622 research outputs found
A Deep Optical Observation for an Enigmatic Unidentified Gamma-Ray Source 3EG J1835+5918
We report a deep optical imaging observation by the Subaru telescope for a
very soft X-ray source RX J1836.2+5925, which has been suspected to be an
isolated neutron star associated with the brightest as-yet unidentified EGRET
source outside the Galactic plane, 3EG J1835+5918. An extended source having a
complex, bipolar shape is found at B ~ 26, and this might be an extended pulsar
nebular whose flux is about 5-6 orders of magnitude lower than gamma-ray flux,
although finding a galaxy of this magnitude by chance in the error circle is of
order unity. We have found two even fainter, possibly point sources at B ~ 28,
although their detections are not firm because of low signal-to-noise. If the
extended object of B ~ 26 is a galaxy and not related to 3EG J1835+5918, a
lower limit on X-ray/optical flux ratio is set as f_X/f_B >~ 2700, giving a
further strong support of the neutron-star identification of 3EG J1835+5918.
Interestingly, if either of the two sources at B ~ 28 is the real counterpart
of RX J1836.2+5925 and thermal emission from the surface of an isolated neutron
star, the temperature and distance to the source become ~ 4 x 10^5K and ~300pc,
respectively, showing a striking similarity of its spectral energy distribution
to the proto-type radio-quiet gamma-ray pulsar Geminga. No detection of
nonthermal hard X-ray emission is consistent with the ASCA upper limit, if the
nonthermal flux of 3EG J1835+5918/RX J1836.2+5925 is at a similar level with
that of Gemiga.Comment: PASJ Letters in press. (Received March 26; Accepted May 17
Model-independent constraints on reionization from large-scale CMB polarization
On large angular scales, the polarization of the CMB contains information
about the evolution of the average ionization during the epoch of reionization.
Interpretation of the polarization spectrum usually requires the assumption of
a fixed functional form for the evolution, e.g. instantaneous reionization. We
develop a model-independent method where a small set of principal components
completely encapsulate the effects of reionization on the large-angle E-mode
polarization for any reionization history within an adjustable range in
redshift. Using Markov Chain Monte Carlo methods, we apply this approach to
both the 3-year WMAP data and simulated future data. WMAP data constrain two
principal components of the reionization history, approximately corresponding
to the total optical depth and the difference between the contributions to the
optical depth at high and low redshifts. The optical depth is consistent with
the constraint found in previous analyses of WMAP data that assume
instantaneous reionization, with only slightly larger uncertainty due to the
expanded set of models. Using the principal component approach, WMAP data also
place a 95% CL upper limit of 0.08 on the contribution to the optical depth
from redshifts z>20. With improvements in polarization sensitivity and
foreground modeling, approximately five of the principal components can
ultimately be measured. Constraints on the principal components, which probe
the entire reionization history, can test models of reionization, provide
model-independent constraints on the optical depth, and detect signatures of
high-redshift reionization.Comment: 14 pages, 13 figures; submitted to Ap
GRBs as Probes of the IGM
Gamma-ray Bursts (GRBs) are the most powerful explosions known, capable of
outshining the rest of gamma-ray sky during their short-lived prompt emission.
Their cosmological nature makes them the best tool to explore the final stages
in the lives of very massive stars up to the highest redshifts. Furthermore,
studying the emission from their low-energy counterparts (optical and infrared)
via rapid spectroscopy, we have been able to pin down the exact location of the
most distant galaxies as well as placing stringent constraints on their host
galaxies and intervening systems at low and high-redshift (e.g. metallicity and
neutral hydrogen fraction). In fact, each GRB spectrum contains absorption
features imprinted by metals in the host interstellar medium (ISM) as well as
the intervening intergalactic medium (IGM) along the line of sight. In this
chapter we summarize the progress made using a large dataset of GRB spectra in
understanding the nature of both these absorbers and how GRBs can be used to
study the early Universe, in particular to measure the neutral hydrogen
fraction and the escape fraction of UV photons before and during the epoch of
re-ionization.Comment: 18 pages; 5 Figures. Accepted for publication in Space Science
Review
The prompt optical/near-infrared flare of GRB 050904: the most luminous transient ever detected
With a redshift of z=6.295, GRB 050904 is the most distant gamma-ray burst
ever discovered. It was an energetic event at all wavelengths and the afterglow
was observed in detail in the near-infrared bands. We gathered all available
optical and NIR afterglow photometry of this GRB to construct a composite NIR
light curve spanning several decades in time and flux density. Transforming the
NIR light curve into the optical, we find that the afterglow of GRB 050904 was
more luminous at early times than any other GRB afterglow in the
pre-\emph{Swift} era, making it at these wavelengths the most luminous
transient ever detected. Given the intrinsic properties of GRB 050904 and its
afterglow, we discuss if this burst is markedly different from other GRBs at
lower redshifts.Comment: The Astronomical Journal, in press; revised version, including the
comments of the referee (one figure added, text restructured, all conclusions
unchanged), 7 pages, 3 figure
Are There Any Redshift >8 Gamma-Ray Bursts in the BATSE Catalog?
