astro-grb

P. Noterdaeme, P. Laursen, P. Petitjean, S. D. Vergani, M.-J. Maureira, C. Ledoux, J. P. U. Fynbo, S. López, R. Srianand

We present the detection of Ly-alpha, [OIII] and H-alpha emission associated
with an extremely strong DLA system (N(HI) = 10^22.10 cm^-2) at z=2.207 towards
the quasar SDSS J113520-001053. This is the largest HI column density ever
measured along a QSO line of sight, though typical of what is seen in GRB-DLAs.
This absorption system also classifies as ultrastrong MgII system with
W2796_r=3.6 A. The mean metallicity of the gas ([Zn/H]=-1.1) and dust depletion
factors ([Zn/Fe]=0.72, [Zn/Cr]=0.49) are consistent with (and only marginally
larger than) the mean values found in the general QSO-DLA population. The
[OIII]-Ha emitting region has a very small impact parameter with respect to the
QSO line of sight, b=0.1″, and is unresolved. From the Ha line, we measure
SFR=25 Msun/yr. The Ly-a line is double-peaked and is spatially extended. More
strikingly, the blue and red Ly-a peaks arise from distinct regions extended
over a few kpc on either side of the star-forming region. We propose that this
is the consequence of Ly-a transfer in outflowing gas. The presence of
starburst-driven outflows is also in agreement with the large SFR together with
a small size and low mass of the galaxy (Mvir~10^10 Msun). From the stellar UV
continuum luminosity of the galaxy, we estimate an age of at most a few 10^7
yr, again consistent with a recent starburst scenario. We interpret the data as
the observation of a young, gas rich, compact starburst galaxy, from which
material is expelled through collimated winds powered by the vigorous star
formation activity. We substantiate this picture by modelling the radiative
transfer of Ly-a photons in the galactic counterpart. Though our model (a
spherical galaxy with bipolar outflowing jets) is a simplistic representation
of the true gas distribution and velocity field, the agreement between the
observed and simulated properties is particularly good. [abridged]

http://arxiv.org/abs/1202.0280

L.P. Xin, A. Pozanenko, D. A. Kann, D. Xu, J. Gorosabel, G. Leloudas, J. Y. Wei, M. Andreev, S. F. Qin, M. Ibrahimov, X. H. Han, A. de Ugarte Postigo, Y. L. Qiu, J. S. Deng, A. Volnova, P. Jakobsson, A. J. Castro-Tirado, F. Aceituno, J. P. U. Fynbo, J. Wang, R. Sanchez-Ramirez, V. Kouprianov, W. K. Zheng, J. C. Tello, C. Wu

The energy injection model is usually proposed to interpret the shallow-decay
phase in Swift GRB X-ray afterglows. However, very few GRBs have simultaneous
signatures of energy injection in their optical and X-ray afterglows. Here, we
report optical observations of GRB 090529A from 2000 sec to $\sim10^6$ sec
after the burst, in which an achromatic decay is seen at both wavelengths. The
optical light curve shows a decay from 0.37 to 0.99 with a break at $\sim10^5$
sec. In the same time interval, the decay indices of the X-ray light curve
changed from 0.04 to 1.2. Comparing these values with the closure relations,
the segment after 3$\times10^{4}$ sec is consistent with the prediction of the
forward shock in an ISM medium without any energy injection. The shallow-decay
phase between 2000 to 3$\times10^{4}$ sec could be due to the external shock in
a wind-type-like medium with an energy injection under the condition of $\nu_o
< \nu_c < \nu_x$. However, the constraint of the spectral region is not well
consistent with the multi-band observations. For this shallow-decay phase,
other models are also possible, such as energy injection with evolving
microphysical parameters, or a jet viewed off-axis,etc.

