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NASA NTRS1992

Gamma-ray bursts and the birthrate of bare neutron stars

Accretion of matter onto the surface of a white dwarf in a binary system can push it over the Chandrasekhar mass limit and may cause it to collapse into a neutron star without mass ejection or with the ejection of only a small mass. Such an optically quiet stellar collapse should be accompanied by a neutrino burst which could be detected with underground neutrino detectors if the collapse took place in our own Galaxy or in very close nearby galaxies. However, the frequency of such collapses is not known. Here we show that, if the ejected mass is less than 3 x 10 exp -4 solar mass, the electron-positron pairs resulting from neutrino-antineutrino annihilations outside the neutrinosphere produce a gamma-ray burst which could be observed out to distances of at least 300 Mpc, and that the observed rate of gamma-ray bursts sets stringent upper limits on the frequency of bare or nearlly bare neutron star births.

Ramaty, R.↗

NASA NTRS1993

Indications for common origin and gravitational interaction in three binary LMC clusters

Three close pair clusters of the LMC, NGC 2006/SL 538, NGC 2011 a/b, and NGC 2042 a/b, have been studied in order to establish their binarity. The stellar content in the outer region of each cluster has been investigated by means of low resolution objective UK Schmidt prism spectra, the cores of the clusters have been examined using low resolution integrated IUE spectra, whereas their density profiles and their observed dynamical parameters have been derived by means of star counts. The integrated spectra of their cores and the stellar content of their outer cluster regions have shown a common origin and a very young age (less than 2 x 10 exp 7 yr) for each member of the pairs, whereas their dynamical study has shown that they are gravitationally interacting. Comparing the age of their stellar content with their dynamical and relaxation times, it has been found that these clusters are physically associated and had no time to relax by stellar encounters, no time to merge, and no time to be destroyed by dynamical friction.

Kontizas, E.↗

NASA NTRS2010

X-Ray Variation Statistics and Wind Clumping in Vela X-1

We investigate the structure of the wind in the neutron star X-ray binary system Vela X-1 by analyzing its flaring behavior. Vela X-1 shows constant flaring, with some flares reaching fluxes of more than 3.0 Crab between 20-60 keV for several 100 seconds, while the average flux is around 250 mCrab. We analyzed all archival INTEGRAL data, calculating the brightness distribution in the 20-60 keV band, which, as we show, closely follows a log-normal distribution. Orbital resolved analysis shows that the structure is strongly variable, explainable by shocks and a fluctuating accretion wake. Analysis of RXTE ASM data suggests a strong orbital change of N. Accreted clump masses derived from the INTEGRAL data are on the order of 5 x 10(exp 19)-10(exp 21) g. We show that the lightcurve can be described with a model of multiplicative random numbers. In the course of the simulation we calculate the power spectral density of the system in the 20-100 keV energy band and show that it follows a red-noise power law. We suggest that a mixture of a clumpy wind, shocks, and turbulence can explain the measured mass distribution. As the recently discovered class of supergiant fast X-ray transients (SFXT) seems to show the same parameters for the wind, the link between persistent HMXB like Vela X-1 and SFXT is further strengthened.

Furst, Felix↗

NASA NTRS2017

Studying the Accretion Geometry of EXO 2030+375 at Luminosities Close to the Propeller Regime

The Be X-ray binary EXO2030+375 was in an extended low-luminosity state during most of 2016. We observed this state with NuSTAR and Swift, supported by INTEGRAL observations and optical spectroscopy with the Nordic Optical Telescope (NOT). We present a comprehensive spectral and timing analysis of these data here to study the accretion geometry and investigate a possible onset of the propeller e ect. The H data show that the circumstellar disk of the Be-star is still present. We measure equivalent widths similar to values found during more active phases in the past, indicating that the low-luminosity state is not simply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keV luminosity of 6:8 1035 erg s-1 (for a distance of 7.1 kpc), are nicely described by standard accreting pulsar models such as an absorbed power law with a high-energy cuto. We find that pulsations are still clearly visible at these luminosities, indicating that accretion is continuing despite the very low mass transfer rate. In phaseresolved spectroscopy we find a peculiar variation of the photon index from 1.5 to 2.5 over only about 3% of the rotational period. This variation is similar to that observed with XMM-Newton at much higher luminosities. It may be connected to the accretion column passing through our line of sight. With Swift/XRT we observe luminosities as low as 1034 erg s-1 where the data quality did not allow us to search for pulsations, but the spectrum is much softer and well described by either a blackbody or soft power-law continuum. This softer spectrum might be due to the accretion being stopped by the propeller e ect and we only observe the neutron star surface cooling.

