Engineering topics

Duncan, Robert C.

Publications and source records attributed to Duncan, Robert C..

17 records

NASA NTRS2001

Temporal and Spectral Characteristics of Short Busts from the Soft Gamma Repeaters 1806-20 And 1900+14

We study the temporal and coarse spectral properties of 268 bursts from SGR 1806-20 and 679 bursts from SGR 1900+14, all observed with the Rossi X-Ray Timing Explorer/Proportional Counter Array. Hardness ratios and temporal parameters, such as T(sub 90) durations and tau(sub 90) emission times are determined for these bursts. We find a lognormal distribution of burst durations, ranging over more than two orders of magnitude: T(sub 90) equals approximately 10(exp -2) to > 1 s, with a peak at approximately 0.1 s. The burst light curves tend to be asymmetrical, with more than half of all events showing rise times t(sub r) < 0.3 T(sub 90). We find that there exists a correlation between the duration and fluence of bursts from both sources. We also find a significant anticorrelation between hardness ratio and fluence for SGR 1806-20 bursts and a marginal anticorrelation for SGR 1900+14 events. Finally, we discuss possible physical implications of these results within the framework of the magnetar model.

Goegues, Ersin↗

NASA NTRS2001

Temporal Characteristics of SGR 1806-20 and SGR 1900+14 Bursts

We study the temporal properties of 268 bursts from SGR 1806$-$20 and 679 bursts from SGR 1900+14 all observed with the Rossi X-Ray Timing Explorer/Proportional Counter Array. Temporal parameters, such as T$_{90}$ durations and $\tau$$_{90}$ emission times are implemented. We find a lognormal distribution of burst durations, ranging over more than two orders of magnitude: T$_{90}$ $\sims$ 10$ {-2}$ to $\gtrsim$ 1 s, with a peak $\sim$ 0.1 s. The burst light curves tend to be asymmetrical, with more than half of all events showing rise times t$_{r}$ $<$ 0.2 T$_{90}$. We find that there exists a correlation between the duration and fluence of bursts from both sources. We also find a significant anti-correlation between hardness ratio and fluence of SGR bursts. Finally we discuss possible physical implications of these results within the framework of the magnetar theory.

Gogus, Ersin↗

NASA NTRS2000

Statistical Properties of SGR 1806-20 Bursts

We present statistics of SGR 1806-20 bursts, combining 290 events detected with the Rossi X-Ray Timing Explorer/Proportional Counter Array, 111 events detected with the Burst and Transient Source Experiment, and 134 events detected with the International Cometary, Explorer. We find that the fluence distribution of bursts observed with each instrument are well described by power laws with indices 1.43, 1.76, and 1.67, respectively. The distribution of time intervals between successive bursts from SGR 1806-20 is described by a lognormal function with a peak at 103 s. There is no correlation between the burst intensity and either the waiting times until the next burst or the time elapsed since the previous burst. In all these statistical properties, SGR 1806-20 bursts resemble a self-organized critical system, similar to earthquakes and solar flares. Our results thus support the hypothesis that the energy source for soft gamma repeater bursts is crustquakes due to the evolving, strong magnetic field of the neutron star, rather than any accretion or nuclear power.

Gogus, Ersin↗

NASA NTRS2000

Timing Noise in SGR 1806-20

We have phase connected a sequence of Rossi X-ray Timing Explorer Proportional Counter Array observations of SGR 1806-20 covering 178 days. We find a simple secular spin-down model does not adequately fit the data. The period derivative varies gradually during the observations between 8.1 and 11.7 x 10(exp -11) s/s (approx. 40% larger than the long term trend), while the average burst rate as seen with the Burst and Transient Source Experiment drops throughout the time interval. The phase residuals show no evidence for periodicity, but more closely resemble timing noise as seen ill radio pulsars. The magnitude of the timing noise., however, is large relative to the noise level typically found in radio pulsars (Delta(sub 8) = 4.8; frequency derivative average power approx. = 8 X 10(exp -20) Hz/sq s). Combined with the noise levels measured for some AXPs. we find all magnetar candidates have A8 values larger than those expected from a simple extrapolation of the correlation found in radio pulsars. We find the timing noise ill SGR 1806-20 is greater than or equal to the levels found in some accreting systems (e.g., Vela X-1, 4U 1538-52 and 4U 1626-67). Alternatively, an orbital model with a period P(sub orb) = 733 days provides a statistically acceptable fit to the data. If the phase residuals are created by Doppler shifts from a gravitationally bound companion. then the allowed parameter space for the mass function (small) and orbital separation (large) rule out the possibility of accretion from the companion sufficient to power the persistent emission from the SGR.

