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NASA NTRS2003

Chandra X-Ray Observatory Observations of the Globular Cluster M28 and its Millisecond Pulsar PSR B1821-24

We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). We detect 46 X-ray sources of which 12 lie within one core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources for which we determine the X-ray luminosity function down to a limit of about 6 x 10(exp 30) erg/s. We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius of r(sub c,x) approx. 11 sec. We obtain the best-fit mass of the X-ray sources to be M(sub x) approx. 1.9 solar masses. We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821-24 and find it best described by a power law with photon-index Gamma approx. equal to 1.2. We find marginal evidence of an emission line centered at 3.3 kev in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar's polar cap if the the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5-8.0 keV band is approx. 3.5 x 10(exp -13) ergs/s/sq cm. We present spectral analyses of the 5 brightest unidentified sources. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T(sup infinity)(sub eff) = 90(sup +30)(sub -10) eV and the radius R(sup infinity)(sub NS) = 14.5(sup +6.9)(sub -3.8) km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core. Using the Chandra-derived positions, we also present a preliminary report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.

Becker, Werner↗
NASA NTRS2003

Chandra X-Ray Observatory Observations of the Globular Cluster M28 and its Milisecond Pulsar PSR B1821-24

We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). We detect 46 X-ray sources, of which 12 lie within 1 core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources, for which we determine the X-ray luminosity function down to a limit of about 6 x 10(exp 30) ergs/s. We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius of r(sub c,X) approx. = 5 11". We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05 ms pulsar B1821-24 and find that it is best described by a power law with photon index Gamma approx. = 1.2. We find marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar s polar cap if the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.1 - 8.0 keV band is approx. = 3.5 x 10(exp -13) ergs/s sq cm. We present spectral analyses of the five brightest unidentified sources. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T(sub eff, sup infinity) = 90(sup +30, sub -10) eV and the radius R(sub NS, sup infinity) = 14.5(sup +6.9, sub -3.8) km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.

Becker, Werner↗
NASA NTRS1991

Shedding light on baryonic dark matter

Halo dark matter, if it is baryonic, may plausibly consist of compact stellar remnants. Jeans mass clouds containing 10 to the 6th to 10 to the 8th solar masses could have efficiently formed stars in the early universe and could plausibly have generated, for a suitably top-heavy stellar initial mass function, a high abundance of neutron stars as well as a small admixture of long-lived low mass stars. Within the resulting clusters of dark remnants, which eventually are tidally disrupted when halos eventually form, captures of neutron stars by nondegenerate stars resulted in formation of close binaries. These evolve to produce, by the present epoch, an observable X-ray signal associated with dark matter aggregations in galaxy cluster cores.

Silk, Joseph↗
NASA NTRS2000

Search for the Centrifugal Barrier in Transient X-Ray Pulsars

During the period 15 April 1997 through 15 April 2000 we have been supported in part by NASA grant NAG5-4115 to pursue a 'Search for the Centrifugal Barrier in Transient X-ray Pulsars' with the Rossi X-Ray Timing Explorer (RXTE). The RXTE observations associated with this research involved monitoring the X-ray intensities and pulse profiles of six transient X-ray pulsars on five different occasions. The goal was to detennine whether the so-called 'centrifugal barrier' effect exists for accretion-powered X-ray pulsars. In this long-postulated effect, accretion onto the neutron star should be inhibited when the accretion rate falls below the point where the magnetospheric boundary lies outside the neutron star's corotation radius. Our RXTE observations yielded detections of two of the six X-ray pulsars under study, but were insufficient, overall, to yield any new insight into the centrifugal barrier effect. The data from the two detected pulsars will be utilized by Professor Deepto Chakrabarty as part of his overall campaign to study the long-term behavior of accretion-powered X-ray sources.

Jefferson, Adrian↗
NASA NTRS2000

Search for the Centrifugal Barrier in Transient X-ray Pulsars

This paper discusses a search for the centrifugal barrier in transient X-ray pulsars with RXTE. The RXTE observation associated with this research involved monitoring the X-ray intensities and pulse profiles of six transient x-ray pulsars on five different occasions. The goal was to determine whether the so-called "centrifugal barrier" effects exists for accretion-powered X-ray pulsars. In this long-postulated effects, accretion onto the neutron star should be inhibited when the accretion rate falls below the point where the magnetospheric boundary lies outside the neutron star's corotation radius.

Rappaport, Saul↗
NASA NTRS2008

Fast X-Ray Oscillations During Magnetar Flares

I will review recent studies of high frequency variability during magnetar giant flares. These oscillations may represent the first observations of global shear oscillations in neutron star crusts. I will also discuss how the observation of crust vibrations can provide a new tool to study neutron star structure.