http://arxiv.org/abs/1202.0347

V. C. Busti, R. C. Santos, J. A. S. Lima

The existence of inhomogeneities in the observed Universe modifies the
distance-redshift relations thereby affecting the results of cosmological tests
in comparison to the ones derived assuming spatially uniform models. By
modeling the inhomogeneities through a generalized
Zeldovich-Kantowski-Dyer-Roeder (ZKDR) approach which is phenomenologically
characterized by a smoothness parameter $\alpha$, we rediscuss the constraints
on the cosmic parameters based on Supernovae type Ia and Gamma-Ray Bursts
(GRBs) data. The present analysis is restricted to a flat $\Lambda$CDM model
with the reasonable assumption that $\Lambda$ does not clump. A
$\chi^{2}$-analysis using 557 SNe Ia data from the Union2 Compilation Data
(Amanullah {\it et al.} 2010) constrains the pair of parameters ($\Omega_m,
\alpha$) to $\Omega_m=0.27_{-0.03}^{+0.08}$($2\sigma$) and $\alpha \geq 0.25$.
A similar analysis based only on 59 Hymnium GRBs (Wei 2010) constrains the
matter density parameter to be $\Omega_m= 0.35^{+0.62}_{-0.24}$ ($2\sigma$)
while all values for the smoothness parameter are allowed. By performing a
joint analysis, it is found that $\Omega_m = 0.27^{+0.06}_{-0.03}$ and $\alpha
\geq 0.52$. As a general result, although considering that current GRB data
alone cannot constrain the smoothness $\alpha$ parameter our analysis provides
an interesting cosmological probe for dark energy even in the presence of
inhomogeneities.

http://arxiv.org/abs/1202.0449

Michele Trenti, Rosalba Perna, Emily M. Levesque, J. Michael Shull, John T. Stocke

Measuring the star formation rate (SFR) at high redshift is crucial for
understanding cosmic reionization and formation of galaxies and black holes.
Two common complementary approaches are Lyman-Break-Galaxy (LBG) surveys for
large samples, and Gamma-Ray-Burst (GRB) observations for sensitivity to SFR in
small galaxies. The z>4 GRB-inferred SFR is higher than the LBG rate, but the
origin of the difference is difficult to understand, as both methods rely on
several modeling assumptions. Using a physically motivated galaxy luminosity
function model, with star formation in dark-matter halos with virial
temperature T_vir>2e4 K (M_DM>2e8 M_sun), we show that GRB and LBG-derived SFRs
are consistent if GRBs extend to faint galaxies (M_AB<-11). To test star
formation below the detection limit L_lim~0.05L^*_{z=3} of LBG surveys, we
propose to measure the fraction f_det(L>L_lim,z) of GRB hosts with L>L_lim.
This fraction quantifies the missing star formation fraction in LBG surveys and
constrains the mass-suppression scale for galaxy formation, with weak
dependence on modeling assumptions. Because f_det(L>L_lim,z) corresponds to the
ratio of star formation rates derived from LBG and GRB surveys, if these
estimators are unbiased, measuring f_det(L>L_lim,z) also constrains the
redshift evolution of the GRB production rate per unit mass of star formation,
typically modeled as (1+z)^beta. Our analysis predicts significant success in
detecting GRB hosts at z~5 with f_det(L>L_lim,z)~0.4, but rarer detections at
z>6. A GRB-host survey reaching M_AB=-18 at z>4 (within reach of Hubble Space
Telescope observations) will elucidate the nature and properties of star
formation sites during the epoch of reionization.

http://arxiv.org/abs/1202.0010

S. Basa, J. G. Cuby, S. Savaglio, S. Boissier, B. Clement, H. Flores, D. Le Borgne, A. Mazure

Long duration Gamma-Ray Bursts (GRBs) allow us to explore the distant
Universe, and are potentially the best tracer of the most distant objects. Our
current knowledge of the properties of GRB host galaxies at redshifts > 5 is
very scarce, and more observations of high-redshift hosts are required to
better understand their properties and the implications of using GRBs as probes
of the high-redshift universe. We performed very deep photometric observations
of three high-redshift GRB host galaxies, GRB 080913 at z = 6.7, GRB 060927 at
z = 5.5 and GRB 060522 at z = 5.1. Our FORS2 and HAWK-I observations at the VLT
targeted the rest-frame UV continuum of these galaxies, allowing us to
constrain their star formation rates. In addition, we performed deep
spectroscopic observations of the GRB080913 host galaxy with X-Shooter at the
VLT to search for Ly-{\alpha} emission. For the sake of the discussion, we make
use of published results on another GRB host, GRB 050904 at z = 6.3. The sample
of GRB host galaxies studied in this paper consists of 4 out of the 6
spectroscopically confirmed GRB host galaxies at z > 5. Despite being the
deepest observations ever reported of high-redshift GRB host galaxies, we do
not detect any of the hosts, neither in photometry nor in spectroscopy in the
case of GRB 080913. These observations indicate that the GRB host galaxies seem
to evolve with time and to have lower star formation rates (SFR) at z > 5 than
they have at z < 2. In addition, the host galaxy of GRB 080913 at z = 6.7 does
not show Ly-{\alpha} emission. While the measured properties of the galaxies in
our sample are in agreement with the properties of the general galaxy
population at z > 5, our observations are not sensitive enough to allow us to
infer further conclusions on whether this specific population is representative
of the general one.

