2,098 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
Galactic Globular Cluster Metallicity Scale from the Calcium Triplet. II. Rankings, Comparisons and Puzzles
We compare our compilation of the W' calcium index for 71 Galactic globular
clusters to the widely used Zinn and West (1984 ApJS, 55, 45) [Fe/H] scale and
to Carretta and Gratton's (1997 A&A Supplement 121, 95) scale from
high-dispersion spectra analyzed with Kurucz (1992, private communication)
model atmospheres. We find our calcium ranking to be tightly correlated with
each comparison set, in a non-linear and a linear fashion, respectively. By
combining our calcium index information with the Zinn and West ranking, we are
able to rank the globular clusters in our sample with a typical precision of
+/- 0.05 dex for [Fe/H] < -0.5 on the Zinn and West scale; for clusters more
metal rich than this, the ranking is less precise. The significant differences
between these metallicity scales raise important questions about our
understanding of Galactic formation and chemical enrichment processes.
Furthermore, in spite of the apparent improvement in metallicity ranking for
the Galactic globular clusters that results from our addition of information
from the Ca II triplet lines to the potpourri of other metallicity indicators,
caution -- perhaps considerable -- may be advisable when using W' as a
surrogate for metallicity, especially for systems where ranges in age and
metallicity are likely.Comment: To appear in the August 1997 issue of PASP Also available at
http://www.hia.nrc.ca/eprints.htm
Measurement of the Radius of Neutron Stars with High S/N Quiescent Low-mass X-ray Binaries in Globular Clusters
This paper presents the measurement of the neutron star (NS) radius using the
thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) inside globular
clusters (GCs). Recent observations of NSs have presented evidence that cold
ultra dense matter -- present in the core of NSs -- is best described by
"normal matter" equations of state (EoSs). Such EoSs predict that the radii of
NSs, Rns, are quasi-constant (within measurement errors, of ~10%) for
astrophysically relevant masses (Mns > 0.5 Msun). The present work adopts this
theoretical prediction as an assumption, and uses it to constrain a single Rns
value from five qLMXB targets with available high signal-to-noise X-ray
spectroscopic data. Employing a Markov-Chain Monte-Carlo approach, we produce
the marginalized posterior distribution for Rns, constrained to be the same
value for all five NSs in the sample. An effort was made to include all
quantifiable sources of uncertainty into the uncertainty of the quoted radius
measurement. These include the uncertainties in the distances to the GCs, the
uncertainties due to the Galactic absorption in the direction of the GCs, and
the possibility of a hard power-law spectral component for count excesses at
high photon energy, which are observed in some qLMXBs in the Galactic plane.
Using conservative assumptions,we found that the radius, common to the five
qLMXBs and constant for a wide range of masses, lies in the low range of
possible NS radii, Rns=9.1(+1.3)(-1.5) km (90%-confidence). Such a value is
consistent with low-res equations of state. We compare this result with
previous radius measurements of NSs from various analyses of different types of
systems. In addition, we compare the spectral analyses of individual qLMXBs to
previous works.Comment: Accepted to Apj. 31 pages, 17 figures, 8 table
Chandra Observations of 1RXS J141256.0+792204 (Calvera)
We report the results of a 30 ks Chandra ACIS-S observation of the isolated
compact object 1RXS J141256.0+792204 (Calvera). The X-ray spectrum is
adequately described by an absorbed neutron star hydrogen atmosphere model with
an effective temperature at infinity of 88.3 +/- 0.8 eV and radiation radius at
infinity of 4.1 +/- 0.1 km/kpc. The best-fit blackbody spectrum yields
parameters consistent with previous measurements; although the fit itself is
not statistically acceptable, systematic uncertainties in the pile-up
correction may contribute to this. We find marginal evidence for narrow
spectral features in the X-ray spectrum between 0.3 and 1.0 keV. In one
interpretation, we find evidence at 81%-confidence for an absorption edge at
0.64 (+0.08) (-0.06) keV with an equivalent width of ~70 eV; if this feature is
real, it is reminiscent of features seen in the isolated neutron stars RX
J1605.3+3249, RX J0720.4-3125, and 1RXS J130848.6+212708 (RBS 1223). In an
alternative approach, we find evidence at 88%-confidence for an unresolved
emission line at energy 0.53 +/- 0.02 keV, with an equivalent width of ~28 eV;
the interpretation of this feature, if real, is uncertain. We search for
coherent pulsations up to the Nyquist frequency of 1.13 Hz and set an upper
limit of 8.0% rms on the strength of any such modulation. We derive an improved
position for the source and set the most rigorous limits to-date on any
associated extended emission on arcsecond scales. Our analysis confirms the
basic picture of Calvera as the first isolated compact object in the
ROSAT/Bright Source Catalog discovered in six years, the hottest such object
known, and an intriguing target for multiwavelength study.Comment: Submitted to ApJ. AASTeX, 19 pages, 2 figure
No Persistent Pulsations in Aquila X-1 as it Fades into Quiescence
