Engineering topics

Rothschild, Richard E.

Publications and source records attributed to Rothschild, Richard E..

17 records

NASA NTRS2019

The Physics of Accretion Onto Highly Magnetized Neutron Stars

Studying the physical processes occurring in the region just above the magnetic polesof strongly magnetized, accreting binary neutron stars is essential to our understanding of stellarand binary system evolution. Perhaps more importantly, it provides us with a natural laboratoryfor studying the physics of high temperature and density plasmas exposed to extreme radiation,gravitational, and magnetic fields. Observations over the past decade have shed new light on themanner in which plasma falling at near the speed of light onto a neutron star surface is halted. Recentadvances in modeling these processes have resulted in direct measurement of the magnetic fieldsand plasma properties. On the other hand, numerous physical processes have been identified thatchallenge our current picture of how the accretion process onto neutron stars works. Observationand theory are our essential tools in this regime because the extreme conditions cannot be duplicatedon Earth. This white paper gives an overview of the current theory, the outstanding theoreticaland observational challenges, and the importance of addressing them in contemporary astrophysicsresearch.

Wolff, Michael T.↗

NASA NTRS2019

The First NuSTAR Observation of 4U 1538-522: Updated Orbital Ephemeris and a Strengthened Case for an Evolving Cyclotron Line Energy

We have performed a comprehensive spectral and timing analysis of the first NuSTAR observation of the high-mass X-ray binary 4U 1538−522. The observation covers the X-ray eclipse of the source, plus the eclipse ingress and egress. We use the new measurement of the mid-eclipse time to update the orbital parameters of the system and find marginally significant evolution in the orbital period, with P(sub orb) / P(sub orb) = (-0.95 +/- 0.37) x 10(exp -6) yr(exp -1). The cyclotron line energy is found to be approximately 1.2 keV higher than RXTE measurements from 1997 to 2003, in line with the increased energy observed by Suzaku in 2012 and strengthening the case for secular evolution of 4U 1538−522's CRSF. We additionally characterize the behavior of the iron fluorescence and emission lines and line-of-sight absorption as the source moves into and out of eclipse.

Hemphill, Paul B.↗

NASA NTRS2017

Evidence for Different Accretion Regimes in GRO J1008-57

We present a comprehensive spectral analysis of the BeXRB GRO J1008-57 over a luminosity range of three orders of magnitude using NuSTAR, Suzaku, and RXTE data. We find significant evolution of the spectral parameters with luminosity. In particular, the photon index hardens with increasing luminosity at intermediate luminosities in the range 10(sup 36) to 10 (sup 37) ergs per second. This evolution is stable and repeatedly observed over different outbursts. However, at the extreme ends of the observed luminosity range, we find that the correlation breaks down, with a significance level of at least 3:7sigma. We conclude that these changes indicate transitions to different accretion regimes, which are characterized by different deceleration processes, such as Coulomb or radiation breaking. We compare our observed luminosity levels of these transitions to theoretical predications and discuss the variation of those theoretical luminosity values with fundamental neutron star parameters. Finally, we present detailed spectroscopy of the unique "triple peaked" outburst in 2014/15 which does not fit in the general parameter evolution with luminosity. The pulse profile on the other hand is consistent with what is expected at this luminosity level, arguing against a change in accretion geometry. In summary, GRO J1008-57 is an ideal target to study different accretion regimes due to the well-constrained evolution of its broad-band spectral continuum over several orders of magnitude in luminosity.

