1,752 research outputs found
Impact of the orbital uncertainties on the timing of pulsars in binary systems
The detection of pulsations from an X-ray binary is an unambiguous signature
of the presence of a neutron star in the system. When the pulsations are missed
in the radio band, their detection at other wavelengths, like X-ray or
gamma-rays, requires orbital demodulation, since the length of the observations
are often comparable to, or longer than the system orbital period. The detailed
knowledge of the orbital parameters of binary systems plays a crucial role in
the detection of the spin period of pulsars, since any uncertainty in their
determination translates into a loss in the coherence of the signal during the
demodulation process. In this paper, we present an analytical study aimed at
unveiling how the uncertainties in the orbital parameters might impact on
periodicity searches. We find a correlation between the power of the signal in
the demodulated arrival time series and the uncertainty in each of the orbital
parameters. This correlation is also a function of the pulsar frequency. We
test our analytical results with numerical simulations, finding good agreement
between them. Finally, we apply our study to the cases of LS 5039 and LS I +61
303 and consider the current level of uncertainties in the orbital parameters
of these systems and their impact on a possible detection of a hosted pulsar.
We also discuss the possible appearance of a sideband ambiguity in real data.
The latter can occur when, due to the use of uncertain orbital parameters, the
power of a putative pulsar is distributed in frequencies lying nearby the
pulsar period. Even if the appearance of a sideband is already a signature of a
pulsar component, it may introduce an ambiguity in the determination of its
period. We present here a method to solve the sideband issue.Comment: Accepted 2012 September 08 by MNRAS. The paper contains 18 figures
and 5 table
Correlating Fermi gamma-ray sources with ultra-high energy cosmic rays
The origin of ultra-high energy cosmic rays (UHECRs) is one of the enduring
mysteries of high-energy astrophysics. To investigate this, we cross-correlate
the recently released Fermi Large Area Telescope First Source Catalog (1FGL)
with the public sample of UHECRs made available by the Pierre Auger
collaboration. Of the 27 UHECRs in the sample, we find 12 events that arrived
within 3.1 degrees of Fermi sources. However, we find similar or larger number
of matches in 63 out of 100 artificial UHECR samples constructed using
positions randomly drawn from the BATSE 4B catalog of gamma-ray bursts (GRBs)
collected from 1991 until 1996. Based on our analysis, we find no evidence that
UHECRs are associated with Fermi sources. We conclude with some remarks about
the astrophysical origin of cosmic rays.Comment: Revised version that considers a smaller deflection angle for UHECRs
(3.1 degrees) and a declination range for the artificial samples that extends
to decl.= +24.8 in response to the referee comments. 4 pages, 2 figures,
submitted to MNRA
New Neighbours: Modelling the Growing Population of Gamma-ray Millisecond Pulsars
The Fermi Large Area Telescope, in collaboration with several groups from the
radio community, have had marvellous success at uncovering new gamma-ray
millisecond pulsars (MSPs). In fact, MSPs now make up a sizable fraction of the
total number of known gamma-ray pulsars. The MSP population is characterized by
a variety of pulse profile shapes, peak separations, and radio-to-gamma phase
lags, with some members exhibiting nearly phase-aligned radio and gamma-ray
light curves (LCs). The MSPs' short spin periods underline the importance of
including special relativistic effects in LC calculations, even for emission
originating from near the stellar surface. We present results on modelling and
classification of MSP LCs using standard pulsar model geometries.Comment: 4 pages, 2 figures, proceedings of the ICREA Workshop on The
High-Energy Emission from Pulsars and their Systems (HEEPS), Sant Cugat,
Spai
First Detection of the Crab Pulsar above 100 GeV
We present the detection of pulsed gamma-ray emission from the Crab pulsar
above 100 GeV with the VERITAS array of atmospheric Cherenkov telescopes.
