Richard E. Rothschild

In-Flight Performance of the High Energy X-Ray Timing Experiment on the Rossi X-Ray Timing Explorer

The High Energy X-Ray Timing Experiment (HEXTE) is one of three scientific instruments aboard the Rossi X-Ray Timing Explorer (RXTE), which was launched on 1995 December 30. RXTE performs timing and spectral studies of bright X-ray sources to determine the physical parameters of these systems. The HEXTE consists of two independent clusters of detectors, each cluster containing four NaI(Tl)/CsI(Na) phoswich scintillation counters sharing a common 1° FWHM field of view. The field of view of each cluster is switched on and off source to provide near real-time background measurements. The net open area of the eight detectors is 1600 cm2, and each detector covers the energy range 15-250 keV with an average energy resolution of 15.4% at 60 keV. The in-flight performance of the HEXTE is described, the light curve and spectrum of the Crab Nebula/pulsar is given, and the 15-240 keV spectrum of the weak source, active galaxy MCG +8-11-11 is presented to demonstrate the weak source spectral capabilities of HEXTE.

Magnetic Fields of Accreting X-Ray Pulsars with the Rossi X-Ray Timing Explorer

Using a consistent set of models, we parameterized the X-ray spectra of all accreting pulsars in the Rossi X-Ray Timing Explorer (RXTE) database that exhibit cyclotron resonance scattering features (CRSFs, or cyclotron lines). These sources in our sample are Her X-1, 4U 0115+63, Cen X-3, 4U 1626-67, XTE J1946-274, Vela X-1, 4U 1907+09, 4U 1538-52, GX 301-2, and 4U 0352+309 (X Per). We searched for correlations among the spectral parameters, concentrating on how the cyclotron line energy relates to the continuum and therefore how the neutron star B-field influences the X-ray emission. As expected, we found a correlation between the CRSF energy and the spectral cutoff energy. However, with our consistent set of fits we found that the relationship is more complex than what has been reported previously. Also, we found not only that the width of the cyclotron line correlates with the energy (as suggested by theory), but also that the width scaled by the energy correlates with the depth of the feature. We discuss the implications of these results, including the possibility that accretion directly affects the relative alignment of the neutron star spin and dipole axes. Finally, we comment on the current state of fitting phenomenological models to spectra in the RXTE/BeppoSAX era and the need for better theoretical models of the X-ray continua of accreting pulsars.

The diffuse X-ray background spectrum from 3 to 50 keV

The spectrum of the extragalactic diffuse X-ray background has been measured with the GSFC Cosmic X-Ray Experiment on HEAO 1 for regions of the sky away from known point sources and more than 20 /sup 0/ from the galactic plane. A total exposure of 80 m/sup 2/-s-sr is available at present. Free-free emission from an optically thin plasma of 40 +- 5 keV provides an excellent description of the observed spectrum from 3 to 50 keV. This spectral shape is confirmed by measurements from five separate layers of three independent detectors. With an estimated absolute precision of approx.10%, the intensity of the emission at 10 keV is 3.2 keV keV/sup -1/ cm/sup -2/ s/sup -1/ sr/sup -1/, a value consistent with the average of previously reported spectra. No other spectral features, such as iron line emission, are evident. This spectrum is not typical of known extra-galactic objects. A uniform hot intergalactic medium of approximately 36% of the closure density of the universe would produce such a flux, although nonuniform models indicating less total matter are probably more realistic.

The discovery of rapid X-ray oscillations in the tail of the SGR 1806-20 hyperflare

We have discovered rapid quasi-periodic oscillations (QPOs) in RXTE/PCA measurements of the pulsating tail of the 2004 December 27 giant flare of SGR 1806-20. QPOs at ~92.5 Hz are detected in a 50 s interval starting 170 s after the onset of the giant flare. These QPOs appear to be associated with increased emission by a relatively hard unpulsed component and are seen only over phases of the 7.56 s spin period pulsations away from the main peak. QPOs at ~18 and ~30 Hz are also detected ~200-300 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected in the flux of a soft gamma-ray repeater or any other magnetar candidate. We interpret the highest QPOs in terms of the coupling of toroidal seismic modes with Alfven waves propagating along magnetospheric field lines. The lowest frequency QPO might instead provide indirect evidence on the strength of the internal magnetic field of the magnetar.