Kühnel, Matthias↗

NASA NTRS2016

Discovery and Modelling of a Flattening of the Positive Cyclotron Line/luminosity Relation in GX 304-1 with RXTE

The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting Xray binary transient source, GX 304−1 in 2010 and 2011. We present results of detailed 3-100 keV spectral analysis of 69 separate observations, and report a positive correlation between cyclotron line parameters, as well as other spectral parameters, with power-law flux. The cyclotron line energy, width and depth versus flux, and thus luminosity, correlations show a flattening of the relationships with increasing luminosity, which are well described by quasi-spherical or disc accretion that yield the surface magnetic field to be ∼5 × 1012 Gauss. Since HEXTE (High Energy X-ray Timing Experiment) cluster A was fixed aligned with the Proportional Counter Array field of view and cluster B was fixed viewing a background region 1. ◦ 5 off of the source direction during these observations near the end of the RXTE mission, the cluster A background was estimated from cluster B events using HEXTEBACKEST. This made possible the detection of the ∼55 keV cyclotron line and an accurate measurement of the continuum. Correlations of all spectral parameters with the primary 2-10 keV power-law flux reveal it to be the primary driver of the spectral shape. The accretion is found to be in the collisionless shock braking regime.

Rothschild, Richard E.↗

NASA NTRS2013

Measurements of Cyclotron Features and Pulse Periods in the High-Mass X-Ray Binaries 4U 1538-522 and 4U 1907+09 with the International Gamma-Ray Astrophysics Laboratory

We present a spectral and timing analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of two high-mass X-ray binaries, 4U 1538−522 and 4U 1907+09. Our timing measurements for 4U 1538−522 find the pulse period to have exhibited a spin-up trend until approximately 2009, after which there is evidence for a torque reversal, with the source beginning to spin down to the most recently measured period of 525.407 plus or minus 0.001 seconds. The most recent INTEGRAL observations of 4U 1907+09 are not found to yield statistically significant pulse periods due to the significantly lower flux from the source compared with 4U 1538−522. A spectral model consisting of a power-law continuum with an exponential cutoff and modified by two cyclotron resonance scattering features is found to fit both sources well, with the cyclotron scattering features detected at approximately 22 and approximately 49 kiloelectronvolts for 4U 1538−522 and at approximately 18 and approximately 36 kiloelectronvolts for 4U 1907+09. The spectral parameters of 4U 1538−522 are generally not found to vary significantly with flux and there is little to no variation across the torque reversal. Examining our results in conjunction with previous work, we find no evidence for a correlation between cyclotron line energy and luminosity for 4U 1538−522. 4U 1907+09 shows evidence for a positive correlation between cyclotron line energy and luminosity, which would make it the fourth, and lowest luminosity, cyclotron line source to exhibit this relationship.

stars: magnetic field↗

NASA NTRS2012

Broadband Spectroscopy Using Two Suzaku Observations of the HMXB GX 301-2

We present the analysis of two Suzaku observations of GX 301-2 at two orbital phases after the periastron passage. Variations in the column density of the line-of-sight absorber are observed, consistent with accretion from a clumpy wind. In addition to a CRSF, multiple fluorescence emission lines were detected in both observations. The variations in the pulse profiles and the CRSF throughout the pulse phase have a signature of a magnetic dipole field. Using a simple dipole model we calculated the expected magnetic field values for different pulse phases and were able to extract a set of geometrical angles, loosely constraining the dipole geometry in the neutron star. From the variation of the CRSF width and energy, we found a geometrical solution for the dipole, making the inclination consistent with previously published values.

Suchy, Slawomir↗

NASA NTRS2012

The Reawakening of the Sleeping X-ray Pulsar XTE J1946+274

We report on a series of outbursts of the high mass X-ray binary XTE 11946+274 in 2010/2011 as observed with INTEGRAL, RXTE, and Swift. We discuss possible mechanisms resulting in the extraordinary outburst behavior of this source. The X-ray spectra can be described by standard phenomenological models, enhanced by an absorption feature of unknown origin at about 10 keV and a narrow iron K alpha fluorescence line at 6.4keV, which are variable in flux and pulse phase. We find possible evidence for the presence of a cyclotron resonance scattering feature at about 25 keV at the 93% level. The presence of a strong cyclotron line at 35 keV seen in data from the source's 1998 outburst and confirmed by a reanalysis of these data can be excluded. This result indicates that the cyclotron line feature in XTE 11946+274 is variable between individual outbursts.

