172 research outputs found
The influence of the jet opening angle on the appearance of relativistic jets
We reinvestigate the problem of the appearance of relativistic jets when
geometrical opening is taken into account. We propose a new criterion to define
apparent velocities and Doppler factors, which we think being determined by the
brightest zone of the jet. We numerically compute the apparent velocity and the
Doppler factor of a non homokinetic jet using different velocity profiles. We
argue that if the motion is relativistic, the high superluminal velocities
beta_{app} ~ gamma, expected in the case of an homokinetic jet, are only
possible for geometrical collimation smaller than the relativistic beaming
angle 1/gamma. This is relatively independent of the jet velocity profile. For
jet collimation angles larger than 1/gamma, the apparent image of the jet will
always be dominated by parts of the jet traveling directly towards the observer
at lorentz factors < gamma resulting in maximal apparent velocities smaller
than gamma}. Furthermore, getting rid of the homokinetic hypothesis yields a
complex relation between the observing angle and the Doppler factor, resulting
in important consequences for the numerical computation of AGN population and
unification scheme model.Comment: Accepted in MNRAS, 12 pages and 9 Figure
kHz Quasi-Periodic Oscillations in the low-mass X-ray binary 4U 0614+09
We report on a comprehensive analysis of the kilohertz (above 300 Hz)
quasi-periodic oscillations (kHz QPOs) detected from the neutron star low-mass
X-ray binary 4U0614+09 with the Rossi X-ray Timing Explorer (RXTE). With a much
larger data set than previously analyzed (all archival data from February 1996
up to October 2007), we first investigate the reality of the 1330 Hz QPO
reported by van Straaten et al. (2000). This QPO would be of particular
interest since it has the highest frequency reported for any source. A thorough
analysis of the same observation fails to confirm the detection. On the other
hand, over our extended data set, the highest QPO frequency we measure for the
upper kHz QPO is at about 1224 Hz; a value which is fully consistent with the
maximum values observed in similar systems. Second, we demonstrate that the
frequency dependence of the quality factor and amplitude of the lower and upper
kHz QPOs follow the systematic trends seen in similar systems (Barret et al.,
2006). In particular, 4U0614+09 shows a drop of the quality factor of the lower
kHz QPO above 700 Hz. If this is due to an approach to the innermost stable
circular orbit, it implies a neutron star mass of about 1.9 solar masses.
Finally, when analyzing the data over fixed durations, we have found a gap in
the frequency distribution of the upper QPO, associated with a local minimum of
its amplitude. A similar gap is not present in the distribution of the lower
QPO frequencies, suggesting some cautions when interpreting frequency ratio
distributions, based on the occurrence of the lower QPO only.Comment: 10 pages, 6 color figures, 2 tables, Accepted for publication in
MNRA
Magnetospheric accretion-ejection processes in the classical T Tauri star AA
From a long time series of high resolution (R=115,000) HARPS spectra and
simultaneous broad-band photometry, we report new evidence for magnetospheric
accretion as well as ejection processes in the nearly edge-on classical T Tauri
star AA Tau.
AA Tau's light curve is modulated with a period of 8.22d. The recurrent
luminosity dips are due to the periodic occultation of the central star by the
magnetically-warped inner disk edge located at about 9 Rstar. Balmer line
profiles exhibit a clear rotational modulation of high-velocity redshifted
absorption components with a period of 8.22 days as well, with a maximum
strength when the main accretion funnel flow passes through the line of sight.
At the same time, the luminosity of the system decreases by about 1 mag,
indicative of circumstellar absorption of the stellar photosphere by the
magnetically-warped, corotating inner disk edge. The photospheric and HeI
radial velocities also exhibit periodic variations, and the veiling is
modulated by the appearance of the accretion shock at the bottom of the
accretion funnel. Diagnostics of hot winds and their temporal behaviour are
also presented.
The peculiar geometry of the young AA Tau system (nearly edge-on) allows us
to uniquely probe the acretion-ejection region close to the star. We find that
most spectral and photometric diagnostics vary as expected from models of
magnetically-channelled accretion in young stars, with a large scale
magnetosphere tilted by 20 deg onto the star's spin axis. We also find evidence
for time variability of the magnetospheric accretion flow on a timescale of a
few rotational periods.Comment: 14 page
Morphological analysis on the coherence of kHz QPOs
We take the recently published data of twin kHz quasi-period oscillations
(QPOs) in neutron star (NS) lowmass X-ray binaries (LMXBs) as the samples, and
investigate the morphology of the samples, which focuses on the quality factor,
peak frequency of kHz QPOs, and try to infer their physical mechanism. We
notice that: (1) The quality factors of upper kHz QPOs are low (2 ~ 20 in
general) and increase with the kHz QPO peak frequencies for both Z and Atoll
sources. (2) The distribution of quality factor versus frequency for the lower
kHz QPOs are quite different between Z and Atoll sources. For most Z source
samples, the quality factors of lower kHz QPOs are low (usually lower than 15)
and rise steadily with the peak frequencies except for Sco X-1, which drop
abruptly at the frequency of about 750 Hz. While for most Atoll sources, the
quality factors of lower kHz QPOs are very high (from 2 to 200) and usually
have a rising part, a maximum and an abrupt drop. (3) There are three Atoll
sources (4U 1728-34, 4U 1636-53 and 4U 1608-52) of displaying very high quality
factors for lower kHz QPOs. These three sources have been detected with the
spin frequencies and sidebands, in which the source with higher spin frequency
presents higher quality factor of lower kHz QPOs and lower difference between
sideband frequency and lower kHz QPO frequency.Comment: 8 pages, 8 figures, publishe
An inhomogeneous jet model for the rapid variability of TeV blazars
We present a new time-dependent inhomogeneous jet model of non-thermal blazar
emission, which reproduces the entire spectral energy distribution together
with the rapid gamma-ray variability. Ultra-relativistic leptons are injected
at the base of a jet and propagate along the jet structure. We assume
continuous reacceleration and cooling, producing a relativistic
quasi-maxwellian (or "pile-up") particle energy distribution. The synchrotron
and Synchrotron-Self Compton jet emissivity are computed at each altitude.