http://arxiv.org/abs/1201.6383

N. R. Tanvir, A. J. Levan, A. S. Fruchter, J. P. U. Fynbo, J. Hjorth, K. Wiersema, M. N. Bremer, J. Rhoads, P. Jakobsson, P. T. O'Brien, E. R. Stanway, D. Bersier, P. Natarajan, J. Greiner, D. Watson, A. J. Castro-Tirado, R. A. M. J. Wijers, R. L. C. Starling, K. Misra, J. F. Graham

We present late-time Hubble Space Telescope imaging of the fields of six
Swift GRBs lying at 5.0<z<9.5. Our data includes very deep observations of the
field of the most distant spectroscopically confirmed burst, GRB 090423, at
z=8.2. Using the precise positions afforded by their afterglows we can place
stringent limits on the luminosities of their host galaxies. In one case, that
of GRB 060522 at z=5.11, there is a marginal excess of flux close to the GRB
position which may be a detection of a host at a magnitude J(AB)=28.5. None of
the others are significantly detected meaning that all the hosts lie below
L\star at their respective redshifts, with star formation rates SFR<4Mo/yr in
all cases. Indeed, stacking the five fields with WFC3-IR data we conclude a
mean SFR<0.17Mo/yr per galaxy. These results support the proposition that the
bulk of star formation, and hence integrated UV luminosity, at high redshifts
arises in galaxies below the detection limits of deep-field observations.
Making the reasonable assumption that GRB rate is proportional to UV luminosity
at early times allows us to compare our limits with expectations based on
galaxy luminosity functions derived from the Hubble Ultra-Deep Field (HUDF) and
other deep fields. We infer that a luminosity function which is evolving
rapidly towards steeper faint-end slope (alpha) and decreasing characteristic
luminosity (L\star), as suggested by some other studies, is consistent with our
observations, whereas a non-evolving LF shape is ruled out at >90% confidence.
Although it is not yet possible to make stronger statements, in the future,
with larger samples and a fuller understanding of the conditions required for
GRB production, studies like this hold great potential for probing the nature
of star formation, the shape of the galaxy luminosity function, and the supply
of ionizing photons in the early universe.

http://arxiv.org/abs/1201.6074

He Gao, Bin-Bin Zhang, Bing Zhang (UNLV)

Gamma-ray bursts (GRBs) have variable lightcurves. Although most models
attribute the observed variability to one physical origin (e.g. central engine
activity, clumpy circumburst medium, relativistic turbulence), some models
invoke two physically distinct variability components. We develop a method,
namely, the stepwise filter correlation (SFC) method, to decompose the
variability components in a GRB lightcurve. Based on a low-pass filter
technique, we progressively filter the high frequency signals from the
lightcurve, and then perform a correlation analysis between each adjunct pair
of filtered lightcurves. Our simulations suggest that if a mock lightcurve
contains a slow variability component superposed on a rapidly varying time
sequence, the correlation coefficient as a function of the filter frequency
would display a prominent dip feature around the frequency of the slow
component. Through simulations, we demonstrate that this method can identify
significant clustering structures of a lightcurve in the frequency domain, and
proved that it can catch superposed signals that are otherwise not easy to
retrieve based on other methods (e.g. the power density spectrum analysis
method). We apply this method to 266 BATSE bright GRBs. We find that the
majority of the bursts have clear evidence of such a superposition effect. We
perform a statistical analysis of the identified variability components, and
discuss the implications for GRB physics.

http://arxiv.org/abs/1103.0074

D. Watson (1), J. French (1), L. Christensen (2 and 3), B. O'Halloran (4), M. Michałowski (5), J. Hjorth (1), D. Malesani (1), J. P. U. Fynbo (1), K. D. Gordon (6), J. M. Castro Cerón (1 and 7) ((1) DARK, (2) T. U. München, (3) ESO, (4) Imperial College London, (5) IoA Edinburgh, (6) STScI, (7) ESAC/ESA)

