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NASA NTRS1989

Surface structure of neutron stars with high magnetic fields

The equation of state of cold dense matter in strong magnetic fields is calculated in the Thomas-Fermi and Thomas-Fermi-Dirac approximations. For use in the latter calculation, a new expression is derived for the exchange energy of the uniform electron gas in a strong magnetic field. Detailed calculations of the density profile in the surface region of a neutron star are described for a variety of equations of state, and these show that the surface density profile is strongly affected by the magnetic field, irrespective of whether or not matter in a magnetic field has a condensed state bound with respect to isolated atoms. It is also shown that, as a consequence of the field dependence of the screening potential, magnetic fields can significantly increase nuclear reaction rates.

Fushiki, I.↗

NASA NTRS1992

Gamma-ray bursts from colliding neutron stars

Three distance scales to gamma-ray bursters are discussed: about 300 pc, about 2 - 50 kpc, and about 1 Gpc, corresponding to the Galactic disk, Galactic halo, and extragalactic origin. No compelling evidence is found in favor of any of them. The BATSE experiment on GRO should determine the distance scale by determining the angular distribution of very weak bursts. The rate of collisions between the neutron stars is about 0.0001/yr in our Galaxy, and about 10 exp 5/yr within the Hubble distance. The collisions are the final phases of binary orbit decay driven by gravitational radiation, and may produce gamma-ray bursts detectable at extra-galactic distances. If strange stars exist then their collisions must release about 10 exp 49 erg in gamma-rays over about 10 seconds. Such events should be detectable out to about 1 Gpc with the current instruments.

Paczynski, Bohdan↗

NASA NTRS2002

Two Years of Chandra Observations: Neutron Stars and Pulsars with Emphasis on the Pulsar in the Crab Nebula

The Chandra X-Ray Observatory is entering its third year of operation. The Observatory, the premiere x-ray telescope for high-resolution imaging, has exceeded all expectations. The sub-arc second angular resolution together with other instrumental capabilities has allowed for new insights into the understanding of compact x-ray emitting objects including neutron stars and pulsars. We briefly review the Chandra Program and the first two years of observation with emphasis on these interesting objects. We detail the results of our observations of the pulsar in the Crab Nebula including the first continuum spectrum that is virtually uncontaminated by any dust-scattered radiation.

Weisskopf, Martin C.↗

NASA NTRS1991

Gamma-ray emission from young neutron stars

The emission models of Cheng et al. (1986) and Harding (1981) are employed to determine likely candidates for pulsed gamma-ray emission on the basis of recent radio data of pulsars. The recent detection of pulsed gamma rays from PSR 1951+32 lends observational support to the scenario of Cheng et al. which also suggests that PSR 1855+09 might be another excellent gamma-ray pulsar candidate. The possible contribution of young neutron stars to the diffuse high energy glow is also discussed.

Hartmann, Dieter H.↗

NASA NTRS1993

Quark matter droplets in neutron stars

We show that, for physically reasonable bulk and surface properties, the lowest energy state of dense matter consists of quark matter coexisting with nuclear matter in the presence of an essentially uniform background of electrons. We estimate the size and nature of spatial structure in this phase, and show that at the lowest densities the quark matter forms droplets embedded in nuclear matter, whereas at higher densities it can exhibit a variety of different topologies. A finite fraction of the interior of neutron stars could consist of matter in this new phase, which would provide new mechanisms for glitches and cooling.

Heiselberg, H.↗

NASA NTRS2005

Discovery of a Neutron Star with Spin Frequency 530 Hz in A1744-361

We report the detection with the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) of 530 Hz burst oscillations in a thermonuclear (Type I) burst from the transient X-ray source A1744-361. This is only the second burst ever observed from this source, and the first to be seen in any detail. Our results confirm that A1744-361 is a low mass X-ray binary (LMXB) system harboring a rapidly rotating neutron star. The oscillations are first detected along the rising edge of the burst, and show evidence for frequency evolution of a magnitude similar to that seen in other burst sources. The modulation amplitude and its increase with photon energy are also typical of burst oscillations. The lack of any strong indication of photospheric radius expansion during the burst suggests a 9 kpc upper limit of the source distance. We also find energy dependent dips, establishing A1744-361 as a high inclination, dipping LMXB. The timescale between the two episodes of observed dips suggests an orbital period of approx. 97 min. We have also detected a 2 - 4 Hz quasi-periodic-oscillation (QPO) for the first time from this source. This QPO appears consistent with approx. 1 Hz QPOs seen from other high inclination systems. We searched for kilohertz QPOs, and found a suggestive 2.3 sigma feature at 800 Hz in one observation. The frequency, strength and quality factor are consistent with that of a lower frequency kilohertz QPO, but the relatively low significance argues for caution, so we consider this a tentative detection requiring confirmation.

