27,896 research outputs found
High-resolution absorption spectroscopy of the circumgalactic medium of the Milky Way
In this article we discuss the importance of high-resolution absorption
spectroscopy for our understanding of the distribution and physical nature of
the gaseous circumgalactic medium (CGM) that surrounds the Milky Way.
Observational and theoretical studies indicate a high complexity of the gas
kinematics and an extreme multi-phase nature of the CGM in low-redshift
galaxies. High-precision absorption-line measurements of the Milky Way's gas
environment thus are essential to explore fundamental parameters of
circumgalactic gas in the local Universe, such as mass, chemical composition,
and spatial distribution. We shortly review important characteristics of the
Milky Way's CGM and discuss recent results from our multi-wavelength
observations of the Magellanic Stream. Finally, we discuss the potential of
studying the warm-hot phase of the Milky Way's CGM by searching for extremely
weak [FeX] l6374.5 and [FeIVX] l5302.9 absorption in optical QSO spectra.Comment: 7 pages, 4 figures; accepted for publication in Astronomical Notes
(paper version of a talk presented at the 10th Thinkshop, Potsdam, 2013
Modeling GRB 050904: Autopsy of a Massive Stellar Explosion at z=6.29
GRB 050904 at redshift z=6.29, discovered and observed by Swift and with
spectroscopic redshift from the Subaru telescope, is the first gamma-ray burst
to be identified from beyond the epoch of reionization. Since the progenitors
of long gamma-ray bursts have been identified as massive stars, this event
offers a unique opportunity to investigate star formation environments at this
epoch. Apart from its record redshift, the burst is remarkable in two respects:
first, it exhibits fast-evolving X-ray and optical flares that peak
simultaneously at t~470 s in the observer frame, and may thus originate in the
same emission region; and second, its afterglow exhibits an accelerated decay
in the near-infrared (NIR) from t~10^4 s to t~3 10^4 s after the burst,
coincident with repeated and energetic X-ray flaring activity. We make a
complete analysis of available X-ray, NIR, and radio observations, utilizing
afterglow models that incorporate a range of physical effects not previously
considered for this or any other GRB afterglow, and quantifying our model
uncertainties in detail via Markov Chain Monte Carlo analysis. In the process,
we explore the possibility that the early optical and X-ray flare is due to
synchrotron and inverse Compton emission from the reverse shock regions of the
outflow. We suggest that the period of accelerated decay in the NIR may be due
to suppression of synchrotron radiation by inverse Compton interaction of X-ray
flare photons with electrons in the forward shock; a subsequent interval of
slow decay would then be due to a progressive decline in this suppression. The
range of acceptable models demonstrates that the kinetic energy and circumburst
density of GRB 050904 are well above the typical values found for low-redshift
GRBs.Comment: 45 pages, 7 figures, and ApJ accepted. Revised version, minor
modifications and 1 extra figur
Joint Astrophysics Nascent Universe Satellite:. utilizing GRBs as high redshift probes
The Joint Astrophysics Nascent Universe Satellite (JANUS) is a multiwavelength cosmology mission designed to address fundamental questions about the cosmic dawn. It has three primary science objectives: (1) measure the massive star formation rate over 5 †z †12 by discovering and observing high-z gamma-ray bursts (GRBs) and their afterglows, (2) enable detailed studies of the history of reionization and metal enrichment in the early Universe, and (3) map the growth of the first supermassive black holes by discovering and observing the brightest quasars at z â„ 6. A rapidly slewing spacecraft and three science instruments â the X-ray Coded Aperture Telescope (XCAT), the Near InfraRed Telescope (NIRT), and the GAmma-ray Transient Experiment for Students (GATES) â make-up the JANUS observatory and are responsible for realizing the three primary science objectives. The XCAT (0.5â20 keV) is a wide field of view instrument responsible for detecting and localizing âŒ60 z â„ 5 GRBs, including âŒ8 z â„ 8 GRBs, during a 2-year mission. The NIRT (0.7â1.7 ”m) refines the GRB positions and provides rapid (†30 min) redshift information to the astronomical community. Concurrently, the NIRT performs a 20, 000 deg2 survey of the extragalactic sky discovering and localizing âŒ300 z â„ 6 quasars, including âŒ50 at z â„ 7, over a two-year period. The GATES provides high-energy (15 keV â1.0 MeV) spectroscopy as well as 60â500 keV polarimetry of bright GRBs. Here we outline the JANUS instrumentation and the mission science motivations