Mueller, Sebastian↗

NASA NTRS2011

Orbital Parameters and Spectroscopy of the Transient X-Ray Pulsar 4U 0115+63

We report on an outburst of the high mass X-ray binary 4U 0115+63 with a pulse period of 3.6s in spring 2008 as observed with INTEGRAL and RXTE. By analyzing the lightcurves we derive an updated orbital- and pulse period ephemeris of the neutron star. We also study the pulse profile variations as a function of time and energy as well as the variability of the spectral parameters. We find clear evidence for at least three cyclotron line features. In agreement with previous observations of 4U 0115+63, we detect an anti-correlation between the luminosity and the fundamental cyclotron line energy.

Mueller, Sebastian↗

NASA NTRS2011

Suzaku Observations of the HMXB 1A 1118-61

We present broad band analysis of the Be/X-ray transient lA 1118-61 by Suzaku at the peak of its 3rd observed outburst in January 2009 and 2 weeks later when the source flux had decayed by an order of magnitude. The continuum was modeled with a cutoffpl model as well as a compTT model, with both cases requiring an additional black body component at lower energies, We confirm the detection of a cyclotron line at approx. 55 keV and discuss the possibility of a first harmonic at approx. 110 keV. Pulse profile comparisons show a change in the profile structure at lower energies, an indication for possible changes in the accretion geometry, Phase resolved spectroscopy in the outburst data show a change in the continuum throughout the pulse period. The decrease in the CRSF centroid energy also indicates that the viewing angle on the accretion column is changing throughout the pulse period. Subject headings: X-rays: stars - X-rays: binaries - stars: pulsars: individual (lA 1118-61) - stars: magnetic fields

Suchy, Slawomir↗

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 NTRS2010

4U 1909+07: A Hidden Pearl

We present a detailed spectral and timing analysis of the High Mass X-ray Binary (HMXB) 4U 1909+07 with INTEGRAL and RXTE. 4U 1909+07 is a persistent accreting X-ray pulsar with a period of approximately 605 s. The period changes erratically consistent with a random walk expected for a wind accreting system. INTEGRAL detects the source with an average of 2.4 cps (corresponding to 15 mCrab), but sometimes exhibits flaring activity up to 50 cps (i.e. 300 mCrab). The strongly energy dependent pulse profile shows a double peaked structure at low energies and only a single narrow peak at energies above 20 keV. The phase averaged spectrum is well described by a powerlaw modified at higher energies by an exponential cutoff and photoelectric absorption at low energies. In addition at 6.4 keV a strong iron fluorescence line and at lower energies a black body component are present. We performed phase resolved spectroscopy to study the pulse phase dependence of the spectral parameters: while most spectral parameters are constant within uncertainties, the blackbody normalization and the cutoff folding energy vary strongly with phase.

Kreykenbohm, Ingo↗

NASA NTRS2009

Temporal Variations of Strength and Location of the South Atlantic Anomaly as Measured by RXTE

The evolution of the particle background at an altitude of approx.540km during the time interval between 1996 and 2007 is studied using the particle monitor of the High Energy X-ray Timing Experiment on board NASA's Rossi X-ray Timing Explorer. A special emphasis of this study is the location and strength of the South Atlantic Anomaly (SAA). The size and strength of the SAA are anti-correlated with the the 10.7 cm radio flux of the Sun, which leads the SAA strength by approx.1 year reflecting variations in solar heating of the upper atmosphere. The location of the SAA is also found to drift westwards with an average drift rate of about 0.3deg/yr following the drift of the geomagnetic field configuration. Superimposed to this drift rate are irregularities, where the SAA suddenly moves eastwards and where furthermore the speed of the drift changes. The most prominent of these irregularities is found in the second quarter of 2003 and another event took place in 1999. We suggest that these events are previously unrecognized manifestations of the geomagnetic jerks of the Earth's magnetic field. Key words: space radiation environment, South Atlantic Anomaly, radiation monitors, Rossi X-ray Timing Explorer

Wilms, Jorn↗

NASA NTRS1991

UCSD High Energy X-ray Timing Experiment magnetic shield design and test results

Results are reported from an effort to define a passive magnetic field concept for the High Energy X-ray Timing Experiment (HEXTE), in the interest of reducing the detector-gain variations due to 0.5-1.0-sec timescale magnetic field variations. This will allow a sensitivity of the order of 1 percent of the HEXTE background. While aperture modulation and automatic gain control will minimize effects on timescales of tens of seconds and longer, passive magnetic shielding of the photomultiplier tubes will address 1-sec timescale variations due to aperture motions.

