4,598 research outputs found
Microlensing variability in time-delay quasars
We have searched for microlensing variability in the light curves of five
gravitationally lensed quasars with well-determined time delays: SBS 1520+530,
FBQ 0951+2635, RX J0911+0551, B1600+434 and HE 2149-2745. By comparing the
light curve of the leading image with a suitably time offset light curve of a
trailing image we find that two (SBS 1520+530 and FBQ 0951+2635) out of the
five quasars have significant long-term (years) and short-term (100 days)
brightness variations that may be attributed to microlensing.The short-term
variations may be due to nanolenses, relativistic hot or cold spots in the
quasar accretion disks, or coherent microlensing at large optical depth.Comment: 12 pages, 5 figures, uses natbib.sty and aa.cl
Delayed soft X-ray emission lines in the afterglow of GRB 030227
Strong, delayed X-ray line emission is detected in the afterglow of GRB
030227, appearing near the end of the XMM-Newton observation, nearly twenty
hours after the burst. The observed flux in the lines, not simply the
equivalent width, sharply increases from an undetectable level (<1.7e-14
erg/cm^2/s, 3 sigma) to 4.1e-14 erg/cm^2/s in the final 9.7 ks. The line
emission alone has nearly twice as many detected photons as any previous
detection of X-ray lines. The lines correspond well to hydrogen and/or
helium-like emission from Mg, Si, S, Ar and Ca at a redshift z=1.39. There is
no evidence for Fe, Co or Ni--the ultimate iron abundance must be less than a
tenth that of the lighter metals. If the supernova and GRB events are nearly
simultaneous there must be continuing, sporadic power output after the GRB of a
luminosity >~5e46 erg/s, exceeding all but the most powerful quasars.Comment: Submitted to ApJL. 14 pages, 3 figures with AASLaTe
Swift Identification of Dark Gamma-Ray Bursts
We present an optical flux vs. X-ray flux diagram for all known gamma-ray
bursts (GRBs) for which an X-ray afterglow has been detected. We propose an
operational definition of dark bursts as those bursts that are optically
subluminous with respect to the fireball model, i.e., which have an
optical-to-X-ray spectral index beta_OX < 0.5. Out of a sample of 52 GRBs we
identify 5 dark bursts. The definition and diagram serve as a simple and quick
diagnostic tool for identifying dark GRBs based on limited information,
particularly useful for early and objective identification of dark GRBs
observed with the Swift satellite.Comment: 4 pages, 1 figure. ApJ Letters, in pres
Gamma-ray burst host galaxies and the link to star-formation
We briefly review the current status of the study of long-duration gamma-ray
burst (GRB) host galaxies. GRB host galaxies are mainly interesting to study
for two reasons: 1) they may help us understand where and when massive stars
were formed throughout cosmic history, and 2) the properties of host galaxies
and the localisation within the hosts where GRBs are formed may give essential
clues to the precise nature of the progenitors. The main current problem is to
understand to what degree GRBs are biased tracers of star formation. If GRBs
are only formed by low-metallicity stars, then their host galaxies will not
give a representative view of where stars are formed in the Universe (at least
not a low redshifts). On the other hand, if there is no dependency on
metallicity then the nature of the host galaxies leads to the perhaps
surprising conclusion that most stars are formed in dwarf galaxies. In order to
resolve this issue and to fully exploit the potential of GRBs as probes of
star-forming galaxies throughout the observable universe it is mandatory that a
complete sample of bursts with redshifts and host galaxy detections is built.Comment: 9 pages, 3 figures. To appear in the proceedings of the Eleventh
Marcel Grossmann Meeting on General Relativity, eds. H. Kleinert, R. T.
Jantzen & R. Ruffini, World Scientific, Singapore, 200
The host of the SN-less GRB 060505 in high resolution
The spiral host galaxy of GRB 060505 at z=0.089 was the site of a puzzling
long duration burst without an accompanying supernova. Studies of the burst
environment by Th\"one et al. (2008) suggested that this GRB came from the
collapse of a massive star and that the GRB site was a region with properties
different from the rest of the galaxy. We reobserved the galaxy in high spatial
resolution using the VIMOS integral-field unit (IFU) at the VLT with a spaxel
size of 0.67 arcsec. Furthermore, we use long slit high resolution data from
HIRES/Keck at two different slit positions covering the GRB site, the center of
the galaxy and an HII region next to the GRB region. We compare the properties
of different HII regions in the galaxy with the GRB site and study the global
and local kinematic properties of this galaxy. The resolved data show that the
GRB site has the lowest metallicity in the galaxy with around 1/3 Z_solar, but
its specific SFR (SSFR) of 7.4 M_solar/yr/L/L* and age (determined by the
Halpha EW) are similar to other HII regions in the host. The galaxy shows a
gradient in metallicity and SSFR from the bulge to the outskirts as it is
common for spiral galaxies. This gives further support to the theory that GRBs
prefer regions of higher star-formation and lower metallicity, which, in S-type
galaxies, are more easily found in the spiral arms than in the centre.