Fürst, F.↗

NASA NTRS1997

A search for stars passing close to the sun

Hipparcos proper motion and parallax data are combined for nearby stars with ground-based radial velocity measurements in order to identify stars which may have passed, or will pass, close enough to the sun to perturb the Oort cloud. Close stellar encounters could deflect large numbers of comets into the inner solar system, with possible serious consequences for impact hazards on the earth. Only one star, Gliese 710 is found with a predicted closest approach of less than 0.5 pc, although several stars come within 1 pc during a 8.5 M year interval. In most cases, the uncertainty in closest approach distance is dominated by uncertainties in the barycenter motion of binary systems. A program to obtain new radial velocities for stars in the sample with no previously published values is underway.

Garcia-Sanchez, J.↗

NASA NTRS1976

On the structure of contact binaries. I - The contact discontinuity

The problem of the interior structure of contact binaries is reviewed, and a simple resolution of the difficulties which plague the theory is suggested. It is proposed that contact binaries contain a contact discontinuity between the lower surface of the common envelope and the Roche lobe of the cooler star. This discontinuity is maintained against thermal diffusion by fluid flow, and the transition layer is thin to the extent that the dynamical time scale is short in comparison with the thermal time scale. The idealization that the transition layer has infinitesimal thickness allows a simple formulation of the structure equations which are closed by appropriate jump conditions across the discontinuity. The further imposition of the standard boundary conditions suffices to define a unique model for the system once the chemical composition, the masses of the two stars, and the orbital separation are specified.

Shu, F. H.↗

NASA NTRS1971

Statistical studies in stellar rotation 2: A method of analyzing rotational coupling in double stars and an introduction to its applications

The correlation between the equatorial velocities of the components of double stars is studied from a statistical standpoint. A theory of rotational correlation is developed and discussed with regard to its applicability to existing observations. The theory is then applied to a sample of visual binaries which are the least studied for rotational coupling. Consideration of eclipsing systems and spectroscopic binaries is limited to show how the degrees of freedom in the spin parallelism problem can be reduced. The analysis lends support to the existence of synchronism in closely spaced binaries.

Bernacca, P. L.↗

NASA NTRS1981

IUE spectra of the hot close binary V Puppis

Low- and high-dispersion ultraviolet spectra obtained by the IUE of the early B-type close binary V Puppis are reported. The continuum derived from the low-dispersion images is consistent with a solar abundance model of effective temperature 25,000 K and log surface gravity 4.0. The high-dispersion spectra show shallow double lines, representing Doppler-resolved stellar lines, which are broadened as expected for the fast rotational velocities given by Levato (1974). The equivalent widths of the silicon lines, however, do not agree with the temperature derived for the star, while the silicon and carbon absorptions indicate a disordered photosphere in the presence of circumstellar gas. Several interstellar lines are also observed for the first time.

