arXiv subjects

R. E. Rothschild

Publications and source records attributed to R. E. Rothschild.

At least 73 records · Page 4Linked to original sources

arXiv2002

X-ray temporal properties of XTE J1650-500 during outburst decay

We investigated the temporal behavior of the new black hole transient XTE J1650-500 with the Rossi X-ray Timing Explorer (RXTE) as the source made a transition to the low/hard state during the decay of the 2001 outburst. We find QPOs in the 4 - 9 Hz range, enhanced time lags and reduced coherence during the state transition. We also observe a shift in the peak frequency of the noise component with energy during the transition. The evolution of the power spectrum as well as the lag and coherence behavior during the state transition are similar to the state transitions observed for other black hole sources, especially Cyg X-1. The temporal properties during the transition to the low state put constraints on the accretion geometry of XTE J1650-500 and may have implications for all black hole binary systems. We suggest a possible geometry and evolution of a jet+corona+disk system based on enhanced lags and peak frequency shift during the transition.

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arXiv2001

Discovery of a Cyclotron Resonance Scattering Feature in the X-ray Spectrum of XTE J1946+274

Observations of the transient accreting pulsar XTE J1946+274 made with the Rossi X-ray Timing Explorer during the course of the 1998 September-November outburst, reveal a cyclotron resonance scattering feature (or "cyclotron line") in the hard X-ray spectrum near 35 keV. We determine a centroid energy of 36.2 +0.5/-0.7 keV, which implies a magnetic field strength of 3.1(1+z)x10^12 G, where z is the gravitational redshift of the scattering region. The optical depth, Tau = 0.33 +0.07/-0.06, and width, sigma = 3.37 +0.92/-0.75 keV, are typical of known cyclotron lines in other pulsars. This discovery makes XTE J1946+274 one of thirteen pulsars with securely detected cyclotron lines resulting in direct magnetic field measurements.

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arXiv2001

Stability of the Cyclotron Resonance Scattering Feature in Her X-1 with RXTE

Five observations of the hard X-ray spectrum of Her X-1 from \xte show that the $\sim 41$ keV energy of the cyclotron scattering line is constant within statistics of a few percent per observtion. Comparison with other observations over many years indicates strongly that the centroid energy of this absorption line has increased some time between 1991 and 1993 by 23%, from 34 keV to 41 keV. Moreover, the cutoff energy of the spectral continuum increased at the same time from 16 keV to 20 keV, which is, within the statistical error of 5%, in direct proportion to the centroid. This may be a sign thatboth these characteristics of the spectrum are controlled in the same way by the magnetic field strength in the region of scattering.

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arXiv2001

X-Ray Observations of XTE J1550-564 During the Decay of the 2000 Outburst - II. Timing

We investigate the timing behavior of XTE J1550-564 with the Rossi X-ray Timing Explorer (RXTE) as the source made a transition to the hard state during the decay of the 2000 outburst. We detect a high frequency QPO at 65 Hz in one observation with a fractional rms amplitude of 4.9%. This is the first time that a high frequency QPO has been detected in a transition to the hard state during outburst decay. We also observe low frequency QPOs in the 0.36-4.1 Hz range and rich aperiodic variability. The changes in the temporal properties during the decay are very similar to the state transitions observed for other sources. We find a correlation between the QPO frequency and the break frequency in the continuum power spectra. We investigate the energy dependence of rms amplitudes of the QPOs. We compare these timing properties with those previously observed for XTE J1550-564 and other Galactic black hole candidates and discuss the implications for QPO models and black hole accretion.

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arXiv2001

Studies of Hard X-ray Tails in Z sources with RXTE/HEXTE

We report RXTE results of spectral analyses of three (Sco X-1, GX 349+2, and Cyg X-2) out of the 6 known Z sources, with emphasis in the hard X-ray emission. No hard X-ray tails were found for Cyg X-2 (< 8.4E-5 photons cm**-2 s**-1, 50-100 keV, 3 sigma) and for GX 349+2 (< 7.9E-5 photons cm**-2 s**-1, 50-100 keV, 3 sigma). For Sco X-1 a variable hard X-ray tail (with an average flux of 2.0E-3 photons cm**-2 s**-1, 50-100 keV) has already been reported. We compare our results to reported detections of a hard component in the spectrum of Cyg X-2 and GX 349+2. We argue that, taking into account all the results on detections of hard X-ray tails in Sco X-1 and GX 349+2, the appearance of such a component is correlated with the brightness of the thermal component.

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arXiv2001

RXTE monitoring of Centaurus A

We report on the analysis from ~110 ks of X-ray observations of Centaurus A carried out with the Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) instruments on Rossi X-ray Timing Explorer (RXTE) during three monitoring campaigns over the last 4 years (10 ks in 1996, 74 ks in 1998, and 25 ks in 2000). The joint PCA/HEXTE X-ray spectrum can be well described by a heavily absorbed power law with photon index 1.8 and a narrow iron line due to fluorescence of cold matter. The measured column depth decreased by about 30% between 1996 and 2000, while the detected 2-10 keV continuum flux remained constant between 1996 and 1998, but increased by 60% in 2000. Since in all three observations the iron line flux did not vary, a corresponding decrease in equivalent width was noted. No appreciable evidence for a reflection continuum in the spectrum was detected. We present the interpretation of the iron line strength through Monte Carlo computations of various geometries. No significant temporal variability was found in Cen A at time scales from days to tens of minutes.