EVIDENCE OF X-RAY SYNCHROTRON EMISSION FROM ELECTRONS ACCELERATED TO 40 TeV IN THE SUPERNOVA REMNANT CASSIOPEIA A

We present the 2‐ 60 keV spectrum of the supernova remnant Cassiopeia A measured using the Proportional Counter Array and the High Energy X-Ray Timing Experiment on the Rossi X-Ray Timing Explorer satellite. In addition to the previously reported strong emission-line features produced by thermal plasmas, the broadband spectrum has a high-energy “tail” that extends to energies at least as high as 120 keV. This tail may be described by a broken power law that has photon indices of G1 5 1.820.6

Discovery of a flux-related change of the cyclotron line energy in Hercules X-1

Aims. We present the results of ten years of repeated measurements of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and report the discovery of a positive correlation of the centroid energy of this absorption feature in pulse phase averaged spectra with source luminosity. Methods. Our results are based on a uniform analysis of observations by the RXTE satellite from 1996 to 2005, using sufficiently long observations of 12 individual 35-day Main-On states of the source. Results. The mean centroid energy E c of the CRSF in pulse phase averaged spectra of Her X-1 during this time is around 40 keV, with significant variations from one Main-On state to the next. We find that the centroid energy of the CRSF in Her X-1 changes by ~5% in energy for a factor of 2 in luminosity. The correlation is positive, contrary to what is observed in some high luminosity transient pulsars. Conclusions. Our finding is the first significant measurement of a positive correlation between E c and luminosity in any X-ray pulsar. We suggest that this behaviour is expected in the case of sub-Eddington accretion and present a calculation of a quantitative estimate, which is very consistent with the effect observed in Her X-1. We urge that Her X-1 is regularly monitored further and that other X-ray pulsars are investigated for a similar behaviour.We present the results of ten years of repeated measurements of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and report the discovery of a positive correlation of the centroid energy of this absorption feature in pulse phase averaged spectra with source luminosity.Our results are based on a uniform analysis of observations bythe RXTE satellite from 1996 to 2005, using sufficiently long observations of 12 individual 35-day Main-On states of the source. The mean centroid energy E_c of the CRSF in pulse phase averaged spectra of Her X-1 during this time is around 40 keV, with significant variations from one Main-On state to the next. We find that the centroid energy of the CRSF in Her X-1 changes by ~5% in energy for a factor of 2 in luminosity. The correlation is positive, contrary to what is observed in some high luminosity transient pulsars. Our finding is the first significant measurement of a positive correlation between E_c and luminosity in any X-ray pulsar. We suggest that this behaviour is expected in the case of sub-Eddington accretion and present a calculation of a quantitative estimate, which is very consistent with the effect observed in Her X-1.We urge that Her X-1 is regularly monitored further and that other X-ray pulsars are investigated for a similar behaviour.

Nature versus Nurture: The Origin of Soft Gamma-Ray Repeaters and Anomalous X-Ray Pulsars

Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are young and radio-quiet X-ray pulsars that have been rapidly spun-down to slow spin periods clustered in the range 5-12 s. Most of these unusual pulsars also appear to be associated with supernova shell remnants (SNRs) with typical ages less than 30 kyr. By examining the sizes of these remnants versus their ages, we demonstrate that the interstellar media that surrounded the SGR and AXP progenitors and their SNRs were unusually dense compared to the environments around most young radio pulsars and SNRs. We explore the implications of this evidence on magnetar and propeller-based models for the rapid spin-down of SGRs and AXPs. We find that evidence of dense environments is not consistent with the magnetar model unless a causal link can be shown between the development of magnetars and the external interstellar medium. Propeller-driven spin-down by fossil accretion disks for SGRs and AXPs appears to be consistent with dense environments since the environment can facilitate the formation of such a disk. This may occur in two ways: (1) formation of a pushback disk from the innermost ejecta pushed back by prompt reverse shocks from supernova remnant interactions with massive progenitor wind material stalled in dense surrounding gas or (2) acquisition of disks by a high-velocity neutron stars, which may be able to capture sufficient amounts of comoving outflowing ejecta slowed by the prompt reverse shocks in dense environments.

