40,495 research outputs found
Transient Relativistically-Shifted Lines as a Probe of Black Hole Systems
X-ray spectra of Seyfert galaxies have revealed a new type of X-ray spectral
feature, one which appears to offer important new insight into the black hole
system. XMM/Chandra revealed several narrow emission lines redward of Fe Kalpha
in NGC 3516. Since that discovery the phenomenon has been observed in other
Seyfert galaxies, e.g. NGC 7314 and ESO 198-G24. We present new evidence for a
redshifted Fe line in XMM spectra of Mrk 766. These data reveal the first
evidence for a significant shift in the energy of such a line, occurring over a
few tens of kiloseconds. This shift may be interpreted as deceleration of an
ejected blob of gas traveling close to the escape velocity.Comment: 13 pages, 5 figures (4 color) accepted by Ap
A kinetic model describing the processivity of Myosin-V
The precise details of how myosin-V coordinates the biochemical reactions and mechanical motions of its two head elements to engineer effective processive molecular motion along actin filaments remain unresolved. We compare a quantitative kinetic model of the myosin-V walk, consisting of five basic states augmented by two further states to allow for futile hydrolysis and detachments, with experimental results for run lengths, velocities, and dwell times and their dependence on bulk nucleotide concentrations and external loads in both directions. The model reveals how myosin-V can use the internal strain in the molecule to synchronize the motion of the head elements. Estimates for the rate constants in the reaction cycle and the internal strain energy are obtained by a computational comparison scheme involving an extensive exploration of the large parameter space. This scheme exploits the fact that we have obtained analytic results for our reaction network, e.g., for the velocity but also the run length, diffusion constant, and fraction of backward steps. The agreement with experiment is often reasonable but some open problems are highlighted, in particular the inability of such a general model to reproduce the reported dependence of run length on ADP concentration. The novel way that our approach explores parameter space means that any confirmed discrepancies should give new insights into the reaction network model
A measurement of the transverse velocity of Q2237+0305
Determination of microlensing parameters in the gravitationally lensed quasar
Q2237+0305 from the statistics of high magnification events will require
monitoring for more than 100 years (Wambsganss, Paczynski & Schneider 1990).
However we show that the effective transverse velocity of the lensing galaxy
can be determined on a more realistic time-scale through consideration of the
distribution of light-curve derivatives. The 10 years of existing monitoring
data for Q2237+0305 are analysed. These data display strong evidence for
microlensing that is not associated with a high magnification event. An upper
limit of v < 500 km/sec is obtained for the galactic transverse velocity which
is smaller than previously assumed values. The analysis suggests that the
observed microlensing variation may be predominantly due to stellar proper
motions. The statistical significance of the results obtained from our method
will be increased by the addition of data points from current and future
monitoring campaigns. However reduced photometric errors will be more valuable
than an increased sampling rate.Comment: 16 pages, including 17 figures. Accepted for publication in M.N.R.A.
The Two-Screen Measurement Setup to Indirectly Measure Proton Beam Self-Modulation in AWAKE
The goal of the first phase of the AWAKE \cite{AWAKE1,AWAKE2} experiment at
CERN is to measure the self-modulation \cite{SMI} of the long SPS proton bunch into microbunches after traversing
of plasma with a plasma density of
. The two screen measurement setup
\cite{Turner2016} is a proton beam diagnostic that can indirectly prove the
successful development of the self-modulation of the proton beam by imaging
protons that got defocused by the transverse plasma wakefields after passing
through the plasma, at two locations downstream the end of the plasma. This
article describes the design and realization of the two screen measurement
setup integrated in the AWAKE experiment. We discuss the performance and
background response of the system based on measurements performed with an
unmodulated Gaussian SPS proton bunch during the AWAKE beam commissioning in
September and October 2016. We show that the system is fully commissioned and
adapted to eventually image the full profile of a self-modulated SPS proton
bunch in a single shot measurement during the first phase of the AWAKE
experiment.Comment: 5 pages 8 figure
Interpretation of the OGLE Q2237+0305 microlensing light-curve
The four bright images of the gravitationally lensed quasar Q2237+0305 are
being monitored from the ground (eg. OGLE collaboration, Apache Point
Observatory) in the hope of observing a high magnification event (HME). Over
the past three seasons (1997-1999) the OGLE collaboration has produced
microlensing light-curves with unprecedented coverage. These demonstrate
smooth, independent (therefore microlensing) variability between the images
(Wozniak et al. 2000a,b; OGLE web page). We have retrospectively compared
probability functions for high-magnification event parameters with several
observed light-curve features. We conclude that the 1999 image C peak was due
to the source having passed outside of a cusp rather than to a caustic
crossing. In addition, we find that the image C light-curve shows evidence for
a caustic crossing between the 1997 and 1998 observing seasons involving the
appearance of new critical images. Our models predict that the next image C
event is most likely to arrive 500 days following the 1999 peak, but with a
large uncertainty (100-2000 days). Finally, given the image A light-curve
derivative at the end of the 1999 observing season, our modelling suggests that
a caustic crossing will occur between the 1999 and 2000 observing seasons,
implying a minimum for the image A light-curve ~1-1.5 magnitudes fainter than
the November 1999 level.Comment: 11 pages, 15 figures. Accepted for publication in M.N.R.A.