Bhattacharyya, Sudip↗

NASA NTRS1998

Studies of Long Period Variability in Neutron Star HMXB Systems

The purpose of this effort was to investigate the long term, quasi-periodic variability of the X-ray emission from the accreting X-ray pulsars LMC X-4 and SMC X-1. These high mass X-ray binary (HMXB) sources are known to vary in a nearly periodic fashion with cycle lengths of about 30 and 60 days respectively. The prevailing model for such behavior is that it is due to the precession of a tilted accretion disk around the neutron star which is the source of the X-ray emission. As the disk precesses, it periodically obscures the emitting region, resulting in reduced flux as observed at the Earth. The obscuration is not strictly periodic, as the disk precession period changes as the total mass and size of the disk change through variable accretion processes. This model is well established for the long-period variability in the X-ray pulsar Her X-1. With this work, my collaborators and I sought to test whether this model works for LMC X-4 and SMC X-1. Observations with the pointed instruments on the Rossi X-ray Timing Explorer (RXTE) were carried out in order to observe changes in the X-ray spectrum and total flux which were correlated with the long period cycles in these objects. One of the main predictions of the precessing disk model is that the periods of low emission are caused not by changes in the central source, but by increased absorption of that flux as seen at the Earth. Such behavior would be observed in the X-ray spectrum as a relative lack of low energy X-rays (which are more easily absorbed) as compared to high energy X-rays. This is what was observed for SMC X-1, landing strong support to the precessing disk model for this system. For LMC X-1, however, the absorption was consistent with zero at the time of minimum flux. The entire spectrum appeared to be equally reduced. This is not supporting evidence for the model. However, it also does not rule out a precessing disk. If at the time of minimum flux, the intervening disk is so thick that no flux, even high energy X-rays, can penetrate, and if there is material above the disk which can scatter X-rays, then a small scattered spectrum will be seen which is indistinguishable from the unobscured source flux. This is quite possibly the case in LMC X-1.

Heindl, William A.↗

NASA NTRS2017

Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is 5.0 x 10(exp -8). We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of (+1.74 +/- 0.05) s between GRB170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between -3 x 10(exp-16) times the speed of light, (ii) place new bounds on the violation of Lorentz invariance, (iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1 - 1.4 per year during the 2018--2019 observing run and 0.3 - 1.7 per year at design sensitivity.

Gravitational wave↗

NASA NTRS2020

A strongly changing accretion morphology during the outburst decay of the neutron star x-ray binary 4U 1608-52

It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below ∼10−2 of the Eddington limit (LEdd). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608−52, to study changes in the reflection spectrum. We find that the broad Fe–Kα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is ∼1037 erg s−1 (∼0.05 LEdd), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 1035 erg s−1 (∼0.002 LEdd). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise ratio at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disc radius, disc ionization, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disc ionization (log ξ 4.1) or a much decreased reflection fraction. A changing disc truncation alone, however, cannot account for the lack of reprocessed Fe–Kα emission. The required increase in ionization parameter could occur if the inner accretion flow evaporates from a thin disc into a geometrically thicker flow, such as the commonly assumed formation of a radiatively inefficient accretion flow at lower mass accretion rates.

J. van den Eijnden↗

NASA NTRS1991

Multipolar magnetic fields in neutron stars

The small periods and period derivatives of millisecond pulsars permit interesting upper bounds to be placed on the strength of multipole components in their magnetic fields if the loss of rotational energy may be approximated by vacuum electromagnetic radiation formulas. The surface field strength of millisecond pulsars may be as high as about 10 to the 12th G if the dominant multipole component is l not less than 3. Significant multipole moments may help explain the unusual pulse morphology of millisecond pulsars. If the magnetic fields in gamma-ray burst sources (old neutron stars?) have the same evolutionary history as in millisecond pulsars, the about 10 to the 12th G local fields observed in several gamma-ray bursts must be due to multipole moments with l not less than 3.

