66 research outputs found
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Contains research objectives, summary of research and reports on one research project.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)National Aeronautics and Space Administration (Grant NGL-22-009-013
Monitoring Supergiant Fast X-ray Transients with Swift. Results from the first year
Swift has allowed the possibility to give Supergiant Fast X-ray Transients
(SFXTs), the new class of High Mass X-ray Binaries discovered by INTEGRAL, non
serendipitous attention throughout all phases of their life. We present our
results based on the first year of intense Swift monitoring of four SFXTs, IGR
J16479-4514, XTE J1739-302, IGR J17544-2619 and AX J1841.0-0536. We obtain the
first assessment of how long each source spends in each state using a
systematic monitoring with a sensitive instrument. The duty-cycle of inactivity
is 17, 28, 39, 55% (5% uncertainty), for IGR J16479-4514, AX J1841.0-0536, XTE
J1739-302, and IGR J17544-2619, respectively, so that true quiescence is a rare
state. This demonstrates that these transients accrete matter throughout their
life at different rates. AX J1841.0-0536 is the only source which has not
undergone a bright outburst during our campaign. Although individual sources
behave somewhat differently, common X-ray characteristics of this class are
emerging such as outburst lengths well in excess of hours, with a multiple
peaked structure. A high dynamic range (including bright outbursts) of 4 orders
of magnitude has been observed. We performed out-of-outburst intensity-based
spectroscopy. Spectral fits with an absorbed blackbody always result in
blackbody radii of a few hundred meters, consistent with being emitted from a
small portion of the neutron star surface, very likely the neutron star polar
caps. We also present the UVOT data of these sources. (Abridged)Comment: Accepted for publication in MNRAS. 20 pages, 9 figures, 8 table
Antideuteron yield at the AGS and coalescence implications
We present Experiment 864's measurement of invariant antideuteron yields in
11.5A GeV/c Au + Pt collisions. The analysis includes 250 million triggers
representing 14 billion 10% central interactions sampled for events with high
mass candidates. We find (1/2 pi pt) d^(2)N/dydpt = 3.5 +/- 1.5 (stat.)
+0.9,-0.5 (sys.) x 10^(-8) GeV^(-2)c^(2) for 1.8=0.35 GeV/c
(y(cm)=1.6) and 3.7 +/- 2.7 (stat.) +1.4,-1.5 (sys.) x 10^(-8) GeV^(-2)c^(2)
for 1.4=0.26 GeV/c, and a coalescence parameter B2-bar of 4.1 +/-
2.9 (stat.) +2.3,-2.4 (sys.) x 10^(-3) GeV^(2)c^(-3). Implications for the
coalescence model and antimatter annihilation are discussed.Comment: 8 pages, 4 figures, Latex, submitted to Phys. Rev. Let
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
Measurements of Light Nuclei Production in 11.5 A GeV/c Au+Pb Heavy-Ion Collisions
We report on measurements by the E864 experiment at the BNL-AGS of the yields
of light nuclei in collisions of Au(197) with beam momentum of 11.5 A GeV/c on
targets of Pb(208) and Pt(197). The yields are reported for nuclei with baryon
number A=1 up to A=7, and typically cover a rapidity range from y(cm) to
y(cm)+1 and a transverse momentum range of approximately 0.1 < p(T)/A < 0.5
GeV/c. We calculate coalescence scale factors B(A) from which we extract model
dependent source dimensions and collective flow velocities. We also examine the
dependences of the yields on baryon number, spin, and isospin of the produced
nuclei.Comment: 21 figures-to be published in Phys. Rev.