Rothschild, Richard E.↗

NASA NTRS1991

Design and performance of an Automatic Gain Control system for the High Energy X-Ray Timing Experiment

The High Energy X-Ray Timing Experiment (HEXTE), currently under development for the X-Ray Timing Explorer (XTE) mission, employs a closed loop gain control system to attain 0.5 percent stabilization of each of eight-phoswich detector gains. This Automatic Gain Control (AGC) system utilizes a split window discriminator scheme to control the response of each detector pulse height analyzer to gated Am-241 X-ray events at 60 keV. A prototype AGC system has been implemented and tested within the gain perturbation environment expected to be experienced by the HEXTE instrument in flight. The AGC system and test configuration are described. Response, stability and noise characteristics are measured and compared with theoretical predictions. The system is found to be generally suitable for the HEXTE application.

Pelling, Michael R.↗

NASA NTRS1989

The X-ray Timing Explorer Mission

The technical aspects of the X-ray Timing Explorer (XTE) mission are described with particular attention paid to the current scientific problems it will address. It features a low-background, continuous 2-200 keV response with large areas. Rapid response (in hours) to observing opportunities is obtained by virtue of a scanning all-sky monitor, rapid spacecraft maneuverability, and rapid response in the control center. XTE will carry out detailed, energy-resolved studies of phenomena close to neutron stars and black holes utilizing XTE's submillisecond timing, its high telemetry rates, and the high throughput of its data system.

Rothschild, Richard E.↗

NASA NTRS1987

A rapid change of the Hercules X-1 pulse profile and high-state duration

Her X-1 has been observed in the 13-180 keV energy range by the HEAO 1 A-4 Low-Energy Detectors during selected phases of the 35 d on-off cycle. During a pointing observation in September 1978, the pulse profile was observed to change continuously from its normal shape to an anomalous double-pulsed form. Moreover, at this time the main-on state, normally of the duration 10 d, was seen to terminate after only 7 d. Since such an anomalous pulse profile has also been reported by Truemper et al. in 1986 during a short-on state, three short-on states in the HEAO 1 data were also investigated. One short-on observation had the sensitivity to detect pulsing, and the observed profile also had an anomalous double-pulsed form. Current models for the 35 d cycle of Her X-1 have been examined, and a model with a neutron star undergoing free precession could not explain the sudden change of pulse profile within 20 hr at the end of a main-on state.

Soong, Yang↗

NASA NTRS1987

Two binary cycles of GX 301-2

The slow X-ray pulsar, GX 301-2, has been observed throughout two full binary cycles of 41.5 days in the energy range 13-170 keV by the UCSD/MIT Hard X-Ray and Low Energy Gamma-Ray Experiment aboard HEAO 1. Increased intensity and variability was observed for about four days near periastron during the two binary cycles, which were separated by six months in 1978. The spectral shape was not observed to vary with binary phase, with the possible exception of a steepening of the continuum above 20 keV near superior conjunction in one of the two observing epochs. Evidence for a spectral line was found in the data at the 1-percent confidence level, but not confirmed in the remaining data. The temporal variability of GX 301-2 was not consistent with simple models of X-ray production by interaction of the stellar wind with the neutron star, implying that future modeling must include effects on wind density and speed caused by shocks in the wind, the X-ray flux from the neutron star, the gravitational influence of the neutron star, and possible mass accumulation near the neutron star.

Rothschild, Richard E.↗