We searched for coherent X-ray pulsations from Aql X-1 in a series of RXTE observations taken shortly after a recent outburst. During the course of these observations, Aql X-1 passes through an apparent "propeller" phase as its luminosity fades to its quiescent value. No pulsations were detected, and we place upper limits (ranging from 0.52% to 9.0%) on the fractional rms amplitude of any periodic signal contained in the various data sets searched. This result has implications for the geometry of the system, if the quiescent luminosity is due to continued low-level accretion. Alternatively, our result supports the idea that the quiescent luminosity may be due to thermal emission
Freeze-Thaw Cycling as a Chemical Weathering Agent on a Cold and Icy Mars
Liquid water was abundant on early Mars, but whether the climate was warm and wet or cold and icy with punctuated periods of melting is still poorly understood. Modern climate models for Mars tend to predict a colder, icier early climate than previously imagined. In addition, ice and glaciation have been major geologic agents throughout the later Hesperian and Amazonian eras. One process that can act in such climates is repeated freezing and thawing of water on the surface and in the subsurface, and is significant because it can occur anywhere with an active layer and could have persisted for a time after liquid water was no longer stable on Mars surface. As freeze-thaw is the dominant mechanical weathering process in most glacial/periglacial terrains, it was likely a significant geomorphologic driver at local to regional scales during past climates, and would potentially have been most active when day-average surface temperatures exceeded 0 C for part of the year. Indeed, freeze-thaw involving liquid water in the Amazonian is evidenced by abundant geomorphic features including polygonal ground and solifluction lobes requiring seasonal thawing. In addition to physical modification, freezing can drive solutions towards supersaturation and force dissolved solutes out as precipitates. In Mars-like terrains, dissolved solutes are typically dominated by silica. In polar regions on Earth, freeze-thaw cycles have been shown to promote deposition of silica, and freeze-thaw experiments on synthetic solutions found stable amorphous silica that built up over multiple cycles. Freeze-thaw may therefore be an important but overlooked chemical weathering process on Mars. However, our ability to assess its impact on alteration of martian terrains is majorly limited by the current lack of understanding of the alteration phases produced (and formation rates) under controlled freeze-thaw weathering of Mars-relevant materials. To address this knowledge gap, we report results from (1) freeze-thaw weathering products found at a glacial Mars analog site at the Three Sisters, Oregon, and (2) new controlled freeze-thaw experiments on basaltic material
Cosmological Gamma-Ray Bursts and Evolution of Galaxies
Evolution of the rate density of cosmological gamma-ray bursts (GRBs) is
calculated and compared to the BATSE brightness distribution in the context of
binary neutron-star mergers as the source of GRBs, taking account of the
realistic star formation history in the universe and evolution of compact
binary systems. We tried two models of the evolution of cosmic star formation
rate (SFR): one is based on recent observations of SFRs at high redshifts,
while the other is based on a galaxy evolution model of stellar population
synthesis that reproduces the present-day colors of galaxies. It is shown that
the binary merger scenario of GRBs naturally results in the comoving
rate-density evolution of \propto (1+z)^{2-2.5} up to z ~ 1, that has been
suggested independently from the compatibility between the number-brightness
distribution and duration-brightness correlation. If the cosmic SFR has its
peak at z ~ 1--2 as suggested by recent observations, the effective power-index
of GRB photon spectrum, \alpha >~ 1.5$ is favored, that is softer than the
recent observational determination of \alpha = 1.1 \pm 0.3. However, high
redshift starbursts (z >~ 5) in elliptical galaxies, that have not yet been
detected, can alleviate this discrepancy. The redshift of GRB970508 is likely
about 2, just below the upper limit that is recently determined, and the
absorption system at z = 0.835 seems not to be the site of the GRB.Comment: ApJ Lett. in press, very minor change just making clear that the
predicted rate-density evolution is in a comoving sense. (Received 1997 May
15; Accepted 1997 July 2
Oxidation and protection of fiberglass-epoxy composite masts for photovoltaic arrays in the low Earth orbital environment
Fiberglass-epoxy composites are considered for use as structural members for the mast of the space station solar array panel. The low Earth orbital environment in which space station is to operate is composed mainly of atomic oxygen, which has been shown to cause erosion of many organic materials and some metals. Ground based testing in a plasma asher was performed to determine the extent of degradation of fiberglass-epoxy composites when exposed to a simulated atomic oxygen environment. During exposure, the epoxy at the surface of the composite was oxidized, exposing individual glass fibers which could easily be removed. Several methods of protecting the composite were evaluated in an atomic oxygen environment and with thermal cycling and flexing. The protection techniques evaluated to date include an aluminum braid covering, an indium-tin eutectic and a silicone based paint. The open aluminum braid offered little protection while the CV-1144 coating offered some initial protection against atomic oxygen, but appears to develop cracks which accelerate degradation when flexed. Coatings such as the In-Sn eutectic may provide adequate protection by containing the glass fibers even though mass loss still occurs
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