Koch, R. H.↗

NASA NTRS1982

Mass loss from cool stars, facts, fads, and fallacies

The accumulation of observational material (ultraviolet spectroscopic measures, quantitative optical spectroscopy, and X-ray photometry) and its use in discerning the presence and character of mass loss across the cool half of the H-R diagram and establishing constraints on theoretical models are discussed. Analogies with closed and open solar magnetic structures are found. Two determinants of atmospheric wind structure, temperature and gravity, may suffice in a most superficial way to define the wind and atmospheric structure in a star, however it is apparent that there is still a missing parameter which may stem from magnetic activity and its particular configuration. Theories that appear successful in reproducing observed line profiles, wind temperatures, and terminal velocities incorporate Alfven wave heating and momentum deposition. Successive observations of an active binary (lambda and G8III-IV) and a supergiant star, alpha Aqr (G2 Ib) revealed that magnetic activity and perhaps mass loss occur on restricted regions of a stellar surface and that long-term structures are present in the wind. These phenomena are present in the solar atmosphere and wind and may be considered a general characteristic of stellar winds.

Dupree, A. K.↗

NASA NTRS1998

Search for Hard X-Ray Emission from Aquila X-1: High Energy Emission from Gamma-ray Radio Star 2CG 135+1/LSI 61 305

Several investigations supported by these CCRO grant were completed or are close to completion. The study of EGRET data for the unidentified source 2CG 135+01 was very fruitful. We discovered transient gamma-ray emission by combining several data obtained since 1994 through 1997. It is the first time that time variable emission is established for this enigmatic source, and clearly an interpretation in terms of an isolated radio pulsar (Geminga-like) is disfavored now. Our preferred model is a Galactic source, probably an energetic pulsar (such as PSR129-63) in a binary system producing gamma-rays because of pulsar wind/mass outflow interaction. We also accumulated may data concerning the radio source LSI 61 303, the possible counterpart of 2CG 135+01. We show that a possible anti-correlation between radio and gamma-ray emission exists. This anticorrelation is evident only in the energy range above 100 MeV, as demonstrated by the lack of it obtained from OSSE data. If confirmed, this anti-correlation would prove to be very important for the interpretation of the hundreds of unidentified gamma-ray sources currently discovered by EGRET near the Galactic plane, and would point to a new class of sources in addition to AGNs and isolated pulsars. We also completed the analysis of several time variable gamma-ray sources near the Galactic plane, with the discussion of evidence for transient emission from 2EG J1813-12 and 2EG J1828+01. We completed several investigations regarding gamma-ray bursts (GRBs), including the study of the brightness distribution for different spectral/duration GRB sub-classes, an investigation of acceleration processes and their consequences for GRB afterglow emission [61, the application of the synchrotron shock model of GRBs to X-ray energies.

Tavani, Marco↗

NASA NTRS2018

Detection of Reflection Features in the Neutron Star Low-mass X-Ray Binary Serpens X-1 with NICER

We present Neutron Star Interior Composition Explorer (NICER) observations of the neutron star (NS) low-mass X-ray binary Serpens X-1 during the early mission phase in 2017. With the high spectral sensitivity and low-energy X-ray passband of NICER, we are able to detect the Fe L line complex in addition to the signature broad, asymmetric Fe K line. We confirm the presence of these lines by comparing the NICER data to archival observations with XMM-Newton/Reflection Grating Spectrometer (RGS) and NuSTAR. Both features originate close to the innermost stable circular orbit (ISCO). When modeling the lines with the relativistic line model RELLINE, we find that the Fe L blend requires an inner disk radius of 1.4(sup 0.2, sub -0.1)R(sub ISCO) and Fe K is at 1.03(sup 0.13, sub -0.03)R(sub ISCO) (errors quoted at 90%). This corresponds to a position of 17(sup 2.5, sub -1.2)km and 12(sup 1.6, sub -0.4)km for a canonical NS mass (M(sub NS)=1.4 solar mass) and dimensionless spin value of a = 0. Additionally, we employ a new version of the RELXILL model tailored for NS(sub s) and determine that these features arise from a dense disk and supersolar Fe abundance.

