arXiv subjects

R. E. Rothschild

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

85 records · Page 5Linked to original sources

arXiv1999

Disappearing Pulses in Vela X-1

We present results from a 20 h RXTE observation of Vela X-1, ncluding a peculiar low state of a few hours duration, during which the pulsation of the X-ray emission ceased, while significant non-pulsed emission remained. This ``quiescent state'' was preceded by a ``normal state'' without any unusual signs and followed by a ``high state'' of several hours of increased activity with strong, flaring pulsations. while there is clear spectral evolution from the normal state to the low state, the spectra of the following high state are surprisingly similar to those of the low state.

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arXiv1999

Discovery of a Third Harmonic Cyclotron Resonance Scattering Feature in the X-ray Spectrum of 4U 0115+63

We have discovered a third harmonic cyclotron resonance scattering feature (CRSF) in observations of the recent outburst of 4U 0115+63 with the Rossi X-ray Timing Explorer (RXTE). The spectrum in a narrow pulse phase range shows CRSFs at 12.40+0.65/-0.35, 21.45+0.25/-0.38, and 33.56+0.70/-0.90 keV. With centroid energy ratios to the fundamental of 1.73+/-0.08 and 2.71+/-0.13, the CRSFs are not harmonically spaced. Strong variability of the continuum and CRSFs with pulse phase indicate a complex emission geometry near the neutron star polar cap. In addition, one RXTE observation, which spanned periastron passage, revealed a strong 2 mHz quasi-periodic oscillation (QPO). This is slower by two orders of magnitude than the beat-frequency QPO expected in this system and slower by a factor of more than 5 compared with other QPOs seen in accreting X-ray pulsars.

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arXiv1999

Is SGR 1900+14 a Magnetar?

We present RXTE observations of the soft gamma--ray repeater SGR 1900+14 taken September 4-18, 1996, nearly 2 years before the 1998 active period of the source. The pulsar period (P) of 5.1558199 +/- 0.0000029 s and period derivative (Pdot) of (6.0 +/- 1.0) X 10^-11 s/s measured during the 2-week observation are consistent with the mean Pdot of (6.126 +/- 0.006) X 10^-11 s/s over the time up to the commencement of the active period. This Pdot is less than half that of (12.77 +/- 0.01) X 10^-11 s/s observed during and after the active period. If magnetic dipole radiation were the primary cause of the pulsar spindown, the implied pulsar magnetic field would exceed the critical field of 4.4 X 10^13 G by more than an order of magnitude, and such field estimates for this and other SGRs have been offered as evidence that the SGRs are magnetars, in which the neutron star magnetic energy exceeds the rotational energy. The observed doubling of Pdot, however, would suggest that the pulsar magnetic field energy increased by more than 100% as the source entered an active phase, which seems very hard to reconcile with models in which the SGR bursts are powered by the release of magnetic energy. Because of this, we suggest that the spindown of SGR pulsars is not driven by magnetic dipole radiation, but by some other process, most likely a relativistic wind. The Pdot, therefore, does not provide a measure of the pulsar magnetic field strength, nor evidence for a magnetar.

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arXiv1998

Observation of Cen A by the Rossi X-ray Timing Explorer

The Rossi X-ray Timing Explorer made a short (10 ks) observation of the radio galaxy Centaurus A on 14 August 1996. Analysis of the combined 2.5-240 keV spectrum has revealed a heavily absorbed(NH=9.42+/-0.24 e22 cm-2) primary power law (index=1.86+/-0.015) and an iron line due to fluorescence of cold matter (EW=162+/-25 eV). Flux from either a jet, primary flux scattered into the line of sight, or primary flux seen through a partial absorber was not required. The iron line width is unresolved at the 95% confidence level (sigma < 0.54 keV). No significant variability in the iron line flux is seen from measurements over the last two decades, while the overall continuum flux varied by more than a factor of four, which implies that the line emission region is distant from that of the primary emission. While radio-quiet Seyfert galaxies exhibit spectral components attributable to Compton reflection from cold matter, Cen A reveals no such component (exposed solid angle ratio < 0.09). This supports unified models of active galaxies that have little difference between Seyfert 2 and low luminosity radio galaxies

