Engineering topics

Mattox, J. R.

Publications and source records attributed to Mattox, J. R..

At least 19 records

NASA NTRS2001

Timing the Geminga Pulsar with EGRET Data

The pulsation of Geminga has been detected to date only at high energies (E greater than 0.1 keV). Since X-ray exposures are short and Geminga is at best only marginally detected in gamma-rays at E less than 30 MeV, the primary means of timing Geminga is with high-energy gamma-rays. The EGRET observations of Geminga now span 4 years. These data are analyzed to determine the 1995 ephemeris for Geminga which is provided here. We continue to count every revolution of Geminga during the GRO mission with a rotational phase resolution which improves with additional exposure. Proper motion is now apparent in the gamma-ray timing, consistent with the optical measurement of Bignami et al. With improved statistics, two addition peaks are tentatively detected in the "minor bridge" region. More exposure is required to confirm them. If found to be real, they are difficult to understand with polar cap models, but are expected for the outer gap model, and provide sorely needed constraints.

Mattox, J. R.↗

NASA NTRS2001

Timing the Geminga Pulsar with Gamma-Ray Observations

We present the COS-B/EGRET 1997 ephemeris for the rotation of the Geminga pulsar. This ephemeris is derived from high-energy gamma-ray observations that span 24 yr. The recently obtained accurate position and proper motion are assumed. A cubic ephemeris predicts the rotational phase of Geminga with errors smaller than 50 milliperiods for all existing high-energy gamma-ray observations that span a 24.2 yr timing baseline. The braking index obtained is 17 +/- 1. Further observation is required to ascertain whether this high value truly reflects the rotational energy loss mechanism, or whether it is a manifestation of timing noise. The ephemeris parameters are sufficiently constrained so that timing noise will be the limitation on forward extrapolation. If Geminga continues to rotate without a glitch, as it has for at least 23 yr, we expect this ephemeris to continue to describe the phase, with an error of less than 100 milliperiods, until 2008. Statistically significant timing residuals are detected in the EGRET data that depart from the cubic ephemeris at a level of 30 milliperiods. Although this could simply be an additional manifestation of timing noise, the EGRET timing residuals appear to have a sinusoidal modulation that is consistent with a planet of mass 1.7/sin i solar mass, orbiting Geminga at a radius of 3.3 AU.

Mattox, J. R.↗

NASA NTRS2000

High-Frequency Observations of Blazars

We report on the results of high-frequency VLBA observations of 42 gamma-ray bright blazars monitored at 22 and 43 GHz between 1993.9 and 1997.6. In 1997 the observations included polarization-sensitive imaging. The cores of gamma-ray blazars are only weakly polarized, with EVPAs (electric-vector position angles) usually within 40 deg of the local direction of the jet. The EVPAs of the jet components are usually within 20 deg of the local jet direction. The apparent speeds of the gamma-ray bright blazars are considerably faster than in the general population of bright compact radio sources. Two X-ray flares (observed with RXTE) of the quasar PKS 1510-089 appear to be related to radio flares, but with the radio leading the X-ray variations by about 2 weeks. This can be explained either by synchrotron self-Compton emission in a component whose variations are limited by light travel time or by the Mirror Compton model.

Marscher, A. P.↗

NASA NTRS2000

High-Frequency Observations of Blazars

We report on the results of high-frequency VLBA observations of 42 gamma ray bright blazars monitored at 22 and 43 GHz between 1993.9 and 1997-6. In 1997 the observations included polarization-sensitive imaging. The cores of gamma ray blazars are only weakly polarized, with EVPAs (electric-vector position angles) usually within 40 degrees of the local direction of the jet. The EVPAs of the jet components are usually within 20 degrees of the local jet direction. The apparent speeds of the gamma ray bright blazars are considerably faster than in the general population of bright compact radio sources. Two X-ray flares (observed with RXTE) of the quasar PKS 1510-089 appear to be related to radio flares, but with the radio leading the X-ray variations by about 2 weeks. This can be explained either by synchrotron self-Compton emission in a component whose variations are limited by light travel time or by the Mirror Compton model.

