705 research outputs found
Gamma-Ray Bursts and the Cosmic Star Formation Rate
We have tested several models of GRB luminosity and redshift distribution
functions for compatibility with the BATSE 4B number versus peak flux relation.
Our results disagree with recent claims that current GRB observations can be
used to strongly constrain the cosmic star formation history. Instead, we find
that relaxing the assumption that GRBs are standard candles renders a very
broad range of models consistent with the BATSE number-flux relation. We
explicitly construct two sample distributions, one tracing the star formation
history and one with a constant comoving density. We show that both
distributions are compatible with the observed fluxes and redshifts of the
bursts GRB970508, GRB971214, and GRB980703, and we discuss the measurements
required to distinguish the two models.Comment: 12 pages, 2 postscript figures, uses AAS LaTex macros v4.0. To be
published in Astrophysical Journal Letters, accepted August 20, 1998. Revised
for publicatio
The Nature of Active Galactic Nuclei with Velocity Offset Emission Lines
We obtained Keck/OSIRIS near-IR adaptive optics-assisted integral-field
spectroscopy to probe the morphology and kinematics of the ionized gas in four
velocity-offset active galactic nuclei (AGNs) from the Sloan Digital Sky
Survey. These objects possess optical emission lines that are offset in
velocity from systemic as measured from stellar absorption features. At a
resolution of ~0.18", OSIRIS allows us to distinguish which velocity offset
emission lines are produced by the motion of an AGN in a dual supermassive
black hole system, and which are produced by outflows or other kinematic
structures. In three galaxies, J1018+2941, J1055+1520 and J1346+5228, the
spectral offset of the emission lines is caused by AGN-driven outflows. In the
remaining galaxy, J1117+6140, a counterrotating nuclear disk is observed that
contains the peak of Pa emission 0.2" from the center of the galaxy.
The most plausible explanation for the origin of this spatially and
kinematically offset peak is that it is a region of enhanced Pa
emission located at the intersection zone between the nuclear disk and the bar
of the galaxy. In all four objects, the peak of ionized gas emission is not
spatially coincident with the center of the galaxy as traced by the peak of the
near-IR continuum emission. The peaks of ionized gas emission are spatially
offset from the galaxy centers by 0.1"-0.4" (0.1-0.7 kpc). We find that the
velocity offset originates at the location of this peak of emission, and the
value of the offset can be directly measured in the velocity maps. The
emission-line ratios of these four velocity-offset AGNs can be reproduced only
with a mixture of shocks and AGN photoionization. Shocks provide a natural
explanation for the origin of the spatially and spectrally offset peaks of
ionized gas emission in these galaxies.Comment: 14 pages, 12 figures, accepted for publication in Ap
Optimization of graded multilayer designs for astronomical x-ray telescopes
We developed a systematic method for optimizing the design of depth-graded multilayers for astronomical hard-x-ray and soft-γ-ray telescopes based on the instrument’s bandpass and the field of view. We apply these methods to the design of the conical-approximation Wolter I optics employed by the balloon-borne High Energy Focusing Telescope, using W/Si as the multilayer materials. In addition, we present optimized performance calculations of mirrors, using other material pairs that are capable of extending performance to photon energies above the W K-absorption edge (69.5 keV), including Pt/C, Ni/C, Cu/Si, and Mo/Si
Determining the Gamma-Ray Burst Distance Scale: Observational Prospects
The BATSE instrument on the Compton Gamma-Ray Observatory has demonstrated that we live near the center of an isotropic but bounded distribution of gamma-ray burst sources but has left unsettled whether the bursts occur in our own Galaxy or at cosmological distances. Because a distance and energy scale is crucial to constraining burst models, this distance ambiguity must be resolved. The key experiment that would distinguish the possibilities is a search for bursts from the halo of M31 or other nearby galaxies. We discuss the observational prospects for this test, showing that no telescope now in orbit or scheduled for launch can settle the debate, but that an experiment could be done with a low-cost, dedicated instrument
The ASCA X-Ray Spectrum of the Unusual Binary LSI +61°303
The unusual Be binary LSI +61°303 was observed by the ASCA X-ray satellite twice during the 26 day orbital-outburst cycle. Here we present the results of the spectral analysis of the ASCA data. The emission spectrum can be characterized by an absorbed power law and is inconsistent with other single component models such as blackbody or hot plasma emission. The data show both flux and spectral
variability between the two observations. The column density is determined for LSI 61°303 , and we put upper limits on Fe line emission and Fe absorption edge optical depth. The power-law index is similar to that seen for classical X-ray pulsars ; however, the low X-ray luminosity and the power-law emission in other wavebands indicate that the emission is not from an optically thick accretion column but rather synchrotron or inverse-Compton emission from relativistic particles. We argue that the sub-eV emission is synchrotron, the super-eV emission is inverse Compton and that the magnetic field in the emission region is a few hundred gauss
Constraints on the average magnetic field strength of relic radio sources 0917+75 and 1401-33 from XMM-Newton observations
We observed two relic radio sources, 0917+75 and 1401-33, with the XMM-Newton
X-ray observatory. We did not detect any X-ray emission, thermal or
non-thermal, in excess of the local background level from either target. This
imposes new upper limits on the X-ray flux due to inverse Compton scattering of
photons from the cosmic microwave background by relativistic electrons in the
relic sources, and new lower limits on the magnetic field strength from the
relative strength of the radio and X-ray emission. The combination of radio and
X-ray observations provides a measure of the magnetic field independent of
equipartition or minimum energy assumptions. Due to increasing sensitivity of
radio observations, the known population of cluster relics has been growing;
however, studies of non-thermal X-ray emission from relics remain scarce. Our
study adds to the small sample of relics studied in X-rays. In both relics, our
field strength lower limits are slightly larger than estimates of the
equipartition magnetic field.Comment: 11 pages, 5 figures. Accepted by MNRA
NuSTAR observations of the young, energetic radio pulsar PSR B1509-58
We report on Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray
observations of the young rotation-powered radio pulsar PSR B150958 in the
supernova remnant MSH 1552. We confirm the previously reported curvature in
the hard X-ray spectrum, showing that a log parabolic model provides a
statistically superior fit to the spectrum compared with the standard power
law. The log parabolic model describes the NuSTAR data, as well as previously
published gamma-ray data obtained with COMPTEL and AGILE, all together spanning
3 keV through 500 MeV. Our spectral modelling allows us to constrain the peak
of the broadband high energy spectrum to be at 2.60.8 MeV, an improvement
of nearly an order of magnitude in precision over previous measurements. In
addition, we calculate NuSTAR spectra in 26 pulse phase bins and confirm
previously reported variations of photon indices with phase. Finally, we
measure the pulsed fraction of PSR B150958 in the hard X-ray energy band for
the first time. Using the energy resolved pulsed fraction results, we estimate
that the pulsar's off-pulse emission has a photon index value between 1.26 and
1.96. Our results support a model in which the pulsar's lack of GeV emission is
due to viewing geometry, with the X-rays originating from synchrotron emission
from secondary pairs in the magnetosphere.Comment: 10 pages, 8 figures, 6 tables, ApJ accepte
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