Three New Long Period X-ray Pulsars Discovered in the Small Magellanic Cloud
The Small Magellanic Cloud is increasingly an invaluable laboratory for
studying accreting and isolated X-ray pulsars. We add to the class of compact
SMC objects by reporting the discovery of three new long period X-ray pulsars
detected with the {\it Chandra X-ray Observatory}. The pulsars, with periods of
152, 304 and 565 seconds, all show hard X-ray spectra over the range from 0.6 -
7.5 keV. The source positions of the three pulsars are consistent with known
H-alpha emission sources, indicating they are likely to be Be type X-ray binary
star systems.Comment: Accepted for publication in the Astrophysical Journa
Hot melt adhesive attachment pad
A hot melt adhesive attachment pad for releasably securing distinct elements together is described which is particularly useful in the construction industry or a spatial vacuum environment. The attachment pad consists primarily of a cloth selectively impregnated with a charge of hot melt adhesive, a thermo-foil heater, and a thermo-cooler. These components are securely mounted in a mounting assembly. In operation, the operator activates the heating cycle transforming the hot melt adhesive to a substantially liquid state, positions the pad against the attachment surface, and activates the cooling cycle solidifying the adhesive and forming a strong, releasable bond
Hubble Space Telescope and Ground-Based Optical and Ultraviolet Observations of GRB010222
We report on Hubble Space Telescope WFPC2 optical and STIS near ultraviolet
MAMA observations, and ground-based optical observations of GRB010222, spanning
15 hrs to 71 days. The observations are well-described by a relativistic
blast-wave model with a hard electron-energy distribution, p = 1.57, and a jet
transition at t_j=0.93 days. These values are slightly larger than previously
found as a result of a correction for the contribution from the host galaxy to
the late-time ground-based observations and the larger temporal baseline
provided by the Hubble Space Telescope observations. The host galaxy is found
to contain a very compact core (size <0.25 arcsec) which coincides with the
position of the optical transient. The STIS near ultraviolet MAMA observations
allow for an investigation of the extinction properties along the line of sight
to GRB010222. We find that the far ultraviolet curvature component (c_4) is
rather large. In combination with the low optical extinction A_V =0.11 mag,
when compared to the Hydrogen column inferred from X-ray observations, we
suggest that this is evidence for dust destruction.Comment: ApJ, in pres
Plasticization and antiplasticization of polymer melts diluted by low molar mass species
An analysis of glass formation for polymer melts that are diluted by
structured molecular additives is derived by using the generalized entropy
theory, which involves a combination of the Adam-Gibbs model and the direct
computation of the configurational entropy based on a lattice model of polymer
melts that includes monomer structural effects. Antiplasticization is
accompanied by a "toughening" of the glass mixture relative to the pure
polymer, and this effect is found to occur when the diluents are small species
with strongly attractive interactions with the polymer matrix. Plasticization
leads to a decreased glass transition temperature T_g and a "softening" of the
fragile host polymer in the glass state. Plasticization is prompted by small
additives with weakly attractive interactions with the polymer matrix. The
shifts in T_g of polystyrene diluted by fully flexible short oligomers are
evaluated from the computations, along with the relative changes in the
isothermal compressibility at T_g to characterize the extent to which the
additives act as antiplasticizers or plasticizers. The theory predicts that a
decreased fragility can accompany both antiplasticization and plasticization of
the glass by molecular additives. The general reduction in the T_g and
fragility of polymers by these molecular additives is rationalized by analyzing
the influence of the diluent's properties (cohesive energy, chain length, and
stiffness) on glass formation in diluted polymer melts. The description of
glass formation at fixed temperature that is induced upon change the fluid
composition directly implies the Angell equation for the structural relaxation
time as function of the polymer concentration, and the computed "zero mobility
concentration" scales linearly with the inverse polymerization index N.Comment: 12 pages, 15 figure
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