arXiv subjects

E. H. Morgan

Publications and source records attributed to E. H. Morgan.

29 records · Page 2Linked to original sources

arXiv1999

X-ray Nova XTE J1550-564: Discovery of a QPO Near 185 Hz

We have investigated the X-ray timing properties of XTE J1550-564 during 60 RXTE PCA observations made between 1998 September 18 and November 28. We detect quasi-periodic oscillations (QPOs) near 185 Hz during four time intervals. The QPO widths (FWHM) are near 50 Hz, and the rms amplitudes are about 1% of the mean flux at 2-30 keV. This is the third Galactic black hole candidate known to exhibit a transient X-ray timing signature above 50 Hz, following the 67 Hz QPO in GRS1915+105 and the 300 Hz QPO in GRO J1655-40. However, unlike the previous cases, which appear to show stationary frequencies, the QPO frequency in XTE J1550-564 must vary by at least 10% to be consistent with observations. The occurrences and properties of the QPO were insensitive to large changes in the X-ray intensity (1.5 to 6.8 Crab). However, the QPO appearance was accompanied by changes in the energy spectrum, namely, an increase in the temperature and a decrease in the normalization of the thermal component. The QPO is also closely related to the hard X-ray power-law component of the energy spectrum since the fractional amplitude of the QPO increases with photon energy. The fast QPOs in accreting black hole binaries are thought to be effects of general relativity; however, the relevance of the specific physical models that have been proposed remains largely uncertain. Low frequency QPOs in the range 3-13 Hz were often observed. Occasionally at high luminosity the rms QPO amplitude was 15% of the flux, a level previously reached only by GRS1915+105.

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arXiv1999

X-ray Nova XTE J1550-564: RXTE Spectral Observations

Excellent coverage of the 1998 outburst of the X-ray Nova XTE J1550-564 was provided by the Rossi X-ray Timing Explorer. XTE J1550-564 exhibited an intense (6.8 Crab) flare on 1998 September 19 (UT), making it the brightest new X-ray source observed with RXTE. We present a spectral analysis utilizing 60 Proportional Counter Array spectra from 2.5-20 keV spanning 71 days, and a nearly continuous All Sky Monitor light curve. The spectra were fit to a model including multicolor blackbody disk and power-law components. XTE J1550-564 is observed in the very high, high/soft, and intermediate canonical outburst states of Black Hole X-ray Novae.

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arXiv1998

Strong Aperiodic X-ray Variability and Quasi-Periodic Oscillation in X-ray Nova XTE J1550-564

We report the discovery of strong aperiodic X-ray variability and quasi-periodic oscillation (QPO) in the X-ray light curves of a new X-ray Nova, XTE J1550-564, and the evolution of the observed temporal properties during the rise of the recent X-ray outburst. The power spectral analysis of the first observation reveals strong aperiodic X-ray variability of the source (~28%), as well as the presence of a QPO at ~82 mHz with fractional rms amplitude ~14% over the 2-60 keV energy range. Also apparent is the first harmonic of the QPO with the amplitude ~9%. As the X-ray flux increases, the source tends to become less variable, and the QPO frequency increases rapidly, from 82 mHz to 4 Hz, over the flux (2-50 keV) range of 1.73-5.75 x 10^{-8} ergs cm^{-2} s^{-1}. The amplitude of the fundamental component of the QPO varies little, while that of the harmonic follows a decreasing trend. The fundamental component strengthens toward high energies, while its harmonic weakens. Initially, the power spectrum is roughly flat at low frequencies and turns into a power law at high frequencies, with the QPO harmonic sitting roughly at the break. In later observations, however, the high-frequency portion of the continuum can actually be better described by a broken power law (as opposed to a simple power law). This effect becomes more apparent at higher energies. The overall amplitude of the continuum shows a similar energy dependence to that of the fundamental component of the QPO. Strong rapid X-ray variability, as well as hard energy spectrum, makes XTE J1550-564 a good black hole candidate. We compare its temporal properties with those of other black hole candidates.