Krolik, Julian H.↗

NASA NTRS1992

Rapid cooling of neutron stars by hyperons and Delta isobars

Direct Urca processes with hyperons and/or nucleon isobars can occur in dense matter as long as the concentration of Lambda hyperons exceeds a critical value that is less than 3 percent and is typically about 0.1 percent. The neutrino luminosities from the hyperon Urca processes are about 5-100 times less than the typical luminosity from the nucleon direct Urca process, if the latter process is not forbidden, but they are larger than those expected from other sources. These direct Urca processes provide avenues for rapid cooling of neutron stars which invoke neither exotic states nor the large proton fraction (of order 0.11-0.15) required for the nucleon direct Urca process.

Prakash, Madappa↗

NASA NTRS2017

An XMM-Newton and NuSTAR Study of IGR J18214-1318: A Non-Pulsating High-Mass X-Ray Binary with a Neutron Star

IGR J18214-1318, a Galactic source discovered by the International Gamma-Ray Astrophysics Laboratory, is a high-mass X-ray binary (HMXB) with a supergiant O-type stellar donor. We report on the XMM-Newton and NuSTAR observations that were undertaken to determine the nature of the compact object in this system. This source exhibits high levels of aperiodic variability, but no periodic pulsations are detected with a 90% confidence upper limit of 2% fractional rms between 0.00003-88 Hz, a frequency range that includes the typical pulse periods of neutron stars (NSs) in HMXBs (0.1-103 s). Although the lack of pulsations prevents us from definitively identifying the compact object in IGR J18214-1318, the presence of an exponential cutoff with e-folding energy < 30 keV in its 0.3-79 keV spectrum strongly suggests that the compact object is an NS. The X-ray spectrum also shows a Fe Kα emission line and a soft excess, which can be accounted for by either a partial-covering absorber with NH approximately equals 10(exp23)cm(exp−2), which could be due to the inhomogeneous supergiant wind, or a blackbody component with = - kT 1.74+0.05 0.04 keV and R >> 0.3 BB km, which may originate from NS hot spots. Although neither explanation for the soft excess can be excluded, the former is more consistent with the properties observed in other supergiant HMXBs. We compare IGR J18214-1318 to other HMXBs that lack pulsations or have long pulsation periods beyond the range covered by our observations.

Fornasini, Francesca M.↗

NASA NTRS2010

X-Rays from Galaxies Teeming with Black Holes and Neutron Stars

Thanks to more than forty years of investment in space-based technology capable of observing the Universe in the x-ray band (0.5 - 100 keV), we have learned quite a bit about the X-ray universe. It has become clear that most of the glow of the X-ray sky is attributed to accretion onto supermassive black holes. However, as we push ever fainter in our detection methods, we find an interesting population of very faint sources arising. These are normal "Milky-way-type" galaxies that also glow in X-rays. The X-ray emission from these galaxies arises from populations of accreting black holes and neutron stars contained in binary systems. This talk will describe our understanding of this population, including some strange regularity in the production of such accreting binary systems. The future, including new technology planned for the next 5-10 years and anticipated theoretical advancements, will also be discussed.

Hornschemeier, Ann↗

NASA NTRS1971

Properties of hadron matter. II - Dense baryon matter and neutron stars.

In this article we have provided certain details of a nuclear-matter computation, based on the Brueckner-Bethe-Goldstone theory of nuclear reaction, which leads to an equation of state for matter in the density region of 10 to 500 trillion g/cu cm. We also explore the possibilities that at very high baryon densities or for very short baryon separations, the net baryon-baryon interaction may be negligible so that the results of dynamical models, like the statistical bootstrap model and the dual-resonance model, may be applicable to the study of dense baryon matter. Several plausible equations of state are constructed, and their effect on the limiting mass of the neutron star is examined.