Limits on the microlens mass function of Q2237+0305
Gravitational microlensing at cosmological distances is potentially a
powerful tool for probing the mass functions of stars and compact objects in
other galaxies. In the case of multiply-imaged quasars, microlensing data has
been used to determine the average microlens mass. However the measurements
have relied on an assumed transverse velocity for the lensing galaxy. Since the
measured mass scales with the square of the transverse velocity, published mass
limits are quite uncertain. In the case of Q2237+0305 we have properly
constrained this uncertainty. The distribution of light curve derivatives
allows quantitative treatment of the relative rates of microlensing due to
proper motions of microlenses, the orbital stream motion of microlenses and the
bulk galactic transverse velocity. By demanding that the microlensing rate due
to the motions of microlenses is the minimum that should be observed we
determine lower limits for the average mass of stars and compact objects in the
bulge of Q2237+0305. If microlenses are assumed to move in an orbital stream
the lower limit ranges between 0.005 and 0.023 solar masses where the the
systematic dependence is due to the fraction of smooth matter and the size of
photometric error assumed for published monitoring data. However, if the
microlenses are assumed to move according to an isotropic velocity dispersion
then a larger lower limit of 0.019-0.11 solar masses is obtained. A significant
contribution of Jupiter mass compact objects to the mass distribution of the
galactic bulge of Q2237+0305 is therefore unambiguously ruled out.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Society. New version has improved presentatio
The Variable X-ray Spectrum of Markarian 766 - II. Time-Resolved Spectroscopy
CONTEXT: The variable X-ray spectra of AGN systematically show steep
power-law high states and hard-spectrum low states. The hard low state has
previously been found to be a component with only weak variability. The origin
of this component and the relative importance of effects such as absorption and
relativistic blurring are currently not clear. AIMS: In a follow-up of previous
principal components analysis, we aim to determine the relative importance of
scattering and absorption effects on the time-varying X-ray spectrum of the
narrow-line Seyfert 1 galaxy Mrk~766. METHODS: Time-resolved spectroscopy,
slicing XMM and Suzaku data down to 25 ks elements, is used to investigate
whether absorption or scattering components dominate the spectral variations in
Mrk 766.Time-resolved spectroscopy confirms that spectral variability in Mrk
766 can be explained by either of two interpretations of principal components
analysis. Detailed investigation confirm rapid changes in the relative
strengths of scattered and direct emission or rapid changes in absorber
covering fraction provide good explanations of most of the spectral
variability. However, a strong correlation between the 6.97 keV absorption line
and the primary continuum together with rapid opacity changes show that
variations in a complex and multi-layered absorber, most likely a disk wind,
are the dominant source of spectral variability in Mrk 76
Predicting caustic crossing high magnification events in Q2237+0305
The central regions of the gravitationally lensed quasar Q2237+0305 can be
indirectly resolved on nano-arcsecond scales if viewed spectrophotometricly
during a microlensing high magnification event (HME). Q2237+0305 is currently
being monitored from the ground (eg. OGLE collaboration, Apache Point
Observatory), with the goal, among others, of triggering ground and spacecraft
based target of opportunity (TOO) observations of an HME. In this work we
investigate the rate of change (trigger) in image brightness that signals an
imminent HME and importantly, the separation between the trigger and the event
peak. In addition, we produce colour dependent model light-curves by combining
high-resolution microlensing simulations with a realistic model for a thermal
accretion disc source. We make hypothetical target of opportunity spectroscopic
observations using our determination of the appropriate trigger as a guide. We
find that if the source spectrum varies with source radius, a 3 observation TOO
program should be able to observe a microlensing change in the continuum slope
following a light-curve trigger with a success rate of >80%.Comment: 17 pages, 16 figures, accepted for publication in M.N.R.A.
- …