3,090 research outputs found
Periodic Modulations in an X-ray Flare from Sagittarius A*
We present the highly significant detection of a quasi-periodic flux
modulation with a period of 22.2 min seen in the X-ray data of the Sgr A* flare
of 2004 August 31. This flaring event, which lasted a total of about three
hours, was detected simultaneously by EPIC on XMM-Newton and the NICMOS
near-infrared camera on the HST. Given the inherent difficulty in, and the lack
of readily available methods for quantifying the probability of a periodic
signal detected over only several cycles in a data set where red noise can be
important, we developed a general method for quantifying the likelihood that
such a modulation is indeed intrinsic to the source and does not arise from
background fluctuations. We here describe this Monte Carlo based method, and
discuss the results obtained by its application to a other XMM-Newton data
sets. Under the simplest hypothesis that we witnessed a transient event that
evolved, peaked and decayed near the marginally stable orbit of the
supermassive black hole, this result implies that for a mass of 3.5 x 10^{6}
Msun, the central object must have an angular momentum corresponding to a spin
parameter of a=0.22.Comment: 4 pages, 6 figures, submitted to ApJ
Event Weighted Tests for Detecting Periodicity in Photon Arrival Times
This paper treats the problem of detecting periodicity in a sequence of
photon arrival times, which occurs, for example, in attempting to detect
gamma-ray pulsars. A particular focus is on how auxiliary information,
typically source intensity, background intensity, and incidence angles and
energies associated with each photon arrival should be used to maximize the
detection power. We construct a class of likelihood-based tests, score tests,
which give rise to event weighting in a principled and natural way, and derive
expressions quantifying the power of the tests. These results can be used to
compare the efficacies of different weight functions, including cuts in energy
and incidence angle. The test is targeted toward a template for the periodic
lightcurve, and we quantify how deviation from that template affects the power
of detection
Shadowing Effects on the Nuclear Suppression Factor, R_dAu, in d+Au Interactions
We explore how nuclear modifications to the nucleon parton distributions
affect production of high transverse momentum hadrons in deuteron-nucleus
collisions. We calculate the charged hadron spectra to leading order using
standard fragmentation functions and shadowing parameterizations. We obtain the
d+Au to pp ratio both in minimum bias collisions and as a function of
centrality. The minimum bias results agree reasonably well with the BRAHMS data
while the calculated centrality dependence underestimates the data and is a
stronger function of p_T than the data indicate.Comment: 18 pages, 3 figures, final version, Phys. Rev. C in pres
Ventricular perforation as a complication of percutaneous valve replacement
Aortic stenosis (AS) is the most commonly encountered valvular disease in developed countries. Once symptomatic, this disease carries a dismal prognosis. Patients unfit for surgical valve replacement may require transcatheter aortic valve implantations (TAVI). We present a complication of this novel approach where a prosthetic valve is delivered through the femoral artery by means of a Retroflex delivery system developed by Edwards Lifesciences
X-Ray Evidence for Flare Density Variations and Continual Chromospheric Evaporation in Proxima Centauri
Using the XMM-Newton X-ray observatory to monitor the nearest star to the
Sun, Proxima Centauri, we recorded the weakest X-ray flares on a magnetically
active star ever observed. Correlated X-ray and optical variability provide
strong support for coronal energy and mass supply by a nearly continuous
sequence of rapid explosive energy releases. Variable emission line fluxes were
observed in the He-like triplets of OVII and NeIX during a giant flare. They
give direct X-ray evidence for density variations, implying densities between
2x10^{10} - 4x10^{11} cm^{-3} and providing estimates of the mass and the
volume of the line-emitting plasma. We discuss the data in the context of the
chromospheric evaporation scenario.Comment: 10 pages, 2 figures, accepted by The Astrophysical Journal, Letters;
improved calculations of radiative loss of cool plasma (toward end of paper
The composition and nature of the dust shell surrounding the binary AFGL 4106
We present infrared spectroscopy and imaging of AFGL~4106. The 2.4-5 micron
ISO-SWS spectrum reveals the presence of a cool, luminous star (T_eff ~ 3750 K)
in addition to an almost equally luminous F star (T_eff ~ 7250 K). The 5-195
micron SWS and LWS spectra are dominated by strong emission from circumstellar
dust. We find that the dust consists of amorphous silicates, with a minor but
significant contribution from crystalline silicates. The amorphous silicates
consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be
excluded but if present they contain much less Fe than the amorphous olivines.