Kinematic measurements of the galaxy do not show evidence for large
perturbations but a minor merger in the past cannot be excluded. This study
confirms the collapsar origin of GRB060505 but reveals that the properties of
the HII region surrounding the GRB were not unique to that galaxy. Spatially
resolved observations are key to know the implications and interpretations of
unresolved GRB hosts observations at higher redshifts.Comment: 15 pages, 16 figures, 6 tables; resubmitted to MNRAS after minor
revision
The Redshift Distribution of the TOUGH Survey
We present the redshift results from a Very Large Telescope program aimed at
optimizing the legacy value of the Swift mission: to characterize a
homogeneous, X-ray selected, sample of 69 GRB host galaxies. 19 new redshifts
have been secured, resulting in a 83% (57/69) redshift completion, making the
survey the most comprehensive in terms of redshift completeness of any sample
to the full Swift depth, available to date. We present the cumulative redshift
distribution and derive a conservative, yet small, associated uncertainty. We
constrain the fraction of Swift GRBs at high redshift to a maximum of 10% (5%)
for z > 6 (z > 7). The mean redshift of the host sample is assessed to be >
2.2. Using this more complete sample, we confirm previous findings that the GRB
rate at high redshift (z > 3) appears to be in excess of predictions based on
assumptions that it should follow conventional determinations of the star
formation history of the universe, combined with an estimate of its likely
metallicity dependence. This suggests that either star formation at high
redshifts has been significantly underestimated, for example due to a dominant
contribution from faint, undetected galaxies, or that GRB production is
enhanced in the conditions of early star formation, beyond those usually
ascribed to lower metallicity.Comment: 7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 34 in eConf
Proceedings C130414
A Micro-Payment Scheme Encouraging Collaboration in Multi-Hop Cellular Networks
We propose a micro-payment scheme for multi-hop cellular networks that encourages collaboration in packet forwarding by letting users benefit from relaying others` packets. At the same time as proposing mechanisms for detecting and rewarding collaboration, we introduce appropriate mechanisms for detecting and punishing various forms of abuse. We show that the resulting scheme -- which is exceptionally light-weight -- makes collaboration rational and cheating undesirable
Money Grows on (Proof-)Trees: The Formal FA1.2 Ledger Standard
Once you have invented digital money, you may need a ledger to track who owns what - along with an interface to that ledger so that users of your money can transact. On the Tezos blockchain this implies: a smart contract (distributed program), storing in its state a ledger to map owner addresses to token quantities; along with standardised entrypoints to query and transact on accounts.
A bank does a similar job - it maps account numbers to account quantities and permits users to transact - but in return the bank demands trust, it incurs expense to maintain a centralised server and staff, it uses a proprietary interface ... and it may speculate using your money and/or display rent-seeking behaviour. A blockchain ledger is by design decentralised, inexpensive, open, and it won\u27t just decide to bet your tokens on risky derivatives (unless you want it to).
The FA1.2 standard is an open standard for ledger-keeping smart contracts on the Tezos blockchain. Several FA1.2 implementations already exist.
Or do they? Is the standard sensible and complete? Are the implementations correct? And what are they implementations of? The FA1.2 standard is written in English, a specification language favoured by wet human brains but notorious for its incompleteness and ambiguity when rendered into dry and unforgiving code.
In this paper we report on a formalisation of the FA1.2 standard as a Coq specification, and on a formal verification of three FA1.2-compliant smart contracts with respect to that specification. Errors were found and ambiguities were resolved; but also, there now exists a mathematically precise and battle-tested specification of the FA1.2 ledger standard.
We will describe FA1.2 itself, outline the structure of the Coq theories - which in itself captures some non-trivial and novel design decisions of the development - and review the detailed verification of the implementations
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