Marscher, A. P.↗

NASA NTRS1995

Egret detection of the blazar PKS 0420-014

During its full-sky survey, the Energetic Gamma Ray Experiment Telescope (EGRET) high-energy instrument aboard the Compton Gamma-Ray Observatory detected gamma-ray emission in the energy range above 30 MeV from a source identified as the blazar PKS 0420-014. This object was observed during two separate viewing periods in 1992 February/March and May/June. An intensity decrease above 100 MeV of a factor of at least 1.5 from a maximum of (5.0 +/- 1.4) x 10(exp -7) photons 1 sq cm/s was observed during that time interval indicating extensive variability. The photon spectrum in the range between 30 and 10,000 MeV at the time of the maximum intensity is well represented by a power law with an exponent of -1.9 +/- 0.3. Some similarities with other EGRET detected blazars are briefly discussed.

Radecke, H.-D.↗

NASA NTRS1995

Timing the Geminga pulsar with EGRET data

The pulsation of Geminga has been detected to date only at high energies (E greater than 0.1 keV). Since x-ray exposures are short and Geminga is at best only marginally detected in gamma-rays at E less than 30 MeV, the primary means of timing Geminga is with high-energy gamma-rays. The EGRET observations of Geminga now span 4 years. These data are analyzed to determine the 1995 ephemeris for Geminga which is provided here. We continue to count every revolution of Geminga during the GRO mission with a rotational phase resolution which improves with additional exposure. Proper motion is now apparent in gamma-ray timing, consistent with the optical measurement of Bignami et al. With improved statistics, two additional peaks are tentatively detected in the 'minor bridge' region. More exposure is required to confirm them. If found to be real, they are difficult to understand with polar cap models, but are expected for the outer gap model, and provide sorely needed constraints.

Mattox, J. R.↗

NASA NTRS1994

Search of the energetic gamma-ray experiment telescope (EGRET) data for high-energy gamma-ray microsecond bursts

Hawking (1974) and Page & Hawking (1976) investigated theoretically the possibility of detecting high-energy gamma rays produced by the quantum-mechanical decay of a small black hole created in the early universe. They concluded that, at the very end of the life of the small black hole, it would radiate a burst of gamma rays peaked near 250 MeV with a total energy of about 10(exp 34) ergs in the order of a microsecond or less. The characteristics of a black hole are determined by laws of physics beyond the range of current particle accelerators; hence, the search for these short bursts of high-energy gamma rays provides at least the possibility of being the first test of this region of physics. The Compton Observatory Energetic Gamma-Ray Experiment Telescope (EGRET) has the capability of detecting directly the gamma rays from such bursts at a much fainter level than SAS 2, and a search of the EGRET data has led to an upper limit of 5 x 10(exp -2) black hole decays per cu pc per yr, placing constraints on this and other theories predicting microsecond high-energy gamma-ray bursts.

Fichtel, C. E.↗

NASA NTRS1993

Results from the energetic gamma-ray experiment telescope (EGRET) on the Compton Observatory

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) covers the high energy gamma ray energy range, approximately 30 MeV to 30 GeV, with a sensitivity considerably greater than earlier high energy gamma-ray satellites. Thus far, 4 pulsars have been detected and their properties measured, including in 3 cases the energy spectrum as a function of phase. The details of the galactic plane are being mapped and a spectra of the center region has been obtained in good agreement with that expected from cosmic ray interactions. The Magellanic clouds have been examined with the Large Magellanic Cloud (LMC) having been detected at a level consistent with it having a cosmic ray density compatible with quasi-stable equilibrium. Sixteen Active Galactic Nuclei (AGN's) have been seen thus far with a high degree of certainty including 12 quasars and 4 BL Lac objects, but no Seyferts. Time variation has been detected in some of these AGN's

Fichtel, C. E.↗

NASA NTRS1993

EGRET observations of active galactic nuclei - 0836 + 710, 0454 - 234, 0804 + 499, 0906 + 430, 1510-089, and 2356 + 196

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory observed high-energy gamma rays (50 - 2000 MeV) from quasar 0836 + 710 (z = 2.16) during observations in 1992 January, near the time of an optical fare (von Linde et al., 1993). The gamma-ray spectrum can be fitted with a power law with photon number index 2.4 +/- 0.2. EGRET identifies quasars 0454 - 234, 0804 + 499, 0906 + 430, 1510 - 089, and 2356 + 196 at a statistical significance of between 4 and 5 standard deviations.