Woods, Peter M.↗

NASA NTRS1999

Hard Burst Emission from the Soft Gamma Repeater SGR 1900+14

We present evidence for burst emission from SGR 1900 + 14 with a power-law high-energy spectrum extending beyond 500 keV. Unlike previous detections of high-energy photons during bursts from soft gamma repeaters (SGRs), these emissions are not associated with extraordinarily bright flares. Not only is the emission hard, but the spectra are better fitted by D. Band's gamma-ray burst (GRB) function rather than by the traditional optically thin thermal bremsstrahlung model. We find that the spectral evolution within these hard events obeys a hardness/intensity anticorrelation. Temporally, these events are distinct from typical SGR burst emissions in that they are longer (approximately 1 s) and have relatively smooth profiles. Despite a difference in peak luminosity of approximately > 10(exp 11) between these bursts from SGR 1900 + 14 and cosmological GRBs, there are striking temporal and spectral similarities between the two kinds of bursts, aside from spectral evolution. We outline an interpretation of these events in the context of the magnetar model.

Woods, Peter M.↗

NASA NTRS1999

Variable Spin-Down in the Soft Gamma Repeater SGR 1900+14 and Correlations with Burst Activity

We have analyzed Rossi X-ray Timing Explorer Proportional Counter Array observations of the pulsed emission from SGR 1900+ 14 during 1996 September, 1998 June-October, and early 1999. Using these measurements and results reported elsewhere, we construct a period history of this source for 2.5 yr. We find significant deviations from a steady spin-down trend during quiescence and the burst active interval. Burst and Transient Source Experiment observations of the burst emission are presented and correlations between the burst activity and spin-down rate of SGR 1900+14 are discussed. We find an 80 day interval during the summer of 1998 when the average spin-down rate is larger than the rate elsewhere by a factor approximately 2.3. This enhanced spin-down may be the result of a discontinuous spin-down event or "braking glitch" at the time of the giant flare on 1998 August 27. Furthermore, we find a large discrepancy between the pulsar period and average spin-down rate in X-rays as compared to radio observations for 1998 December and 1999 January.

Woods, Peter M.↗

NASA NTRS1999

Discovery of a Magnetar Associated with the Soft Gamma Repeater SGR 1900 + 14

The soft-gamma repeater (SGR) 1900 + 14 became active again on June 1998 after a long period of quiescence; it remained at a low state of activity until August 1998, when it emitted a series of extraordinarily intense outbursts. We have observed the source with RXTE twice, during the onset of each active episode. We confirm the pulsations at the 5.16 s period reported earlier from SCR 1900 + 14 . Here we report the detection of a secular spindown of the pulse period at an average rate of 1.1 x 10(exp -10) s/s. In view of the strong similarities between SGRs, we attribute the spindown of SGR 1900 + 14 to magnetic dipole radiation, possibly accelerated by a quiescent flux, as in the case of SGR 1806 - 20. This allows an estimate of the pulsar dipolar magnetic field, which is 2 - 8 x 10(exp 14) G. Our results confirm that SGRs are magnetars.