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arXiv2001

HST Observations of SGR 0526-66: New Constraints on Accretion and Magnetar Models

Soft Gamma-ray Repeaters (SGRs) are among the most enigmatic sources known today. Exhibiting huge X- and Gamma-ray bursts and flares, as well as soft quiescent X-ray emission, their energy source remains a mystery. Just as mysterious are the Anomalous X-ray pulsars (AXPs), which share many of the same characteristics. Thanks to recent Chandra observations, SGR 0526-66, the first SGR, now appears to be a transition object bridging the two classes, and therefore observations of it have implications for both SGRs and AXPs. The two most popular current models for their persistent emission are accretion of a fossil disk or decay of an enormous (~10^15 G) magnetic field in a magnetar. We show how deep optical observations of SGR 0526-66, the only SGR with small enough optical extinction for meaningful observations, show no evidence of an optical counterpart. These observation place strong new constraints on both accretion disk and magnetar models, and suggest that the spectral energy distribution may peak in the hard-UV. Almost all accretion disks are excluded by the optical data, and a magnetar would require a ~10^15-10^16 G field.

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arXiv2001

Resolution of the Age Discrepancies in Pulsar/SNR Associations

Pulsars associated with supernova remnants (SNRs) are valuable because they provide constraints on the mechanism(s) of pulsar spin-down. Here we discuss two SNR/pulsar associations in which the SNR age is much greater than the age of the pulsar obtained by assuming pure magnetic dipole radiation (MDR) spin-down. The PSR B1757-24/SNR G5.4-1.2 association has a minimum age of ~40 kyr from proper motion upper limits, yet the MDR timing age of the pulsar is only 16 kyr, and the newly discovered pulsar PSR J1846-0258 in the >2 kyr old SNR Kes 75 has an MDR timing age of just 0.7 kyr. These and other pulsar/SNR age discrepancies imply that the pulsar spin-down torque is not due to pure MDR, and we discuss a model for the spin-down of the pulsars similar to the ones recently proposed to explain the spin-down of soft gamma-ray repeaters and anomalous x-ray pulsars.

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arXiv2001

HEXTE Studies of Sco X-1 Spectra: Detections of Hard X-Ray Tails Beyond 200 keV

Using the HEXTE experiment on-board the RXTE satellite, we performed a search for hard X-ray tails in Sco X-1 spectra. We found strong evidence for the presence of such a non-thermal component on several occasions. Using the PCA/RXTE we were able to track the position of the source along the Z diagram, and we observed that the presence of the hard X-ray tail is not confined to a particular region. However, we found a correlation between the power law index of the non-thermal component and the position of the source in the Z diagram, suggesting that the hard X-ray spectrum (i.e., E > 50 keV) becomes flatter as the mass accretion rate increases. We were also able to study the temporal variation of the appearance/absence of the hard X-ray component. With our derived luminosities, we were also able to test the idea that X-ray luminosities can be used to distinguish between X-ray binary systems containing neutron stars and black holes.

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arXiv2001

Discovery of a Cyclotron Resonant Scattering Feature in the RXTE Spectrum of 4U 0352+309 (X Per)

We have discovered a ~29 keV Cyclotron Resonant Scattering Feature (CRSF) in the X-Ray spectrum of 4U 0352+309 (X Per) using observations taken with the Rossi X-Ray Timing Explorer. 4U 0352+309 is a persistent low luminosity (L_x = 4.2x10^34 ergs/s) X-ray pulsar, with a 837 s period and which accretes material from the Be star X Per. The X-Ray spectrum, unusual when compared to brighter accreting pulsars, may be due to the low mass accretion rate and could be typical of the new class of persistent low luminosity Be/X-Ray binary pulsars. We attempted spectral fits with continuum models used historically for 4U 0352+309, and found that all were improved by the addition of a CRSF at ~29 keV. The model that best fit the observations is a combination of a 1.45+/-0.02 keV blackbody with a 5.4x10^8 cm^2 area, and a power-law with a 1.83+/-0.03 photon index modified by the CRSF. In these fits the CRSF energy is 28.6+1.5-1.7 keV, implying a magnetic field strength of 2.5(1+z)x10^12 G in the scattering region (where z is the gravitational redshift). Phase resolved analysis shows that the blackbody and cyclotron line energies are consistent with being constant through the pulse.

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arXiv2000

Nature vs. 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 which 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 <30 kyr. By examining the sizes of these remnants versus their ages, we demonstrate that the interstellar media which 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 ISM. 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'' disks 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 a sufficient amounts of co-moving outflowing ejecta slowed by the prompt reverse shocks in dense environments.