Gamma-ray burst (GRB) host galaxies have been studied extensively in optical
photometry and spectroscopy. Here we present the first mid-infrared spectrum of
a GRB host, HG031203. It is one of the nearest GRB hosts at z=0.1055, allowing
both low and high-resolution spectroscopy with Spitzer-IRS. Medium resolution
UV-to-K-band spectroscopy with the X-shooter spectrograph on the VLT is also
presented, along with Spitzer IRAC and MIPS photometry, as well as radio and
sub-mm observations. These data allow us to construct a UV-to-radio spectral
energy distribution with almost complete spectroscopic coverage from 0.3-35
micron of a GRB host galaxy for the first time, potentially valuable as a
template for future model comparisons. The IRS spectra show strong,
high-ionisation fine structure line emission indicative of a hard radiation
field in the galaxy, suggestive of strong ongoing star-formation and a very
young stellar population. The selection of HG031203 via the presence of a GRB
suggests that it might be a useful analogue of very young star-forming galaxies
in the early universe, and hints that local BCDs may be used as more reliable
analogues of star-formation in the early universe than typical local
starbursts. We look at the current debate on the ages of the dominant stellar
populations in z~7 and z~8 galaxies in this context. The nebular line emission
is so strong in HG031203, that at z~7, it can reproduce the spectral energy
distributions of z-band dropout galaxies with elevated IRAC 3.6 and 4.5 micron
fluxes without the need to invoke a 4000A break.

http://arxiv.org/abs/1010.1783

Salvatore Capozziello, Ruth Lazkoz, Vincenzo Salzano

We study the possibility to extract model independent information about the
dynamics of the universe by using Cosmography. We intend to explore it
systematically, to learn about its limitations and its real possibilities. Here
we are sticking to the series expansion approach on which Cosmography is based.
We apply it to different data sets: Supernovae Type Ia (SNeIa), Hubble
parameter extracted from differential galaxy ages, Gamma Ray Bursts (GRBs) and
the Baryon Acoustic Oscillations (BAO) data. We go beyond past results in the
literature extending the series expansion up to the fourth order in the scale
factor, which implies the analysis of the deceleration, q_{0}, the jerk, j_{0}
and the snap, s_{0}. We use the Markov Chain Monte Carlo Method (MCMC) to
analyze the data statistically. We also try to relate direct results from
Cosmography to dark energy (DE) dynamical models parameterized by the
Chevalier-Polarski-Linder (CPL) model, extracting clues about the matter
content and the dark energy parameters. The main results are: a) even if
relying on a mathematical approximate assumption such as the scale factor
series expansion in terms of time, cosmography can be extremely useful in
assessing dynamical properties of the Universe; b) the deceleration parameter
clearly confirms the present acceleration phase; c) the MCMC method can help
giving narrower constraints in parameter estimation, in particular for higher
order cosmographic parameters (the jerk and the snap), with respect to the
literature; d) both the estimation of the jerk and the DE parameters, reflect
the possibility of a deviation from the LCDM cosmological model.

http://arxiv.org/abs/1104.3096

Bunyo Hatsukade, Tetsuya Hashimoto, Kouji Ohta, Kouichiro Nakanishi, Yoichi Tamura, Kotaro Kohno

We present the results of the 16-cm-waveband continuum observations of four
host galaxies of gamma-ray bursts (GRBs) 990705, 021211, 041006, and 051022
using the Australia Telescope Compact Array. Radio emission was not detected in
any of the host galaxies. The 2sigma upper limits on star-formation rates
derived from the radio observations of the host galaxies are 23, 45, 27, and 26
Msun/yr, respectively, which are less than about 10 times those derived from
UV/optical observations, suggesting that they have no significant dust-obscured
star formation. GRBs 021211 and 051022 are known as the so-called “dark GRBs”
and our results imply that dark GRBs do not always occur in galaxies enshrouded
by dust. Because large dust extinction was not observed in the afterglow of
GRB021211, our result {\bf suggests the possibility} that the cause of the dark
GRB is the intrinsic faintness of the optical afterglow. On the other hand, by
considering the high column density observed in the afterglow of GRB051022, the
likely cause of the dark GRB is the dust extinction in the line of sight of the
GRB.

http://arxiv.org/abs/1201.4947