Kouveliotou, Chryssa↗

NASA NTRS1999

Statistical Properties of SGR 1900+14 Bursts

We study the statistics of soft gamma repeater (SGR) bursts, using a data base of 187 events detected with BATSE and 837 events detected with RXTE PCA, all from SGR 1900+14 during its 1998-1999 active phase. we find that the fluence or energy distribution of bursts is consistent with a power law of index 1.66, over 4 orders of magnitude. This scale-free distribution resembles the Gutenberg-Richter Law for earthquakes, and gives evidence for self-organized criticality in SGRS. The distribution of time intervals between successive bursts from SGR 1900+14 is consistent with a log-normal distribution. There is no correlation between burst intensity and the waiting times till the next burst, but there is some evidence for a correlation between burst intensity and the time elapsed since the previous burst. We also find a correlation between the duration and the energy of the bursts, but with significant scatter. In all these statistical properties, SGR bursts resemble earthquakes and solar flares more closely than they resemble any known accretion-powered or nuclear-powered phenomena. Thus our analysis lends support to the hypothesis that the energy source for SGR bursts is internal to the neutron star, and plausibly magnetic.

Gogus, Ersin↗

NASA NTRS1999

Hard Burst Emission from the Soft Gamma Repeater SGR 1900+14

We present evidence for burst emission from SGR 1900+14 with a power-law high energy spectrum extending beyond 500 kev. Unlike previous detections of high energy photons during bursts from SGRS, these emissions are not associated with high-luminosity burst intervals. Not only is the emission hard, but the spectra are better fit by Band's GRB function rather than by the traditional optically-thin thermal bremsstrahlung model. We find that the spectral evolution within these hard events obeys a hardness/intensity anti-correlation. Temporally, these events are distinct from typical SGR burst emissions in that they are longer (about 1 s) and have relatively smooth profiles. Despite a difference in peak luminosity of > 1E+ll between these bursts from SGR 1900+14 and cosmological GRBS, there are striking temporal and spectral similarities between the two kinds of bursts, aside from spectral evolution. We outline an interpretation of these events in the context of the magnetar model.

Duncan, Robert D.↗

NASA NTRS1999

On the Variable Spin-Down of SGR 1900+14

We consider the physical implications of the rapid spindown of Soft Gamma Repeater 1900+14 reported in a companion paper by Woods et al. During an 80 day interval between June 1998 and the large outburst on August 27, 1998, the mean spin-down rate increased by a factor 2.3 resulting in a positive period offset of delta P/P = 1 X 10(exp -4)/ A radiation hydrodynamical outflow associated with the August 27 event could impart the required torque, but only if the dipole magnetic field is stronger than about 10(exp 14) G and the outflow lasts longer than the observed X-ray flare. A positive period increment is also a natural consequence of a gradual plastic deformation of the neutron star crust by an intense magnetic field. Finally, we discuss the relative stability of the long term spin-down rate of SGR 1900+14, and compare the relative spindown rates of the SGRs and the Anomalous X-ray Pulsars.

Thompson, Christopher↗

NASA NTRS1999

Variable Spin-Down in the Soft Gamma Repeater SGR 1900+14 and Correlations with Burst Activity

We have analyzed Rossi X-ray Timing Explorer Proportional Counter Array observation of the pulsed emission from SGR 1900+14 during September 1996, June-October 1998, and early 1999. Using these measurements and results reported elsewhere. we construct a period history of this source for 2.5 years. We find significant deviations from a steady spin-down trend during quiescence and the burst active interval. Burst And Transient Source Experiment (BATSE) observations of the burst emission are presented and correlations between the burst activity and spin-down rate of SGR 1900 + 14 are discussed. We find an 80 day interval during the summer of 1998 when the average spin-down rate is larger than the rate elsewhere by a factor of about 2.3. This enhanced spin-down may, be the result of a discontinuous spin-down event or "braking glitch" at the time of the giant flare on 27 August 1998. Furthermore find a large discrepancy between the pulsar period and average spin-down rate in X-rays as compared to radio observation for December 1998 and January 1999.

