Gamma-ray burst early optical afterglows: implications for the initial Lorentz factor and the central engine

Early optical afterglows have been observed from GRB 990123, GRB 021004, and GRB 021211, which reveal rich emission features attributed to reverse shocks. It is expected that Swift will discover many more early afterglows. Here we investigate in a unified manner both the forward and the reverse external shock emission components, and introduce a straightforward recipe for directly constraining the initial Lorentz factor of the fireball using early optical afterglow data. The scheme is largely independent of the shock microphysics. We identify two types of combinations of the reverse and forward shock emission, and explore their parameter regimes. We also discuss a possible diagnostic for magnetized ejecta. There is evidence that the central engine of GRB 990123 is strongly magnetized.Comment: emulateapj style, 6 pages, 1 figure. Expanded version accepted for publication in ApJ Part

On the short-term temporal variations of GNSS receiver differential phase biases

As a first step towards studying the ionosphere with the global navigation satellite system (GNSS), leveling the phase to the code geometry-free observations on an arc-by-arc basis yields the ionospheric observables, interpreted as a combination of slant total electron content along with satellite and receiver differential code biases (DCB). The leveling errors in the ionospheric observables may arise during this procedure, which, according to previous studies by other researchers, are due to the combined effects of the code multipath and the intra-day variability in the receiver DCB. In this paper we further identify the short-term temporal variations of receiver differential phase biases (DPB) as another possible cause of leveling errors. Our investigation starts by the development of a method to epoch-wise estimate between-receiver DPB (BR-DPB) employing (inter-receiver) single-differenced, phase-only GNSS observations collected from a pair of receivers creating a zero or short baseline. The key issue for this method is to get rid of the possible discontinuities in the epoch-wise BR-DPB estimates, occurring when satellite assigned as pivot changes. Our numerical tests, carried out using Global Positioning System (GPS, US GNSS) and BeiDou Navigation Satellite System (BDS, Chinese GNSS) observations sampled every 30 s by a dedicatedly selected set of zero and short baselines, suggest two major findings. First, epoch-wise BR-DPB estimates can exhibit remarkable variability over a rather short period of time (e.g. 6 cm over 3 h), thus significant from a statistical point of view. Second, a dominant factor driving this variability is the changes of ambient temperature, instead of the un-modelled phase multipath

P2P XQuery and the StreetTiVo application

In the AmbientDB project, we are building MonetDB/XQuery, an open-source XML DBMS (XDBMS) with support for distributed querying and P2P services. Our work is motivated by the hypothesis that P2P is a disruptive paradigm that should change the nature of database technology. Most of the existing distributed DBMS technologies were developed to be used in (small-scale) local-area networks (LAN). Those technologies usually assume that (i) there is a central controller and/or peers have complete knowledge of the whole system, (ii) peers are uniform and highly available, (iii) placement of data happens in a controlled way and is rarely changed and (iv) a global database schema is used. Peer-to-Peer (P2P) networks have led the distributed DBMS research to reconsider existing technologies in such a new environment, where (i) systems have decentralized architectures, (ii) peers join or leave the network at any time, (iii) placement of data is out of the system's control and it changes frequently, (iv) each peer can have its local database schema (or no schema at all), and (v) data owned by the peers are often incomplete, overlapping and even conflicting

Distributed XQuery and Updates Processing with Heterogeneous XQuery Engines

We demonstrate XRPC, a minimal XQuery extension that enables distributed querying between heterogeneous XQuery engines. The XRPC language extension enhances the existing concept of XQuery functions with the Remote Procedure Call (RPC) paradigm. XRPC is orthogonal to all XQuery features, including the XQuery Update Facility (XQUF). Note that executing xquf updating functions over XRPC leads to the phenomenon of distributed transactions. XRPC achieves heterogeneity by an open SOAP-based network protocol, that can be implemented by any engine, and an XRPC Wrapper that allow

Integrating XQuery and P2P in MonetDB/XQuery*

MonetDB/XQuery* is a fully functional publicly available XML DBMS that has been extended with distributed and P2P data management functionality. Our (minimal) XQuery language extension XRPC adds the concept of RPC to XQuery, and exploits the set-at-a-time database processing model to optimize the networking cost through a technique called Bulk RPC. We describe our approach to include the services offered by diverse P2P network structures (such as DHTs), in a way that avoids any further intrusion in the XQuery language and semantics, and show how this, similarly to Bulk RPC, will lead to further query optimization opportunities where the XDBMS interacts with the underlying P2P network. We also discuss some P2P data management applications were MonetDB/XQuery* is being used (an in-home small scenario and a wide-area collaborative application). As this research is work-in-progress, we outline some research questions on our path towards defining and realizing P2P XDBMS technology

A modified carrier-to-code leveling method for retrieving ionospheric observables and detecting short-term temporal variability of receiver differential code biases

Sensing the ionosphere with the global positioning system involves two sequential tasks, namely the ionospheric observable retrieval and the ionospheric parameter estimation. A prominent source of error has long been identified as short-term variability in receiver differential code bias (rDCB). We modify the carrier-to-code leveling (CCL), a method commonly used to accomplish the first task, through assuming rDCB to be unlinked in time. Aside from the ionospheric observables, which are affected by, among others, the rDCB at one reference epoch, the Modified CCL (MCCL) can also provide the rDCB offsets with respect to the reference epoch as by-products. Two consequences arise. First, MCCL is capable of excluding the effects of time-varying rDCB from the ionospheric observables, which, in turn, improves the quality of ionospheric parameters of interest. Second, MCCL has significant potential as a means to detect between-epoch fluctuations experienced by rDCB of a single receiver

Optical afterglows of short Gamma-ray Bursts and GRB 040924

Short-duration Gamma-ray bursts (GRBs) (\leq 2{\rm s}) have remained a mystery due to the lack of afterglow detection until recently. The models to interpret short GRBs invoke distinct progenitor scenarios. Here we present a generic analysis of short GRB afterglows, and calculate the optical lightcurves of short GRBs within the framework of different progenitor models. We show that all these optical afterglows are bright enough to be detected by the Ultraviolet and Optical Telescope (UVOT) on board the {\em Swift} observatory, and that different models could be distinguished with a well-monitored lightcurve. We also model the afterglow data of the recently discovered short burst GRB 040924. We find that the limited data are consistent with a low medium density environment which is consistent with the pre-concept of the compact-star merger progenitor model, although the models with a collapsar progenitor are not ruled out.Comment: 6 pages (3 figures), expanded version accepted for publication in Ap