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arXiv1998

Discovery of a New 89 Second X-ray Pulsar XTE J1906+09

We report on the discovery of a new pulsating X-ray source during Rossi X-ray Timing Explorer observations of a low galactic latitude field centered at RA (J2000) = 19 hr 05 m 43 s and Dec (J2000) = +08 deg 58 arcmin 48 arcsec. Significant pulsations were detected by both the PCA and HEXTE instruments aboard RXTE at a fundamental period of 89.17 +/- 0.02 seconds, with higher harmonics also visible in the 2-10 keV power spectrum. The folded lightcurve from the source is multiply peaked at lower energies, and changes to single peaked morphology above ~20 keV. The phase averaged spectrum from the source is well fit by strongly absorbed power law or thermal bremsstrahlung spectral models of photon index 1.9 +/- 0.1 or temperature 19.5 +/- 4.6 keV, respectively. The mean neutral hydrogen column density is approximately 10^23 cm^-2, suggesting a distance of >10 kpc to the source and a minimum 2-10 keV X-ray luminosity of 2*10^{35} ergs s^{-1}. By comparison with other pulsars with similar periods and luminosities, we suggest that XTE J1906+09 has a supergiant companion with an underfilled Roche lobe. We speculate further that one of the M stars in a peculiar M star binary system may be the companion.

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arXiv1998

RXTE Observations of the SGR 1806-20 Steady Emission

We discuss observations of the quiescent emission from the soft gamma repeater SGR 1806-20 by the Rossi X-ray Timing Explorer. We find that the 2-20 keV RXTE data is consistent with a constant spectral shape during both active bursting periods and periods of relative quiescence, and is best described by a nonthermal (power law) spectral shape. Using archival ASCA data we find that the quiescent spectrum of SGR 1806-20 is well fit over the energy range 1-30 keV by a power law of photon index 2.31 +/- 0.04, with thermal bremsstrahlung and Raymond-Smith models producing much worse fits to the data.

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arXiv1998

RXTE Absolute Timing Results for the Pulsars B1821-24 and B1509-58

Observations with the Rossi X-ray Timing Explorer and the Jodrell Bank, Parkes, and Green Bank telescopes have enabled us to determine the time delay between radio and X-ray pulses in the two isolated pulsars B1821-24 and B1509-58. For the former we find that the narrow X-ray and radio pulse components are close to being coincident in time, with the radio peak leading by 0.02 period (60 +/- 20 microsec), while the wide X-ray pulse component lags the last of the two wider radio components by about 0.08 period. For the latter pulsar we find, using the standard value for the dispersion measure, that the X-ray pulse lags the radio by about 0.27 period, with no evidence for any energy-dependence in the range 2-100 keV. However, uncertainties in the history of the dispersion measure for this pulsar make a comparison with previous results difficult. It is clear that there are no perceptable variations in either the lag or the dispersion measure at time scales of a year or less.

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arXiv1997

HEXTE Observations of SGR 1806-20 During Outburst

We discuss observations of the soft gamma repeater SGR 1806-20 during the RXTE Target of Opportunity observations made in November 1996. During the ~50 ksec RXTE observation, HEXTE (15-250 keV) detected 17 bursts from the source, with fluxes ranging from 3 x 10^{-9} to 2.2 x 10^{-7} ergs cm^{-2} s^{-1} (20-100 keV). We obtained spectra for the brighter HEXTE by fitting thermal bremsstrahlung and power law functions over the energy range 17 - 200 keV. The best-fit temperatures and photon indices range from 30 - 55 keV and 2.2 - 2.7, respectively. The weighted average temperature of the HEXTE bursts was 41.8 +/- 1.7 keV, which is consistent with previous SGR 1806-20 burst spectra. The persistent emission from SGR 1806-20 was not detected with HEXTE.