Ludlam, R. M.↗

NASA NTRS2018

A NICER Spectrum of MAXI J1535-571: Near-Maximal Black Hole Spin and Potential Disk Warping

We report on a Neutron star Interior Composition Explorer (NICER) observation of the Galactic X-ray binary and stellar-mass black hole candidate, MAXI J1535−571. The source was likely observed in an "intermediate" or "very high" state, with important contributions from both an accretion disk and hard X-ray corona. The 2.3-10 keV spectrum shows clear hallmarks of relativistic disk reflection. Fits with a suitable model strongly indicate a nearmaximal spin parameter of a = cJ/GM(exp 2) = 0.994(2) and a disk that extends close to the innermost stable circular orbit, r/r(sub ISCO) = 1.08(8) (1σ statistical errors). In addition to the relativistic spectrum from the innermost disk, a relatively narrow Fe K emission line is also required. The resolution of NICER reveals that the narrow line may be asymmetric, indicating a specific range of emission radii. Fits with a relativistic line model suggest an inner radius of r = 144 +140/-60 GM/c(exp 2) for the putative second reflection geometry; full reflection models suggest that radii a few times larger are possible. The origin of the narrow line is uncertain, but a warp likely provides the most physically plausible explanation. We discuss our results in terms of the potential for NICER to reveal new features of the inner and intermediate accretion disk around black holes.

Miller, J. M.↗

NASA NTRS1982

IUE spectroscopy, visible-band polarimetry, and radiometry of V641 Mon

This hot, double line, ellipsoidal variable member of NGC 2264 has been shown previously to be either a semi-detached or contact close binary. Low-resolution IUE spectra are best fitted to a Kurucz model atmosphere for very small (approximately 0.08 mag) E(B-V). The familiar interstellar absorption dip near lambda 2200 is apparently absent. A suitable model atmosphere can be fitted to the IUE fluxes, but flux excesses (compared to the model) appear for all the published U through L magnitudes. The spectrum of the B through L excess appears to follow a .0001 lambda dependence. It is shown that this cannot be interpreted as arising from another star fortuitously observed in the visible band or IR. Ground based polarization measures indicate V641 Mon to be a polarization variable. Previous and new V light curves show the amplitude of light variability itself to be variable by about a factor of 2. It is suggested that all these observed characteristics are best explained by postulating "third light" and identifying part of it with Rayleigh scattered starlight very near to the stars. From this same region there arise circumstellar absorptions which give rise to nontheoretical strengths for Si II and Si III lines.

Koch, R. H.↗

NASA NTRS1993

A local instability of tidally distorted accretion disks

Hydrodynamic accretion disks in binary star systems have local, three-dimensional, approximately incompressible instabilities whose growth rate is linearly proportional to the tidal field of the secondary star. The instability occurs throughout the disk but is most rapid in the outer parts. The free energy for the instability derives from the tidal distortion of the disk rather than its differential rotation. The instability will excite m-parallel 1 internal waves of the type discussed by Vishniac et al. (1992), and it should facilitate return of angular momentum from the disk to the secondary star. It is closely related to a recently discovered instability that has been suggested as a mechanism for transition to turbulence in terrestrial and laboratory flows, but whether it will lead to strong turbulence in disks is as yet unclear.

Goodman, Jeremy↗

NASA NTRS1983

Optical radiation associated with gamma-ray bursts

Calculations are made of the approximate characteristics of the reprocessed optical radiation resulting from the absorption of a gamma-ray burst by a nearby star. The overall reprocessing time scale, including contributions from gamma-ray transfer and diffusion of the optical radiation, is estimated. It is noted that diffusive cooling occurs by either a 'self-similar' or a 'transparency' wave of postabsorption stellar surface temperatures greater or less than 10,000 K, respectively. Depending on the combination of stellar and gamma-ray burst properties, the reprocessing time scale can vary from the duration of the gamma-ray burst (approximately 1 s) to the maximum calculated cooling time (approximately 1000 s). Calculations for close binary systems are made of the number of optical photons per square centimeter expected at earth as a function of the observed gamma-ray burst fluences and likely distances. It is predicted that, if all gamma-ray bursters are in close binaries, the number of detections per year by an all sky monitor of 200 photons per sq cm sensitivity will be about 100.