Woods, Peter M.↗

NASA NTRS1993

Neutron star dynamos and the origins of pulsar magnetism

Neutron star convection is a transient phenomenon and has an extremely high magnetic Reynolds number. In this sense, a neutron star dynamo is the quintessential fast dynamo. The convective motions are only mildly turbulent on scales larger than the approximately 100 cm neutrino mean free path, but the turbulence is well developed on smaller scales. Several fundamental issues in the theory of fast dynamos are raised in the study of a neutron star dynamo, in particular the possibility of dynamo action in mirror-symmetric turbulence. It is argued that in any high magnetic Reynolds number dynamo, most of the magnetic energy becomes concentrated in thin flux ropes when the field pressure exceeds the turbulent pressure at the smallest scale of turbulence. In addition, the possibilities for dynamo action during the various (pre-collapse) stages of convective motion that occur in the evolution of a massive star are examined, and the properties of white dwarf and neutron star progenitors are contrasted.

Thompson, Christopher↗

NASA NTRS1992

Formation of very strongly magnetized neutron stars - Implications for gamma-ray bursts

It is proposed that the main observational signature of magnetars, high-field neutron stars, is gamma-ray bursts powered by their vast reservoirs of magnetic energy. If they acquire large recoils, most magnetars are unbound from the Galaxy or reside in an extended, weakly bound Galactic corona. There is evidence that the soft gamma repeaters are young magnetars. It is argued that a convective dynamo can also generate a very strong dipole field after the merger of a neutron star binary, but only if the merged star survives for as long as about 10-100 ms. Several mechanisms which could impart a large recoil to these stars at birth, sufficient to escape from the Galactic disk, are discussed.

Duncan, Robert C.↗

NASA NTRS1991

Expansion-cooled Lyman-alpha clouds

It is shown that recent observations by Pettini et al. (1990) which indicate that low-N H I Ly-alpha forest lines have small velocity widths and that the velocity widths are positively correlated with N (H I) can be understood as the result of adiabatic cooling of expanding clouds. It is argued that expansion cooling can efficiently lower temperatures and velocity widths of diffuse ionized clouds, and that this trend of diminishing temperature and velocity width in a wide range of plausible cloud models is consistent with double-quasar data. Expansion can provide a natural explanation for the steep z-evolution of the cloud numbers.

Duncan, Robert C.↗

NASA NTRS1989

Voids in the Ly-alpha forest

The general issue of using the Ly-alpha absorbers in quasar spectra as tracers of the large-scale matter distribution is discussed, emphasizing complications due to evolutionary effects and line blending. Then, a novel method of statistical analysis of the line data appropriate for studying large-scale structure is described. This method circumvents the problem of line blending by truncating the interval distribution at the low end. It also explicitly takes z-dependent effects into account. As an example, the method is applied to an 18-quasar spectrum sample of 417 lines.

Duncan, Robert C.↗

NASA NTRS1988

Quasar ionization of Lyman-alpha clouds - The proximity effect, a probe of the ultraviolet background at high redshift

The distribution of lines in the Ly-alpha forests in quasar spectra is examined using spectral data from 19 quasars with emission lines redshifts z sub Q ranging from 1.7 to 3.8. The number density of Ly-alpha lines generally increases with resdhift z, but there exists a countervailing trend of diminishing number density within individual quasar spectra as z tends to z sub Q. Evidence is presented that this countervailing trend is due to enhanced ionization of Ly-alpha clouds by the bright nearby quasars in whose spectra they are observed. It is suggested that this proximity effect should be used as a powerful tool to measure locally the ionizing flux emitted by high-redshift objects.

Bajtlik, Stanislaw↗

NASA NTRS1988

Clustering and voids in the Lyman-alpha forest

In a statistical study of quasar absorption lines, no evidence is found for large voids in the Ly-alpha forest, contrary to some recent suggestions. The 28 Mpc gap found by Crotts (1987) in the spectrum of Q0420-388 is not statistically significant, and it is possible, from a sample of 18 quasar spectra, to reject at the 2 sigma level the hypothesis that comoving voids of 30 Mpc fill as much as 15 percent of the universe. However, a significant (at the 99 percent level) excess of small line intervals of velocity width 200-600 km/s is found. This provides evidence either for the gravitationally induced correlations proposed in some models or for the fragmented shells proposed in others.

Ostriker, Jeremiah P.↗