Gamma-ray emission at theses energies was not expected in pulsar models. The
detection of pulsed emission above 100 GeV and the absence of an exponential
cutoff makes it unlikely that curvature radiation is the primary production
mechanism of gamma rays at these energies.Comment: 5 pages, proceedings of the TAUP 2011 conference in Munich, German
D-Foam Phenomenology: Dark Energy, the Velocity of Light and a Possible D-Void
In a D-brane model of space-time foam, there are contributions to the dark
energy that depend on the D-brane velocities and on the density of D-particle
defects. The latter may also reduce the speeds of photons linearly with their
energies, establishing a phenomenological connection with astrophysical probes
of the universality of the velocity of light. Specifically, the cosmological
dark energy density measured at the present epoch may be linked to the apparent
retardation of energetic photons propagating from nearby AGNs. However, this
nascent field of `D-foam phenomenology' may be complicated by a dependence of
the D-particle density on the cosmological epoch. A reduced density of
D-particles at redshifts z ~ 1 - a `D-void' - would increase the dark energy
while suppressing the vacuum refractive index, and thereby might reconcile the
AGN measurements with the relatively small retardation seen for the energetic
photons propagating from GRB 090510, as measured by the Fermi satellite.Comment: 10 pages, 3 figure
The quality of accruals and earnings : the role of components of accrual estimation errors
University of Technology, Sydney. Faculty of Business.I investigate the quality of accrual components by employing the approach used by Dechow and Dichev (2002) to measure the quality of aggregate accruals and extending it to identifiable components of accruals. I provide some initial evidence concerning components of accruals and to what extent the quality of specific accruals components contributes to overall accrual quality. The initial results indicate that Australian firms exhibit similar aggregate accruals behaviour and characteristics to those reported by Dechow and Dichev. Relating to accrual component quality, I find that quality measures relating to receivables and supplier/employee costs generally are associated with firm characteristics, such as operating environment volatility, size, and length of operating cycle, in a manner similar to aggregate accruals quality. Other accrual component quality measures, however, do not appear systematically associated with any firm characteristics. The results also indicate that the aggregate accruals quality measure is consistently and strongly positively associated with the quality of supplier/employee-related accruals, though primarily for firms which report large magnitude working capital changes over time. Interestingly, the same firms tend to exhibit a negative association between aggregate accruals quality and receivables-related quality. Thus, for my sample of Australian firms it appears that aggregate accruals quality is largely driven by the quality of accruals for costs relating to suppliers and employees, rather than revenues
Deep Observation of the Giant Radio Lobes of Centaurus A with the Fermi Large Area Telescope
The detection of high energy (HE) {\gamma}-ray emission up to about 3 GeV
from the giant lobes of the radio galaxy Centaurus A has been recently reported
by the Fermi-LAT Collaboration based on ten months of all-sky survey
observations. A data set more than three times larger is used here to study the
morphology and photon spectrum of the lobes with higher statistics. The larger
data set results in the detection of HE {\gamma}-ray emission (up to about 6
GeV) from the lobes with a significance of more than 10 and 20 {\sigma} for the
North and the South lobe, respectively. Based on a detailed spatial analysis
and comparison with the associated radio lobes, we report evidence for a
substantial extension of the HE {\gamma}-ray emission beyond the WMAP radio
image in the case of the Northern lobe of Cen A. We reconstruct the spectral
energy distribution (SED) of the lobes using radio (WMAP) and Fermi-LAT data
from the same integration region. The implications are discussed in the context
of hadronic and leptonic scenarios
Prospects for Observations of Pulsars and Pulsar Wind Nebulae with CTA
The last few years have seen a revolution in very-high gamma-ray astronomy
(VHE; E>100 GeV) driven largely by a new generation of Cherenkov telescopes
(namely the H.E.S.S. telescope array, the MAGIC and MAGIC-II large telescopes
and the VERITAS telescope array). The Cherenkov Telescope Array (CTA) project
foresees a factor of 5 to 10 improvement in sensitivity above 0.1 TeV,
extending the accessible energy range to higher energies up to 100 TeV, in the
Galactic cut-off regime, and down to a few tens GeV, covering the VHE photon
spectrum with good energy and angular resolution. As a result of the fast
development of the VHE field, the number of pulsar wind nebulae (PWNe) detected
has increased from one PWN in the early '90s to more than two dozen firm
candidates today. Also, the low energy threshold achieved and good sensitivity
at TeV energies has resulted in the detection of pulsed emission from the Crab
Pulsar (or its close environment) opening new and exiting expectations about
the pulsed spectra of the high energy pulsars powering PWNe. Here we discuss
the physics goals we aim to achieve with CTA on pulsar and PWNe physics
evaluating the response of the instrument for different configurations.Comment: accepted for publication in Astroparticle Physic
A Curious Source of Extended X-ray Emission in the Outskirts of Globular Cluster GLIMPSE-C01
We report the discovery of an unusual source of extended X-ray emission CXOU
J184846.3-013040 (`The Stem') located on the outskirts of the globular cluster
GLIMPSE-C01. No point-like source falls within the extended emission which has
an X-ray luminosity L_X =10^{32} ergs/s and a physical size of 0.1 pc at the
inferred distance to the cluster. These X-ray properties are consistent with
the pulsar wind nebula (PWN) of an unseen pulsar located within the 95-percent
confidence error contour of unidentified Fermi gamma-ray source 0FGL
J1848.6-0138. However, we cannot exclude an alternative interpretation that
postulates X-ray emission associated with a bow shock produced from the
interaction of the globular cluster and interstellar gas in the Galactic plane.
Analysis of the X-ray data reveals that `The Stem' is most significant in the
2-5 keV band, which suggests that the emission may be dominated by non-thermal
bremsstrahlung from suprathermal electrons at the bow shock. If the bow shock
interpretation is correct, these observations would provide compelling evidence
that GLIMPSE-C01 is shedding its intracluster gas during a galactic passage.
Such a direct detection of gas stripping would help clarify a crucial step in
the evolutionary history of globular clusters. Intriguingly, the data may also
accommodate a new type of X-ray source.Comment: 7 pages, 5 figures, accepted for publication in MNRA
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