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arXiv1998

RXTE Observations of 0.1-300 Hz QPOs in the Microquasar GRO J1655-40

We have investigated 52 RXTE pointed observations of GRO J1655-40 spanning the X-ray outburst that commenced on 1996 April 25 and lasted for 16 months. Our X-ray timing analyses reveal four types of QPOs: three with relatively stable central frequencies at 300 Hz, 9 Hz, and 0.1 Hz, and a fourth that varied over the range 14-28 Hz. The 300 Hz and 0.1 Hz QPOs appear only at the highest observed luminosities (Lx > 0.15 Ledd), where the power-law component dominates the X-ray spectrum. At lower luminosity, the disk flux exceeds the power-law flux and only two of the QPOs are observed: the spectrally "soft" 9 Hz QPO, and the narrow, "hard" QPO that varies from 14-28 Hz as the hard flux decreases. The 300 Hz QPO is likely to be analogous to the stationary QPO at 67 Hz seen in the microquasar GRS1915+105. We discuss models of these high-frequency QPOs which depend on effects due to general relativity. The 9 Hz QPO displays a spectrum consistent with a thermal origin, but this frequency does not appear to be consistent with any of the natural time scales associated with the disk, or with the inferred values of the mass and rapid spin of the black hole. The mechanism for the 14-28 Hz QPOs appears to be linked to the power-law component, as do the 1-10 Hz QPOs in GRS1915+105. Finally, we show data for GRO J1655-40 and GRS1915+105 as each source teeters between relative stability and a state of intense oscillations at 0.1 Hz. A comparison of the sources' spectral parameters allows us to speculate that the black hole mass in GRS1915+105 is very large, possibly in the range 39-70 Msun.

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arXiv1998

Monitoring the Galactic Microquasars with the Rossi X-ray Timing Explorer

The Galactic microquasars GRS1915+105 and GRO J1655-40 were targets for dedicated monitoring programs with RXTE throughout 1996 and 1997. We review recent results with particular attention to the X-ray QPOs seen in these sources. We confirm the reappearance of 67 Hz oscillations in GRS1915+105 in several different spectral states. Six intervals, each of 1 to 6 months duration, yield QPO detections with a central frequency of 66.7 +- 1.4 Hz and a FWHM of 3.4 +- 1.0 Hz. Comparing these observing intervals, the average X-ray luminosity varies by a factor of 4 with no significant change in the central QPO frequency. GRS1915+105 has been active in X-rays since 1992, and it shows no signs of evolving toward quiescence. In contrast, GRO J1655-40, which began its most recent X-ray outburst on 1996 April 25, is seen to fade below 20 mCrab on 1997 Aug 17. There are no reappearances of the 300 Hz QPO during the entire second half of the X-ray outburst; during this time the X-ray spectrum is dominated by the soft, thermal component. There are still several models seeking to explain the high frequency QPOs in microquasars. While it is likely that the cause invokes some effect of General Relativity, the correct model and the details of the emission mechanism remain uncertain.

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arXiv1997

Peak Luminosities of Bursts from GRO J1744-28 measured with the RXTE PCA

GRO J1744-28, discovered by BATSE in December 95, is the second neutron star system known to produce frequent accretion powered bursts. The system has been regularly monitored with the RXTE PCA since the peak of the first outburst in January 96 at which time the observed persistent and bursting count rates were ~25,000 ct/sec and ~150,000 ct/sec, with corresponding instrumental deadtimes of ~10% and > 50%. We present a model which allows the reconstruction of the true incident count rate in the presence of enormous deadtime and use the model to derive a history of the peak luminosities and fluences of the bursts as a function of time. During the peak of the January 1996 and January 1997 outbursts, when the persistent emission was > 1 Crab, we infer peak luminosities of ~100 times the Eddington luminosity, and a ratio of persistent emission to integrated burst emission of ~34.