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arXiv1997

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 December 30, 1995. 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 degree 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.

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arXiv1997

The X-ray Spectrum of the Plerionic System PSR B1509-58/MSH 15-52

We present the results of observations of the PSR B1509$-$58/MSH 15$-$52 system in X-rays ($2-250$ keV) by the Rossi X-ray Timing Explorer. The spectra of the peak of the pulsed component (radio phase $0.17-0.53$) is fit by a power law of photon index $1.36\pm0.01$, with no evidence of a high energy spectral break seen up to $\sim200$ keV. For the off-pulse spectral component, the spectrum from $2-250$ keV is fit by a power law of photon index $2.215\pm0.005$. An iron emission line at 6.7 keV with an equivalent width of 129 eV improves the fit, but only at a marginal significance. Thermal bremsstrahlung and Raymond-Smith models produce much worse fits to the unpulsed data. The lack of a high energy spectral break in the pulsed emission implies an efficiency of $\geq 3%$ in the conversion of pulsar spindown energy to pulsed X-rays in the system.

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arXiv1997

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 broad-band 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 1.8 +0.5/-0.6 and 3.04 +0.15/-0.13 and a break energy of 15.9 +0.3/-0.4 keV. We argue that the high-energy component, which dominates the spectrum above about 10 keV, is produced by synchrotron radiation from electrons that have energies up to at least 40 TeV. This conclusion supports the hypothesis that Galactic cosmic rays are accelerated predominantly in supernova remnants.

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arXiv1996

X-ray Variability of Cygnus X-1 In Its Soft State

We reported previously that for Cyg X-1 there is a settling period following the transition from hard to soft state (astro-ph/9610071). During the transiton, The low energy spectrum (below ~10 keV) varies significantly from observation to observation while the high energy portion changes little. The source reaches nominal soft-state brightness during the settling period. It can be characterized by a soft low-energy spectrum and significant low-frequency 1/f noise and white noise on the power density spectrum (PDS). The low-energy spectrum becomes even softer, and the PDS is completely dominated by the 1/f noise, when the ``true'' soft state is reached. In this paper, subsequent RXTE observations of Cyg X-1 in the soft state are examined, and the results confirm our earlier conclusions. Furthermore, we show the results from observations taken during a soft-to-hard transition. As expected, the white noise appears again, and accordingly, the 1/f noise becomes less dominant, similar to the settling period at the end of the hard-to-soft transition. The low-frequency 1/f noise has not been observed when Cyg X-1 is in the hard state. Therefore, it seems to be positively correlated with the disk mass accretion rate which is low in the hard state and high in the soft state. The difference in the observed spectral and timing properties between the hard and soft states is qualitatively consistent with a simple ``fluctuating corona'' model (astro-ph/9610071). Here we present more evidence for it.

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arXiv1996

RXTE Observation of Cygnus X-1 In Its High State

We present the results from the RXTE observations of Cygnus X-1 in its high state. In the energy range of 2-200 keV, the observed X-ray spectrum can be described by a model consisting of a soft blackbody component and a broken power-law with a high energy cutoff. The low energy spectrum (below about 11 keV) varies significantly from observation to observation while the high energy portion changes little. The X-ray flux varies on all timescales down to milliseconds. The power density spectrum (PDS) can be characterized by excess red noise (``1/f'') at low frequencies and a white noise component that extends to 1-3 Hz before being cut off. At higher frequencies, the PDS becomes power-law again, with a slope of roughly -2 (i.e., ``1/f^2''). Broad peaks in the range of 3-9 Hz are present, and might be due to quasi-periodic oscillations. The PDS shows interesting spectral dependence: the 1/f component becomes more prominent when the low-energy spectrum becomes softer. The difference in the observed spectral and timing properties between the low and high states is qualitatively consistent with a simple ``fluctuating corona'' model.

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