289 research outputs found
Acceleration of Ultra-High Energy Cosmic Rays in the Colliding Shells of Blazars and GRBs: Constraints from the Fermi Gamma ray Space Telescope
Fermi Gamma ray Space Telescope measurements of spectra, variability time
scale, and maximum photon energy give lower limits to the apparent jet powers
and, through gammagamma opacity arguments, the bulk Lorentz factors of
relativistic jets. The maximum cosmic-ray particle energy is limited by these
two quantities in Fermi acceleration scenarios. Recent data are used to
constrain the maximum energies of cosmic-ray protons and Fe nuclei accelerated
in colliding shells of GRBs and blazars. The Fermi results indicate that Fe
rather than protons are more likely to be accelerated to ultra-high energies in
AGNs, whereas powerful GRBs can accelerate both protons and Fe to >~ 10^{20}
eV. Emissivity of nonthermal radiation from radio galaxies and blazars is
estimated from the First Fermi AGN Catalog, and shown to favor BL Lac objects
and FR1 radio galaxies over flat spectrum radio quasars, FR2 radio galaxies,
and long-duration GRBs as the sources of UHECRs.Comment: 8 pages, 3 figures, ApJ, in pres
The Effects of Shilling on Final Bid Prices in Online Auctions
An increasing number of reports of online auction fraud are of growing concern to auction operators and participants. In this research, we discuss reserve price shilling, where a bidder shills in order to avoid paying auction house fees, rather than to drive up the price of the final bid. We examine the effect that premium bids, since they are linked with reserve price shill bids, have upon the final selling price. We use 10,260 eBay auctions during April 2001, and identify 1,389 auctions involving 493 sellers and 1,314 involved in concurrent auctions that involving the exact same item. We find that premium bidding occurs 23 % of the time, in 263 of the 1,389 auctions. Using a theoretical perspective involving valuation signals, we show that other bidders may view high bids as signals that an item is worth more. Thus, they may be willing to pay more for the item than items that do not receive premium bids. The implications are disturbing in that sellers may be more motivated to enter a shill bid in order to drive up the final price in an online auction. We also examine and report on alternative hypotheses involving winner’s curse and the possibility of reserve price shill bids. Our results are developed in the context of a weighted leas
Understanding the LMXB X2127+119 in M15 I. X-ray eclipses and dips
We present X-ray observations of the high-inclination low-mass X-ray binary
system X2127+119 (AC211) in the globular cluster M15 (NGC 7078). The
observations consist of data acquired in 1996 with the RXTE satellite and in
1995 with the ASCA satellite. Also, the MPC1 data from the 1988 GINGA
observations were de-archived and re-analysed. The phase-folded 2-10 keV
hardness ratios from all three missions differ significantly indicating that
the system can exhibit different spectral behaviours. We find that the X-ray
eclipse profiles can be described relatively well using a simple model where
the secondary star passes in front of a large X-ray emitting region. For this
we require a mass ratio (q=M1/M2) of about one. The radius of this X-ray
emitting region is ~0.8 RL1 and its vertical extent 60 degrees above the
orbital plane. We suggest that if this X-ray emitting region were an optically
thick corona, it would explain various puzzling aspects of this system. We also
show that the X-ray dip observed at phases around 0.65 does not conform with
the idea that the dip is caused by vertically extended material associated with
the stream/disc impact region, but that it could be due to structure in the
inner parts of the disc.Comment: 12 pages, 14 figures, accepted for publication in A&
High-Energy Cosmic Rays from Gamma-Ray Bursts
A model is proposed for the origin of cosmic rays (CRs) from ~10^14 eV to the
highest energies, >10^20 eV. Gamma-Ray Bursts (GRBs) are assumed to inject CR
protons and ions into the interstellar medium of star-forming
galaxies--including the Milky Way--with a power law spectrum extending to a
maximum energy ~10^20 eV. The CR spectrum near the knee is fit with CRs trapped
in the Galactic halo that were accelerated and injected by an earlier Galactic
GRB. These CRs diffuse in the disk and halo of the Galaxy due to gyroresonant
pitch-angle scattering with MHD turbulence in the Galaxy's magnetic field. The
preliminary (2001) KASCADE data through the knee of the CR spectrum are fit by
a model with energy-dependent propagation of CR ions from a single Galactic
GRB. Ultra-high energy CRs (UHECRs), with energies above the ankle are assumed
to propagate rectilinearly with their spectrum modified by photo-pion,