Strohmayer, Tod E.↗
NASA NTRS2019

NICER Observation of Unusual Burst Oscillations in 4U 1728-34

The Neutron Star Interior Composition Explorer has observed seven thermonuclear X-ray bursts from the lowmass X-ray binary neutron star 4U 1728-34 from the start of the mission’s operations until 2019 February. Three of these bursts show oscillations in their decaying tail, with frequencies that are within 1 Hz of the previously detected burst oscillations from this source. Two of these burst oscillations have unusual properties: they have large fractional root mean square (rms) amplitudes of 48% ± 9% and 46% ± 9%, and they are detected only at photon energies above 6 keV. By contrast, the third detected burst oscillation is compatible with previous observations of this source, with a fractional rms amplitude of 7.7% ± 1.5% rms in the 0.3 to 6.2 keV energy band. We discuss the implications of these large-amplitude burst oscillations, finding that they are difficult to explain with the current theoretical models for X-ray burst tail oscillations.

Data analysis methods↗
NASA NTRS2021

Identification of a Local Sample of Gamma-Ray Bursts Consistent with a Magnetar Giant Flare Origin

Cosmological gamma-ray bursts (GRBs) are known to arise from distinct progenitor channels: short GRBs mostly from neutron star mergers and long GRBs from a rare type of core-collapse supernova (CCSN) called collapsars. Highly magnetized neutron stars called magnetars also generate energetic, short-duration gamma-ray transients called magnetar giant flares (MGFs). Three have been observed from the Milky Way and its satellite galaxies, and they have long been suspected to constitute a third class of extragalactic GRBs. We report the unambiguous identification of a distinct population of four local (<5 Mpc) short GRBs, adding GRB 070222 to previously discussed events. While identified solely based on alignment with nearby star-forming galaxies, their rise time and isotropic energy release are independently inconsistent with the larger short GRB population at >99.9% confidence. These properties, the host galaxies, and non-detection in gravitational waves all point to an extragalactic MGF origin. Despite the small sample, the inferred volumetric rates for events above 4 × 10(exp 44) erg of R(sub MGF) =3.8(sup +4.0)(sub -3.1) ×10(exp 5) Gpc(exp -3) yr(exp -1) make MGFs the dominant gamma-ray transient detected from extragalactic sources. As previously suggested, these rates imply that some magnetars produce multiple MGFs, providing a source of repeating GRBs. The rates and host galaxies favor common CCSN as key progenitors of magnetars.

Gamma-ray bursts↗
NASA NTRS1978

Superdense neutron matter

A relativistic theory of high-density matter is presented which takes into account the short-range interaction due to the exchange of spin-2 mesons. An equation of state is derived and used to compute neutron-star properties. The prediction of the theory for the values of maximum mass and moment of inertia for a stable neutron star are 1.75 solar masses and 1.68 by 10 to the 45th power g-sq cm, in very good agreement with the presently known observational bounds. The corresponding radius is found to be 10.7 km. It is found that the inclusion of the spin-2 interaction reduces the disagreement between the relativistic and nonrelativistic theories in their predictions of masses and moments of inertia.

Canuto, V.↗
NASA NTRS1981

Probable optical counterpart of a Gamma-ray burster

Sixteen million seconds, or over a half year, of optical monitoring of three Gamma-ray burst positions using the Harvard College Observatory collection of archival plates are described. The probable optical counterpart of the November 19, 1978 Gamma-ray burster has been discovered on a blue emulsion plate exposed in 1928. Optical searches indicate that the absolute magnitude of the Gamma-ray burst system at quiescence is fainter than 13, and a recurrence rate of about 10 to the -7.5/sec is found from Gamma-ray and optical data. Such a high recurrence rate precludes any model which uses a collision between a neutron star and an asteroid-like body, as well as any model requiring accretion from the interstellar matter onto a neutron star.

Schaefer, B. E.↗
NASA NTRS1985

Ultra-high-energy gamma-rays from Cygnus X-3

Ultrahigh-energy gamma-ray emission with photon energies up to 20 PeV has been detected from the galactic X-ray source Cygnus X-3, implying acceleration of charged particles to even greater energies. A model for acceleration of particles to such energies in a binary containing an accreting neutron star with a magnetic field of about 100 MG is presented. The acceleration occurs in an accretion disk surrounding the neutron star, by an unipolar-inductor mechanism.

Chanmugam, G.↗
NASA NTRS2000

Results of a Deep Chandra Observation of the Crab Nebula and Pulsar

The Crab Nebula and pulsar were observed for a total of 150 ksec with the LETG/HRC-S combination aboard the Chandra X-Ray Observatory in 2000, January. One of the principal aims of the experiment was to study the emission of from the pulsar as a function of pulse phase. Neutron stars are believed to be formed with core temperatures of 10(exp 11). As the pulsar is the youngest known neutron star with an age of only 940 yrs, it should be possible to observe thermal emission from the hot stellar surface which in turn constrains equations of state. The pulsar, on the other hand, is a powerful non-thermal emitter, powering an X-ray bright synchrotron nebula which, in Einstein and ROSAT observations, overshadowed the fainter thermal surface emission. Making use of the high angular resolution provided by Chandra we were able to detect X-rays from the Crab-pulsar at all pulse phases. We discuss whether this detection is indeed of thermal emission or of a faint synchrotron component of the pulsed emission from the magnetosphere. We further report on dynamical effects observed in the pulsar-wind outflow and the analysis of the LETG spectral data, especially near the oxygen edge. The results of the spectral analysis has interesting implications for the composition of the interstellar medium.