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arXiv1997

Evidence for a Disk-Jet Interaction in the Microquasar GRS 1915+105

We report simultaneous X-ray and infrared (IR) observations of the Galactic microquasar GRS1915+105 using XTE and the Palomar 200-inch telescope on August 13-15, 1997 UTC. During the last two nights, the microquasar GRS 1915+105 exhibited quasi-regular X-ray/infrared (IR) flares with a spacing of $\sim 30$ minutes. While the physical mechanism triggering the flares is currently unknown, the one-to-one correspondence and consistent time offset between the X-ray and IR flares establish a close link between the two. At late times in the flares the X-ray and IR bands appear to ``decouple'', with the X-ray band showing large-amplitude fast oscillations while the IR shows a much smoother, more symmetrical decline. In at least three cases, the IR flare has returned to near its minimum while the X-rays continue in the elevated oscillatory state, ruling out thermal reprocessing of the X-ray flux as the source of IR flare. Furthermore, observations of similar IR and radio flares by Fender et al. (1997) imply that the source of the IR flux in such flares is synchrotron emission. The common rise and subsequent decoupling of the X-ray and IR flux and probable synchrotron origin of the IR emission is consistent with a scenario wherein the IR flux originates in a relativistic plasma which has been ejected from the inner accretion disk. In that case, these simultaneous X-ray/IR flares from a black-hole/relativistic-jet system are the first clear observational evidence linking of the time-dependent interaction of the jet and the inner disk in decades of quasar and microquasar studies.

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arXiv1997

Rxte Observations of a New X-Ray Transient: GRS 1737-31

We present the results from RXTE observations of a new X-ray transient, GRS 1737-31, near the Galactic center during an outburst. Over a span of two weeks, the source was seen to vary significantly on time scales down to a few seconds. The rapid flares in the light curves bear resemblance to those observed for Cygnus X-1, a well-known black hole candidate (BHC). The power-density spectrum is also very typical of BHCs in the HARD (or LOW) state: it is flat below a characteristic break frequency of ~0.03 Hz, and becomes roughly a 1/f power law above. The observed X-ray spectrum can be characterized by a simple power law with a photon index of ~1.7 over a broad energy range 2-200 keV, which is remarkably similar to that of Cyg X-1 in the HARD state. The similarities to Cyg X-1 make GRS 1737-31 a likely BHC. However, unlike most transient BHCs, GRS 1737-31 does not seem to reach a ``soft state'' even during an outburst. We discuss this phenomenon in light of the comparison to a class of hard X--ray BHCs including transients GRO J0422+32, V404Cyg and GRO J1719-24, and persistent sources 1E1740.7-2942 and GRS 1758-258.

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arXiv1996

RXTE observations of GRS 1915+105

The galactic superluminal motion source GRS 1915+105 was observed with RXTE at several occasions during its ongoing active state. The observed X-ray intensity changes drastically on a variety of time scales ranging from sub-seconds to days. In particular, the source exhibits quasi-periodic brightness sputters with varying duration and repetition time scale. These episodes occur occasionally, while the more common X-ray intensity variations are faster with much smaller amplitudes. The spectrum during the brightness sputters is remarkably different from the spectrum of the mean high state emission. We argue that such sputtering episodes are possibly caused by a major accretion disk instability. Based on the coincidence in time of two radio flares following the observed X-ray sputtering episodes we speculate that superluminal ejections (as observed from GRS 1915+105 during earlier activity periods) are related to episodes of large amplitude X-ray variations.

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arXiv1996

X-Ray Observations of GRS 1915+105

A comparison of ROSAT observations with simultaneous BATSE monitoring data shows a different intensity evolution in the soft and hard X-ray band of GRS 1915+105 since 1992. In addition, fits of simultaneous ROSAT/BATSE spectra require more than one spectral component. A short description of results of RXTE pointed observations of GRS 1915+105 since April 1996 is also presented.

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arXiv1996

Discovery of Sub-Millisecond Quasi-Periodic Oscillations in the X-Ray Flux of Scorpius X-1

We report the discovery, with NASA's Rossi X-ray Timing Explorer (RXTE), of the first sub-millisecond oscillation found in a celestial X-ray source. The quasi-periodic oscillations (QPO) come from Sco X-1 and have a frequency of approximately 1100 Hz, amplitudes of 0.6-1.2% (rms) and are relatively coherent, with Q up to ~10^2. The frequency of the QPO increases with accretion rate, rising from 1050 to 1130 Hz when the source moves from top to bottom along the normal branch in the X-ray color-color diagram, and shows a strong, approximately linear correlation with the frequency of the well-known 6-20 Hz normal/flaring branch QPO. We also report the discovery of QPO with a frequency near 800 Hz that occurs, simultaneously with the 1100 Hz QPO, in the upper normal branch. We discuss several possible interpretations, one involving a millisecond X-ray pulsar whose pulses we see reflected off accretion-flow inhomogeneities. Finally, we report the discovery of ~45 Hz QPO, most prominent in the middle of the normal branch, which might be magnetospheric beat-frequency QPO.

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