A model for cyclotron resonance scattering features

Aims. We study the physics of cyclotron line formation in the high-energy spectra of accreting X-ray pulsars. In particular, we link numerical predictions for the line profiles to results from observational data analysis. Therefore, first we investigate the theoretical predictions and the significance of our model parameters, and second we aim at the development of a model to fit cyclotron lines in observational data. Methods. Simulations were performed using Monte Carlo methods. The data were extracted with HEADAS 6.1.1 and INTEGRAL OSA 5.1. A convolution model for the cyclotron line shapes was implemented for the XSPEC spectral analysis software package and for data packages compatible with XSPEC local models. Results. We predict the shapes of cyclotron lines for different prescribed physical settings. The calculations assume that the line-forming region is a low-density electron plasma, which is of cylindrical or slab geometry and which is exposed to a uniform, sub-critical magnetic field. We investigate the dependence of the shape of the fundamental line on angle, geometry, optical depth and temperature. We also discuss variations of the line ratios for non-uniform magnetic fields. We have developed a new convolution and interpolation model to simulate line features regardless of any a priori assumed shape of the neutron star continuum. Fitting RXTE and INTEGRAL data of the accreting X-ray pulsar V0332+53 with this model gives a qualitative description of the data. Strong emission wings of the fundamental cyclotron feature as predicted by internally irradiated plasma geometries are in principle observable by todays instruments but are not formed in V0332+53, hinting at a bottom illuminated slab geometry for line formation.

2-165 keV observations of active galaxies and the diffuse background

HEAO 1 spectral observations of 12 active galaxies in the 12-165 keV and 2-50 keV ranges are reported. The spectra of these galaxies in the 2-165 keV range are well represented by a single power law model; within experimental uncertainties a narrow dispersion in power law index attributable to the individual galaxies is observed, while the 2-165 keV luminosities of these galaxies ranged from 3 x 10 to the 43rd to 3 x 10 to the 45th ergs/s. An apparent universality of the spectral form is found which can be interpreted as due to a common electron distribution with a temperature of tens of keV in the Compton scattering region or as a common nonthermal power-law distribution generating the observed flux through synchrotron-Compton processes. The data indicate that relativistic particles are likely to be responsible for the X-rays from cores of active galaxies through synchroton-Compton processes. In addition, it is noted that only weak number evolution, if any at all, is present in active galaxies.

Multiwavelength observations of the Galactic black hole transient 4U 1543-47 during outburst decay: state transitions and jet contribution

Multiwavelength observations of Galactic black hole (GBH) transients during state transitions and in the low/hard state may provide detailed information on the accretion structure of these systems. The object 4U 1543-47 is a GBH transient that was covered exceptionally well in X-rays and the infrared (daily observations) and reasonably well in the optical and radio during its outburst decay in 2002. When all the available information is gathered from the intermediate and the low/hard states, 4U 1543-47 makes an important contribution to our understanding of state transitions and the role of outflows in the high-energy emission properties of black hole binaries. The evolution of the X-ray spectral and temporal properties and the infrared light curve place strong constraints on different models to explain the overall emission from accreting black holes. The overall spectral energy distribution is consistent with a synchrotron origin for the optical and infrared emission; however, the X-ray flux is above the power-law continuation of the optical and infrared flux. The infrared light curve, the HEXTE light curve, and the evolution of the X-ray photon index indicate that the major source of hard X-rays cannot be direct synchrotron radiation from an acceleration region in a jet for most of the outburst decay.