Several luminosity indicators have been found for Gamma-Ray Bursts (GRBs)
wherein measurable light curve and spectral properties are well-correlated with
the peak luminosity. Several papers have each applied one different luminosity
relation to find redshifts for BATSE GRBs and claim to identify specific bursts
with z>8. The existence of such high redshift events is not surprising, as
BATSE has enough sensitivity to see them and GRBs are expected out to the
redshift of the first star formation. To improve results we used five
luminosity relations with updated calibrations to determine redshifts with
error bars. Combining these relations, we calculated the redshifts of 36 BATSE
GRBs with claimed z>8. Our results include 13 bursts with our derived best
redshift z_best>8, which looks promising at first. But the calculated redshift
uncertainties are significantly large in these selected cases. With only one
exception, all of our bursts have z_1siglow<9. The one exception (BATSE trigger
2035) is likely a short duration burst at z>~4. Our best case for a very high
redshift event is BATSE trigger 3142 with z_best>20 and z_1siglow=8.9, however
we can only say z>4.1 at the two-sigma confidence level. In all, we cannot
point toward any one BATSE burst as confidently having z>8. One implication is
to greatly weaken prior claims that GRBs have a steeply rising rate-density out
to high redshifts.Comment: ApJ in press, 18 page
Very Strong TeV Emission as Gamma-Ray Burst Afterglows
Gamma-ray bursts (GRBs) and following afterglows are considered to be
produced by dissipation of kinetic energy of a relativistic fireball and
radiation process is widely believed as synchrotron radiation or inverse
Compton scattering of electrons. We argue that the transfer of kinetic energy
of ejecta into electrons may be inefficient process and hence the total energy
released by a GRB event is much larger than that emitted in soft gamma-rays, by
a factor of \sim (m_p/m_e). We show that, in this case, very strong emission of
TeV gamma-rays is possible due to synchrotron radiation of protons accelerated
up to \sim 10^{21} eV, which are trapped in the magnetic field of afterglow
shock and radiate their energy on an observational time scale of \sim day. This
suggests a possibility that GRBs are most energetic in TeV range and such TeV
gamma-rays may be detectable from GRBs even at cosmological distances, i.e., z
\sim 1, by currently working ground-based telescopes. Furthermore, this model
gives a quantitative explanation for the famous long-duration GeV photons
detected from GRB940217. If TeV gamma-ray emission which is much more energetic
than GRB photons is detected, it provides a strong evidence for acceleration of
protons up to \sim 10^{21} eV.Comment: 10 pages, no figure. To appear in ApJ Letter
Infrared Spectral Energy Distribution of Galaxies in the AKARI All Sky Survey: Correlations with Galaxy Properties, and Their Physical Origin
We have studied the properties of more than 1600 low-redshift galaxies by
utilizing high-quality infrared flux measurements of the AKARI All-Sky Survey
and physical quantities based on optical and 21-cm observations. Our goal is to
understand the physics determining the infrared spectral energy distribution
(SED). The ratio of the total infrared luminosity L_TIR, to the star-formation
rate (SFR) is tightly correlated by a power-law to specific SFR (SSFR), and
L_TIR is a good SFR indicator only for galaxies with the largest SSFR. We
discovered a tight linear correlation for normal galaxies between the radiation
field strength of dust heating, estimated by infrared SED fits (U_h), and that
of galactic-scale infrared emission (U_TIR ~ L_TIR/R^2), where R is the optical
size of a galaxy. The dispersion of U_h along this relation is 0.3 dex,
corresponding to 13% dispersion in the dust temperature. This scaling and the
U_h/U_TIR ratio can be explained physically by a thin layer of heating sources
embedded in a thicker, optically-thick dust screen. The data also indicate that
the heated fraction of the total dust mass is anti-correlated to the dust
column density, supporting this interpretation. In the large U_TIR limit, the
data of circumnuclear starbursts indicate the existence of an upper limit on
U_h, corresponding to the maximum SFR per gas mass of ~ 10 Gyr^{-1}. We find
that the number of galaxies sharply drops when they become optically thin
against dust-heating radiation, suggesting that a feedback process to galaxy
formation (likely by the photoelectric heating) is working when dust-heating
radiation is not self-shielded on a galactic scale. Implications are discussed
for the M_HI-size relation, the Kennicutt-Schmidt relation, and galaxy
formation in the cosmological context.Comment: 29 pages including 28 figures. matches the published version (PASJ
2011 Dec. 25 issue). The E-open option was chosen for this article, i.e., the
official version available from PASJ site
(http://pasj.asj.or.jp/v63/n6/630613/630613-frame.html) without restrictio
Positional Coincidence between the High-latitude Steady Unidentified Gamma-ray Sources and Possibly Merging Clusters of Galaxies
We report an evidence for the first time that merging clusters of galaxies
are a promising candidate for the origin of high galactic-latitude, steady
unidentified EGRET gamma-ray sources. Instead of using past optical catalogs of
eye-selected clusters, we made a matched-filter survey of galaxy clusters over
4\arcdeg \times 4\arcdeg areas around seven steady unidentified EGRET sources
at |b|>45\arcdeg together with a 100 \sq \arcdeg area near the South
Galactic Pole as a control field. In total, 154 Abell-like cluster candidates
and 18 close pairs/groups of these clusters, expected to be possibly merging
clusters, were identified within estimated redshift . Five
among the seven EGRET sources have one or two cluster pairs/groups (CPGs)
within 1\arcdeg from them. We assess the statistical significance of this
result by several methods, and the confidence level of the real excess is
maximally 99.8% and 97.8% in a conservative method. In contrast, we found no
significant correlation with single clusters. In addition to the spatial
correlation, we also found that the richness of CPGs associated with EGRET
sources is considerably larger than those of CPGs in the control field. These
results imply that a part of the steady unidentified EGRET sources at
high-latitude are physically associated with close CPGs, not with single
clusters. We also discuss possible interpretations of these results. We argue
that, if these associations are real, they are difficult to explain by hadronic
processes, but best explained by the inverse-Compton scattering by high energy
electrons accelerated in shocks of cluster formation, as recently proposed.Comment: 9 pages, 2 PostScript figures, uses emulateapj5.sty, added new
analysis and discussion, ApJ accepte
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