windows, ques-tion types and coreference resolution. Weshow that the proposed method outper-forms the baseline of Richardson et al.(2013), and despite its relative simplicity,is comparable to recent work using ma-chine learning. We hope that our approachwill inform future work on this task. Fur-thermore, we argue that MC500 is harderthan MC160 due to the way question an-swer pairs were created
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