Klein-Nishina effects as well as intrinsic gamma-gamma absorption are included
in the computation. Due to the pair production optical depth, considerable
particle density enhancement can occur, particularly during flaring states.
Time-dependent jet emission can be computed by varying the particle injection,
but due to the sensitivity of pair production process, only small variations of
the injected density are required during the flares. The stratification of the
jet emission, together with a pile-up distribution, allows significantly lower
bulk Lorentz factors, compared to one-zone models. Applying this model to the
case of PKS2155-304 and its big TeV flare observed in 2006, we can reproduce
simultaneously the average broad band spectrum of this source as well as the
TeV spectra and TeV light curve of the flare with bulk Lorentz factor lower
than 15
Superluminal motion of a relativistic jet in the neutron star merger GW170817
The binary neutron star merger GW170817 was accompanied by radiation across
the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance
of 41+/-3 Mpc. The radio and X-ray afterglows of GW170817 exhibited delayed
onset, a gradual rise in the emission with time as t^0.8, a peak at about 150
days post-merger, followed by a relatively rapid decline. To date, various
models have been proposed to explain the afterglow emission, including a
choked-jet cocoon and a successful-jet cocoon (a.k.a. structured jet). However,
the observational data have remained inconclusive as to whether GW170817
launched a successful relativistic jet. Here we show, through Very Long
Baseline Interferometry, that the compact radio source associated with GW170817
exhibits superluminal motion between two epochs at 75 and 230 days post-merger.
This measurement breaks the degeneracy between the models and indicates that,
while the early-time radio emission was powered by a wider-angle outflow
(cocoon), the late-time emission was most likely dominated by an energetic and
narrowly-collimated jet, with an opening angle of <5 degrees, and observed from
a viewing angle of about 20 degrees. The imaging of a collimated relativistic
outflow emerging from GW170817 adds substantial weight to the growing evidence
linking binary neutron star mergers and short gamma-ray bursts.Comment: 42 pages, 4 figures (main text), 2 figures (supplementary text), 2
tables. Referee and editor comments incorporate
H.E.S.S. observations of gamma-ray bursts in 2003-2007
Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray
bursts (GRBs) in some scenarios. Exploring this photon energy regime is
necessary for understanding the energetics and properties of GRBs. GRBs have
been one of the prime targets for the H.E.S.S. experiment, which makes use of
four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays.
Dedicated observations of 32 GRB positions were made in the years 2003-2007 and
a search for VHE gamma-ray counterparts of these GRBs was made. Depending on
the visibility and observing conditions, the observations mostly start minutes
to hours after the burst and typically last two hours. Results from
observations of 22 GRB positions are presented and evidence of a VHE signal was
found neither in observations of any individual GRBs, nor from stacking data
from subsets of GRBs with higher expected VHE flux according to a
model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from
the GRB positions were derived. For those GRBs with measured redshifts,
differential upper limits at the energy threshold after correcting for
absorption due to extra-galactic background light are also presented.Comment: 9 pages, 4 tables, 3 figure
Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017
Aims: The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes. Our aim is to understand the radiative processes by investigating the
observed emission and its production mechanism using the High Energy
Stereoscopic System (H.E.S.S.) experiment. Methods: We report recent
observations of the BL Lac source RGB J0152+017 made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cherenkov telescopes. Contemporaneous observations were made in X-rays by the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope. Results: A signal of 173
gamma-ray photons corresponding to a statistical significance of 6.6 sigma was
found in the data. The energy spectrum of the source can be described by a
powerlaw with a spectral index of 2.95+/-0.36stat+/-0.20syst. The integral flux
above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source
spectral energy distribution (SED) can be described using a two-component
non-thermal synchrotron self-Compton (SSC) leptonic model, except in the
optical band, which is dominated by a thermal host galaxy component. The
parameters that are found are very close to those found in similar SSC studies
in TeV blazars. Conclusions: RGB J0152+017 is discovered as a source of VHE
gamma-rays by H.E.S.S. The location of its synchrotron peak, as derived from
the SED in Swift data, allows clearly classification it as a
high-frequency-peaked BL Lac (HBL).Comment: Accepted for publication in A&A Letters (5 pages, 4 figures
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