photo-pair, and expansion losses. We fit the measured UHECR spectrum assuming
comoving luminosity densities of GRB sources consitent with possible star
formation rate histories of the universe. For power-law CR proton injection p>2
this model implies that the nonthermal content in the GRB blast waves is
hadronically dominated by a factor ~60-200. Calculations show that 100 TeV-100
PeV neutrinos could be detected several times per year from all GRBs in
kilometer-scale neutrino detectors such as IceCube, for GRB blast-wave Doppler
factors <~200. GLAST measurements of gamma-ray components and cutoffs will
constrain the product of nonthermal baryon loading and radiative efficiency,
limit the Doppler factor, and test this senario.Comment: 43 pages, 21 figures, to appear in Astropart. Phy
Evaluating 'Prefer not to say' Around Sensitive Disclosures
As people's offline and online lives become increasingly entwined, the sensitivity of personal information disclosed online is increasing. Disclosures often occur through structured disclosure fields (e.g., drop-down lists). Prior research suggests these fields may limit privacy, with non-disclosing users being presumed to be hiding undesirable information. We investigated this around HIV status disclosure in online dating apps used by men who have sex with men. Our online study asked participants (N=183) to rate profiles where HIV status was either disclosed or undisclosed. We tested three designs for displaying undisclosed fields. Visibility of undisclosed fields had a significant effect on the way profiles were rated, and other profile information (e.g., ethnicity) could affect inferences that develop around undisclosed information. Our research highlights complexities around designing for non-disclosure and questions the voluntary nature of these fields. Further work is outlined to ensure disclosure control is appropriately implemented around online sensitive information disclosures
Constraining Jet/Disk Geometry and Radiative Processes in Stellar Black Holes XTE J1118+480 and GX 339-4
We present results from modeling of quasi-simultaneous broad band (radio
through X-ray) observations of the galactic stellar black hole (BH) transient
X-ray binary (XRB) systems XTE J1118+480 and GX 339-4 using an irradiated disc
+ compact jet model. In addition to quantifying the physical properties of the
jet, we have developed a new irradiated disc model which also constrains the
geometry and temperature of the outer accretion disc by assuming a disc heated
by viscous energy release and X-ray irradiation from the inner regions. For the
source XTE J1118+480, which has better spectral coverage of the two in optical
and near-IR (OIR) wavelengths, we show that the entire broad band continuum can
be well described by an outflow-dominated model + an irradiated disc. The
best-fit radius of the outer edge of the disc is consistent with the Roche lobe
geometry of the system, and the temperature of the outer edge of the accretion
disc is similar to those found for other XRBs. Irradiation of the disc by the
jet is found to be negligible for this source. For GX 339-4, the entire
continuum is well described by the jet-dominated model only, with no disc
component required. For the two XRBs, which have very different physical and
orbital parameters and were in different accretion states during the
observations, the sizes of the jet base are similar and both seem to prefer a
high fraction of non-thermal electrons in the acceleration/shock region and a
magnetically dominated plasma in the jet. These results, along with recent
similar results from modeling other galactic XRBs and AGNs, may suggest an
inherent unity in diversity in the geometric and radiative properties of
compact jets from accreting black holes.Comment: Accepted for publication in MNRAS. Uses mn2e.cls. 17 pages, 4
figures, 3 table
Fundamental Parameters of Low Mass X-ray Binaries II: X-Ray Persistent Systems
The determination of fundamental parameters in X-ray luminous (persistent)
X-ray binaries has been classically hampered by the large optical luminosity of
the accretion disc. New methods, based on irradiation of the donor star and
burst oscillations, provide the opportunity to derive dynamical information and
mass constraints in many persistent systems for the first time. These
techniques are here reviewed and the latest results presented.Comment: 8 pages, 4 figures; invited review presented at COSPAR Colloquium
"Spectra & Timing of Compact X-ray Binaries," January 17-20, 2005, Bombay,
India. To be published in Advances in Space Researc
Sequencing strategies in large, competitive, ascending price automobile auctions: An experimental examination
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