Weisskopf, M. C.↗
NASA NTRS1987

Compact X-ray binaries in hierarchical triples. I - Tidal angular momentum loss and GX 17 + 2

A general formalism is developed for the enhanced mass transfer rate expected in a close binary with a (distant) third companion star. Such a hierarchical triple model is applied to the X-ray burster and QPO source GX 17 + 2, consisting of an inner mass-transferring binary comprising a main-sequence secondary and an accreting neutron star, and a more distant companion. The model is shown to account for the anomalously high mass transfer rate for this burster and other systems with short orbital periods. A G star, which does not appear to be the binary companion of the neutron star but is consistent with the sub-arc second radio error box for GX 17 + 2, may be the triple companion. A predicted velocity curve for the triple companion is presented.

Bailyn, Charles D.↗
NASA NTRS1990

Gamma ray pulsars: Models and observations

The two known gamma ray pulsars, the Crab and Vela, were used as guides for the development of models of high-energy radiation from spinning neutron stars. Two general classes of models were developed: those with the gamma radiation originating in the pulsar magnetosphere far from the neutron star surface (outer gap models) and those with the gamma radiation coming from above the polar cap (polar cap models). The goal is to indicate how EGRET can contribute to understanding gamma-ray pulsars, and especially how it can help distinguish between models for emission.

Thompson, David J.↗
NASA NTRS1998

TOO Observations of Soft Gamma Repeaters

SGR 1806-20 turned active late 1996, and in response to this reactivation we triggered our target-of-opportunity observations. The results were extremely rewarding. We found a periodic modulation of the persistent X-ray signal, at a period of 7.47 seconds. We combined our data with similar data obtained by Dr T. Strohmayer, which confirmed the presence of the pulsations, and gave evidence for a spin down on a time scale of some 1000 years. We retrieved the pulsations in archival ASCA data, obtained in 1993 and 1995, and confirmed the long-term spin down of the neutron star in this system. The properties of SGR 1806-20 lead to the conclusion that it is a mgnetar, i.e., a neutron star with a magnetic field some 2 to 3 orders of magnitude higher than those of young radio pulsars and binary X-ray pulsars.

vanParadijs, J. A.↗
NASA NTRS1985

Search for stellar collapse with the MACRO detector at Gran Sasso

It is viewed that in stellar evolution stars in the range of 8 to 12 solar masses evolve gradually as increasingly heavier nuclei are produced and then consumed in a series of exothermic thermonuclear processes ultimately leading to the formation of a core composed almost entirely of nickel and iron. When the mass of this hot iron-nickel core reaches the critical value of approximately 1.4 solar masses, electron degeneracy pressure is no longer able to support the outer layers of the star and a collapse process begins. Since the core has exhausted its thermonuclear fuel, further stages of thermonuclear burning cannot prevent a runaway collapse. As the density reaches 10 to the 10th power gm sub/cm at a temperature near 10 to the 10th power k, most of the heavy nuclei are dissociated into free nucleons and electron capture on free protons leads to a decrease in the degeneracy pressure and further acceleration of the collapse process. Although this general picture has received substantial confirmation over the past two decades with the discovery of radio pulsars (neutron stars), X-ray pulsars (accreting binary neutron stars) and Cyg X-1 (probably an accreting black hole), an actual neutrino burst is not yet convincingly detected.

Steinberg, R.↗
NASA NTRS1985

Gamma-ray bursters

Current theoretical models developed to explain the observational data (from spaceborne detectors) on gamma-ray bursters are summarized and illustrated with drawings, diagrams, graphs, and photographic images. Although the data are fragmentary and often flawed by instrument defects, models involving neutron stars with strong magnetic fields are generally favored, and it is assumed that most observed bursters lie within the Galaxy. The neutron-star origin of the bursts is suggested by their intensity and rapid variability (implying a very compact high-energy source) and the presence in some burster spectra of a line at 420 keV which is explained by the combination of electron-positron annihilation and gravitational reddening. Consideration is also given to optical flashes observed to occur about once per year in the direction of gamma bursters, and the need for further searches for lower-energy emissions from bursters is stressed.

Schaefer, B. E.↗
NASA NTRS1982

A numerical simulation of the magnetospheric gate model for the X-ray bursters

A Lagrangian, stellar interior computer code which assumes radiation transport by diffusion is used to numerically simulate an X-ray burst occurring on a rapidly rotating, 1 solar mass neutron star accreting material from a companion in noncontinuous, episodic fashion. The blob of gas from the companion extends from the surface of the neutron star to a radius of 185 km, is optically thick, has mass of 10 to the -11th solar masses, and is initially at rest with respect to the surface. The gas initially has a temperature of about 10 million K and a surface luminosity of 0.1 that of the sun. The infall results in a burst which lasts about 0.1 second and reaches a peak luminosity and effective temperature of 240,000 suns and nine million K. The burst is followed by a phase of oscillations with a period of 0.2 second. It is found that the burst is too cool and rapid to resemble normal bursts, although the time scale is in agreement with some observations.

Starrfield, S.↗