London, R. A.↗

NASA NTRS1992

Gamma-ray bursts as the death throes of massive binary stars

We propose that gamma-ray bursts are created in the mergers of double neutron star binaries and black hole neutron star binaries at cosmological distances. Two different processes provide the electromagnetic energy for the bursts: neutrino-antineutrino annihilation into electron-position pairs during the merger, and magnetic flares generated by the Parker instability in a postmerger differentially rotating disk. In both cases, an optically thick fireball of size less than or approximately equal to 100 km is initially created, which expands ultrarelativistically to large radii before radiating. The scenario is only qualitative at this time, but it eliminates many previous objections to the cosmological merger model. The strongest bursts should be found close to, but not at the centers of, galaxies at redshifts of order 0.1, and should be accompanied by bursts of gravitational radiation from the spiraling-in binary which could be detected by LIGO.

Narayan, Ramesh↗

NASA NTRS1997

Torque Reversal and Spin-Down of the Accretion-Powered Pulsar 4U 1626-67

Over 5 yr of hard X-ray (20-60 keV) monitoring of the 7.66 s accretion-powered pulsar 4U 1626-67 with the Compton Gamma Ray Observatory/BATSE large-area detectors has revealed that the neutron star is now steadily spinning down, in marked contrast to the steady spin-up and spin-down torques differ by only 15% with the neutron star spin changing on a timescale |v/v| approximately equals 5000 yr in both states. The current spin-down rate is itself decreasing on a timescale |v/v| approximately equals 26 yr. The long-term timing history shows small-amplitude variations on a 4000 day timescale, which are probably due to variations in the mass transfer rate. The pulsed 20-60 keV emission from 4U 1626-67 is well-fitted by a power-law spectrum with photon index gamma = 4.9 and a typical pulsed intensity of 1.5 x 10(exp -10) ergs cm (exp -2)s(exp -1). The low count rates with BATSE prohibited us from constraining the reported 42 minute binary orbit, but we can rule out long-period orbits in the range 2 days < or = P(orb) < or = 900 days. We compare the long-term torque behavior of 4U 1626-67 to other disk-fed accreting pulsars and discuss the implications of our results for the various theories of magnetic accretion torques. The abrupt change in the sign of the torque is difficult to reconcile with the extremely smooth spin-down now observed. The strength of the torque noise in 4U 1626-67, approximately 10(exp -22) Hz(exp 2)s(exp -2) Hz(exp -1), is the smallest ever measured for an accreting X-ray pulsar, and it is comparable to the timing noise seen in young radio pulsars. We close by pointing out that the core temperature and external torque (the two parameters potentially relevant to internal sources of timing noise) of an accreting neutron star are also comparable to those of young radio pulsars.

Chakrabarty, Deepto L.↗

NASA NTRS1981

A UBV photometric study of the 5.2 hour X-ray binary 4U 2129+47

The paper presents results derived from UBV photometric observations of the 5.2 X-ray binary 4U 2129+47 which were made on 11 nights between 1978 October and 1979 August. The light curves observed during this period are relatively stable in shape and amplitude (Delta B = 1.5 mag) and are a surprisingly close match to the time-averaged light curves observed for HZ Her. There is a significant asymmetry in their shape which is independent of color and varies in amplitude by a factor of approximately 2. The UBV colors at minimum light suggest a considerably earlier spectral type (G or earlier) than is inferred by assuming that the star fills its Roche lobe (late K or early M). Optical pulsations were studied, and a 3 sigma limit of 0.5% was set on the fraction of the flux which is pulsed for periods between 60 ms and 300 s. Assuming that the optical companion is a normal main-sequence star filling its Roche lobe, its mass is approximately 0.65 solar mass. Assuming also that the compact object has a mass of 1.3 solar masses and that the orbital inclination of the system is approximately 90 deg, it is found that the orbital separation is approximately 1.9 solar radii and K-opt is approximately 300 km/s.

Mcclintock, J. E.↗