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arXiv2000

HEXTE Detections of Hard X-Ray Tails in Sco X-1

We report the detection of a non-thermal hard X-ray component from Sco X-1 based upon the analysis of 20-220 keV spectra obtained with the HEXTE experiment on-board the RXTE satellite. We find that the addition of a power-law component to a thermal bremsstrahlung model is required to achieve a good fit in 5 of 16 observations analyzed. Using PCA data we were able to track the movement of the source along the Z diagram, and we found that the presence of the hard X-ray tail is not confined to a specific Z position. However, we do observe an indication that the power law index hardens with increasing mass accretion rate, as indicated from the position on the Z diagram. We find that the derived non-thermal luminosities are at order of 10% of that derived for the brightest of the atoll sources.

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arXiv2000

Does Pulsar B1757--24 Have a Fallback Disk?

Radio pulsars are thought to spin-down primarily due to torque from magnetic dipole radiation (MDR) emitted by the time-varying stellar magnetic field as the star rotates. This assumption yields a `characteristic age' for a pulsar which has generally been assumed to be comparable to the actual age. Recent observational limits on the proper motion of pulsar B1757-24, however, revealed that the actual age (>39 kyr) of this pulsar is much greater than its MDR characteristic age (16 kyr) - calling into question the assumption of pure MDR spin-down for this and other pulsars. To explore the possible cause of this discrepancy, we consider a scenario in which the pulsar acquired an accretion disk from supernova ejecta, and the subsequent spin-down occurred under the combined action of MDR and accretion torques. A simplified model of the accretion torque involving a constant mass inflow rate at the pulsar magnetosphere can explain the age and period derivative of the pulsar for reasonable values of the pulsar magnetic field and inflow rate. We discuss testable predictions of this model.

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arXiv2000

The 1999 Hercules X-1 Anomalous Low State

A failed main-on in the 35d cycle of Her X-1 was observed with the Rossi X-Ray Timing Explorer (RXTE) on 1999 April 26. Exceptions to the normal 35d cycle have been seen only twice before; in 1983 and again 1993. We present timing and spectral results of this latest Anomalous Low State (ALS) along with comparisons to the main-on and normal low states. Pulsations were observed in the 3-18 keV band with a fractional RMS variation of (0.037+-0.003). Spectral analysis indicates that the ALS spectrum has the same shape as the main-on but is modified by heavy absorption and scattering. We find that 70% of the observed emission has passed through a cold absorber (N_H=5.0x10^{23}cm^{-2}). This partially absorbing spectral fit can be applied to the normal low state with similar results. We find that the ALS observations may be interpreted as a decrease in inclination of the accretion disk causing the central X-Ray source to be obscured over the entire 35d cycle.

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arXiv2000

Multiple Cyclotron Lines in the Spectrum of 4U 0115+63

We report phase resolved spectroscopy of the transient accreting pulsar, 4U0115+63. For the first time, more than two cylotron resonance scattering features are detected in the spectrum of an X-ray pulsar. The shape of the fundamental line appears to be complex, and this is in agreement with predictions of Monte-Carlo models. As in other pulsars, the line energies and optical depths are strong functions of pulse phase. One possible model for this is an offset of the dipole of the neutron star magnetic field.

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arXiv2000

RXTE Studies of Cyclotron Lines in Accreting Pulsars

In this paper, we summarize the RXTE measurements of cyclotron resonance scattering features in 8 accreting pulsars. The wide bandpass and modest resolution of the RXTE instruments make them ideal for measuring these generally broad features. In particular, the high energy response of HEXTE provides a window for discovery of new lines not detectable with proportional counters such as Ginga or the PCA alone.

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arXiv1999

Magnetic Field Limits on SGRs

We measure the period and spin-down rate for SGR 1900+14 during the quiescient period two years before the recent interval of renewed burst activity. We find that the spin-down rate doubled during the burst activity which is inconsistent with both mangetic dipole driven spin down and a magnetic field energy source for the bursts. We also show that SGRs 1900+14 and 1806-20 have braking indices of $\sim$1 which indicate that the spin-down is due to wind torques and not magnetic dipole radiation. We further show that a combination of dipole radiation, and wind luminosity, coupled with estimated ages and present spin parameters, imply that the magnetic fields of SGRs 1900+14 and 1806-20 are less than the critical field of 4$\times10^{13}$ G and that the efficiency for conversion of wind luminosity to x-ray luminosity is <2%.

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arXiv1999

Magnetic Field Limit on SGR 1900+14

We measured the period and spin-down rate for SGR 1900+14 during the quiescient period two years before the recent interval of renewed burst activity. We have shown that the spin-down age of SGR 1900+14 is consistent with a braking index of ~1 which is appropriate for wind torques and not magnetic dipole radiation. We have shown that a combination of dipole radiation, and wind luminosity, coupled with estimated ages and present spin parameters, imply that the magnetic field for SGR 1900+14 is less than 6 x 10^13 G and that the efficiency for conversion of wind luminosity to x-ray luminosity is <2%.

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