The origin of the early time optical emission of Swift GRB 080310
We present broadband multi-wavelength observations of GRB 080310 at redshift
z = 2.43. This burst was bright and long-lived, and unusual in having extensive
optical and near IR follow-up during the prompt phase. Using these data we
attempt to simultaneously model the gamma-ray, X-ray, optical and IR emission
using a series of prompt pulses and an afterglow component. Initial attempts to
extrapolate the high energy model directly to lower energies for each pulse
reveal that a spectral break is required between the optical regime and 0.3 keV
to avoid over predicting the optical flux. We demonstrate that afterglow
emission alone is insufficient to describe all morphology seen in the optical
and IR data. Allowing the prompt component to dominate the early-time optical
and IR and permitting each pulse to have an independent low energy spectral
indices we produce an alternative scenario which better describes the optical
light curve. This, however, does not describe the spectral shape of GRB 080310
at early times. The fit statistics for the prompt and afterglow dominated
models are nearly identical making it difficult to favour either. However one
enduring result is that both models require a low energy spectral index
consistent with self absorption for at least some of the pulses identified in
the high energy emission model.Comment: 24 pages, 12 figures, 12 tables. Accepted to MNRA
Mass dependence of light nucleus production in ultrarelativistic heavy ion collisions
Light nuclei can be produced in the central reaction zone via coalescence in
relativistic heavy ion collisions. E864 at BNL has measured the production of
ten light nuclei with nuclear number of A=1 to A=7 at rapidity and
. Data were taken with a Au beam of momentum of 11.5 A
on a Pb or Pt target with different experimental settings. The
invariant yields show a striking exponential dependence on nuclear number with
a penalty factor of about 50 per additional nucleon. Detailed analysis reveals
that the production may depend on the spin factor of the nucleus and the
nuclear binding energy as well.Comment: (6 pages, 3 figures), some changes on text, references and figures'
lettering. To be published in PRL (13Dec1999
A Census of Oxygen in Star-Forming Galaxies: An Empirical Model Linking Metallicities, Star Formation Rates and Outflows
In this contribution we present the first census of oxygen in star-forming
galaxies in the local universe. We examine three samples of galaxies with
metallicities and star formation rates at z = 0.07, 0.8 and 2.26, including the
SDSS and DEEP2 surveys. We infer the total mass of oxygen produced and mass of
oxygen found in the gas-phase from our local SDSS sample. The star formation
history is determined by requiring that galaxies evolve along the relation
between stellar mass and star formation rate observed in our three samples. We
show that the observed relation between stellar mass and star formation rate
for our three samples is consistent with other samples in the literature. The
mass-metallicity relation is well established for our three samples and from
this we empirically determine the chemical evolution of star-forming galaxies.
Thus, we are able to simultaneously constrain the star formation rates and
metallicities of galaxies over cosmic time allowing us to estimate the mass of
oxygen locked up in stars. Combining this work with independent measurements
reported in the literature we conclude that the loss of oxygen from the
interstellar medium of local star-forming galaxies is likely to be a ubiquitous
process with the oxygen mass loss scaling (almost) linearly with stellar mass.
We estimate the total baryonic mass loss and argue that only a small fraction
of the baryons inferred from cosmological observations accrete onto galaxies.Comment: 24 pages, 18 figures. Accepted for publication in Ap
The stellar and sub-stellar IMF of simple and composite populations
The current knowledge on the stellar IMF is documented. It appears to become
top-heavy when the star-formation rate density surpasses about 0.1Msun/(yr
pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing
metallicity and in increasingly massive early-type galaxies. It declines quite
steeply below about 0.07Msun with brown dwarfs (BDs) and very low mass stars
having their own IMF. The most massive star of mass mmax formed in an embedded
cluster with stellar mass Mecl correlates strongly with Mecl being a result of
gravitation-driven but resource-limited growth and fragmentation induced
starvation. There is no convincing evidence whatsoever that massive stars do
form in isolation. Various methods of discretising a stellar population are
introduced: optimal sampling leads to a mass distribution that perfectly
represents the exact form of the desired IMF and the mmax-to-Mecl relation,
while random sampling results in statistical variations of the shape of the
IMF. The observed mmax-to-Mecl correlation and the small spread of IMF
power-law indices together suggest that optimally sampling the IMF may be the
more realistic description of star formation than random sampling from a
universal IMF with a constant upper mass limit. Composite populations on galaxy
scales, which are formed from many pc scale star formation events, need to be
described by the integrated galactic IMF. This IGIMF varies systematically from
top-light to top-heavy in dependence of galaxy type and star formation rate,
with dramatic implications for theories of galaxy formation and evolution.Comment: 167 pages, 37 figures, 3 tables, published in Stellar Systems and
Galactic Structure, Vol.5, Springer. This revised version is consistent with
the published version and includes additional references and minor additions
to the text as well as a recomputed Table 1. ISBN 978-90-481-8817-
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