Comparison with laboratory data shows that the pure Mg-end members of the
crystalline olivine and pyroxene solid solution series are present. In
addition, we find strong evidence for simple oxides (FeO and Al2O3) as well as
crystalline H2O ice. Several narrow emission features remain unidentified.
Modelling of the dust emission using a dust radiation transfer code shows that
large grains (~1 micron) must be present and that the abundance of the
crystalline silicates is between 7 and 15% of the total dust mass, depending on
the assumed enstatite to forsterite ratio, which is estimated to be between 1
and 3. The amorphous and crystalline dust components in the shell do not have
the same temperature, implying that the different dust species are not
thermally coupled. We find a dust mass of ~3.9 x 10^-2 M_sol expelled over a
period of 4 x 10^3 years for a distance of 3.3 kpc. The F-star in the AFGL~4106
binary is likely a post-red-supergiant in transition to a blue supergiant or WR
phase.Comment: 22 pages (including 12 figures), accepted by Astronomy and
Astrophysic
Recommended from our members
The solar influence on the probability of relatively cold UK winters in the future
Recent research has suggested that relatively cold UK winters are more common when solar activity is low (Lockwood et al 2010 Environ. Res. Lett. 5 024001). Solar activity during the current sunspot minimum has fallen to levels unknown since the start of the 20th century (Lockwood 2010 Proc. R. Soc. A 466 303â29) and records of past solar variations inferred from cosmogenic isotopes (Abreu et al 2008 Geophys. Res. Lett. 35 L20109) and geomagnetic activity data (Lockwood et al 2009 Astrophys. J. 700 937â44) suggest that the current grand solar maximum is coming to an end and hence that solar activity can be expected to continue to decline. Combining cosmogenic isotope data with the long record of temperatures measured in central England, we estimate how solar change could influence the probability in the future of further UK winters that are cold, relative to the hemispheric mean temperature, if all other factors remain constant. Global warming is taken into account only through the detrending using mean hemispheric temperatures. We show that some predictive skill may be obtained by including the solar effect
DE Canum Venaticorum : a bright, eclipsing red dwarfâwhite dwarf binary
Context. Close white dwarfâred dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarfâred dwarf binary with a relatively short (âŒ8.7 h) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study.
Aims. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters that we discuss in the framework of common-envelope evolution.
Methods. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average lowresolution spectrum of DE CVn, we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system.
Results. The derived ephemeris is HJDmin = 2 452 784.5533(1) + 0.3641394(2) Ă E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8 000 K. The inclination of the system is 86+3⊠â2 and the mass and radius of the red dwarf are 0.41 ± 0.06 M and 0.37+0.06 â0.007 R, respectively, and the mass and radius of the white dwarf are 0.51+0.06
â0.02 M and 0.0136+0.0008 â0.0002 R, respectively.
Conclusions. We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively lowmass star (M †1.6 M). The current age of this system is 3.3â7.3 Ă 109 years, while it will take longer than the Hubble time for DE CVn to evolve into a semi-detached system
X-ray observations of the Large Magellanic Cloud pulsar PSR B0540-69 and its PWN
PSR B0540-69 is a young pulsar in the Large Magellanic Cloud that has similar
properties with respect to the Crab Pulsar, and is embedded in a Pulsar Wind
Nebula. We have analyzed the complete archival RXTE dataset of observations of
this source, together with new Swift-XRT and INTEGRAL-IBIS data. Accurate
lightcurves are produced in various energy bands between 2 and 60 keV, showing
no significant energy variations of the pulse shape. The spectral analysis
shows that the pulsed spectrum is curved, and is best fitted up to 100 keV by a
log-parabolic model: this strengthens the similarities with the Crab pulsar,
and is discussed in the light of a phenomenologic multicomponent model. The
total emission from this source is studied, the relative contributions of the
pulsar and the PWN emission are derived, and discussed in the context of other
INTEGRAL detected pulsar/PWN systems.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
- âŠ