Leung, Y. C.↗

NASA NTRS2005

How to Distinguish Neutron Star and Black Hole X-ray Binaries? Spectral Index and Quasi-Periodic Oscillation Frequency Correlation

Recent studies have revealed strong correlations between 1-10 Hz frequencies of quasiperiodic oscillations (QPOs) and the spectral power law index of several Black Hole (BH) candidate sources when seen in the low/hard state, the steep power-law (soft) state, and in transition between these states. In the soft state these index-QPO frequency correlations show a saturation of the photon index GAMMA approximately equal to 2.7 at high values of the low frequency nu(sub L). This saturation effect was previously identified as a black hole signature. In this paper we argue that this saturation does not occur, at least for one neutron star (NS) source 4U 1728-34, for which the index GAMMA monotonically increases with nu(sub L) to the values of 6 and higher. We base this conclusion on our analysis of approximately 1.5 Msec of RXTE archival data for 4U 1728-34. We reveal the spectral evolution of the Comptonized blackbody spectra when the source transitions from the hard to soft states. The hard state spectrum is a typical thermal Comptonization spectrum of the soft photons which originate in the disk and the NS outer photospheric layers. The hard state photon index is GAMMA approximately 2. The soft state spectrum consists of two blackbody components which are only slightly Comptonized. Thus we can claim (as expected from theory) that in NS sources thermal equilibrium is established for the soft state. To the contrary in BH sources, the equilibrium is never established due to the presence of the BH horizon. The emergent BH spectrum, even in the high/soft state, has a power law component. We also identify the low QPO frequency nu(sub L) as a fundamental frequency of the quasi-spherical component of the transition layer (presumably related to the corona and the NS and disk magnetic closed field lines). The lower frequency nu(sub SL) is identified as the frequency of oscillations of a quasi-cylindrical configuration of the TL (presumably related to the NS and disk magnetic open field lines). We also show that the presence of Fe K(sub alpha), emission-line strengths, QPOs, and the link between them does not depend on radio flux in 4U 1728-34.

Titarchuk, Lev↗

NASA NTRS1978

Period variations in pulsating X-ray sources. I - Accretion flow parameters and neutron star structure from timing observations

Torque fluctuations which can lead to variations in the periods of pulsating X-ray sources are examined. A description of torque variations in terms of noise processes is developed, and the resulting noise models are applied to observations of several pulsating X-ray sources. It is shown that fluctuations in accretion torque could account for the observed period variations and spindown episodes in Her X-1 and Cen X-3. The values of the torque noise strengths inferred from either a nonresonant response or, in the case of Her X-1, a Tkachenko-mode interpretation of the data are found to be consistent with those expected from processes at the magnetospheric boundary of an accreting neutron star. Ways to distinguish among the various interpretations of the period variations are considered. It is noted that fluctuating mass-flow rates may be responsible for other phenomena observed in compact X-ray sources, such as wobble with zero initial amplitude and binary period variations in close binary systems experiencing mass transfer.

Lamb, F. K.↗

NASA NTRS1993

The Guitar nebula - A bow shock from a slow-spin, high-velocity neutron star

The discovery is reported of a prominent nebula produced by the motion of a high-velocity pulsar, PSR 2224 + 65, through partially neutral gas. The pulsar's transverse speed of over about 800 km/s makes it arguably the fastest known star in the Galaxy and guarantees that it will ultimately escape the Galactic potential well. A deep H-alpha image reveals a bright head and a giant limb-brightened 'body' whose variable width suggests that the ambient interstellar gas has density variations on length scales less than 0.1 pc. Thermalization of shock energy occurs at a rate of about 0.01 times the pulsar's spindown loss rate. These observations provide some insights into the likelihood of finding shocks around other pulsars and the use of nebulae to find high-velocity neutron stars either not acting as pulsars or with their radiation beamed away from the earth.

Cordes, James M.↗

NASA NTRS1992

Cyclotron line strength variations in gamma-ray burst GB870303 - Possible evidence of neutron star rotation

An exhaustive search of the Ginga data on gamma-ray burst GB870303 reveals two separate time intervals during which statistically significant line features are evident. One interval shows a single prominent line feature at about 20 keV; a second, shows two line features at about 20 and 40 keV. From model fits to the data, we find that both sets of lines are well-described by cyclotron resonant scattering in a magnetic field B about 1.8 x 10 exp 12 G, and that the differences in the line strengths between the two intervals are significant. The variations are qualitatively similar to those produced by a change in the viewing angle theta relative to the magnetic field. We conjecture that the change in theta is due to rotation of the neutron star, and derive limits P = 45-180 sec on the rotation period P.

Graziani, Carlo↗