Thompson, D. J.↗

NASA NTRS1993

Observation of high-energy gamma rays from the quasi-stellar object CTA 102

The quasar CTA 102 (QSO 2230 + 114) was observed four times in 1991-1992 by the EGRET high-energy gamma-ray telescope on the Compton GRO satellite. In the 1992 January 23-February 6 observation, emission was detected at the level (2.4 +/- 0.5) x 10 exp 7 photons/sq cm s (E is greater than 100 MeV). The other observations produced upper limits or detections with lower significance which are consistent with the same flux. The photon spectrum can be represented by a power law with a number index of 2.6 +/- 0.2, the softest so far observed by EGRET. The emitted gamma-ray luminosity, if isotropic, is 5 x 10 exp 47 ergs/s (H(0) = 75 km/s Mpc , q(0) = 0.5), although there are good reasons to believe that the gamma emission is strongly beamed.

Nolan, P. L.↗

NASA NTRS1993

Time variability in the gamma-ray emission of 3C 279

Following the initial detection of 3C 279 as an intense high-energy gamma-ray emitter during a 1991, June 16-28 observation, the EGRET instrument on the Compton Gamma Ray Observatory made a second observation during 1991, October 3-17. Low-sensitivity observations were also made during 1991, October 12-31 and 1992 April. The relatively intense June fluxes allowed the time structure to be determined down to the level of half-days. During this period a flux enhancement of about a factor of 4 is seen, lasting several days. The October exposures show fluxes somewhat below the minimum June flux, with no significant evidence of time variability. The 1992 April fluxes are well off the main instrument axis, and so the statistics are less compelling, but the evidence appears to show additional time variability. The observations are consistent with a model in which the gamma rays result from Compton scattering of low-energy photons by relativistic electrons in a jet.

Kniffen, D. A.↗

NASA NTRS1993

The EGRET detection of quasar 1633 + 382

In the course of a full-sky survey, the EGRET instrument aboard the Compton Observatory detected an intense, high Galactic latitude source, which is identified as the OVV (optically violent variable) quasar 1633 + 382. The spectrum is represented by a power law with an energy spectral index of 0.9 between 30 MeV and 30 GeV. The gamma-ray power per decade is about 100 times larger than typical values at any longer wavelength. Significant variation of the gamma-ray flux density on a time scale as short as two days was observed, limiting the size of the region in which the gamma-rays are produced. Assuming the X-rays detected by the Einstein Observatory from this quasar are produced in the same region as the gamma-rays, and a similar X-ray flux density was extant at the time of the EGRET observation, the emission must be beamed to avoid a pair-production optical depth of 10 exp 4 which would exist otherwise. If beaming arises from bulk relativistic motion, the minimum Doppler factor required is 7.6.

Mattox, J. R.↗

NASA NTRS1993

Gamma radiation from blazar PKS 0537-441

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) observed high-energy gamma rays from PKS 0537-441 during observations in 1991 July-August. Upper limits from later EGRET observations suggest time variability.

Thompson, D. J.↗

NASA NTRS1993

Detection of high-energy gamma rays from quasar PKS 0528 + 134 by EGRET on the Compton Gamma Ray Observatory

The first several pointing directions of the Compton Gamma Ray Observatory, launched on 1991 April 5, were toward the Galactic anticenter. In addition to the known gamma-ray sources, Crab and Geminga, high-energy gamma-ray emission was observed from the quasar PKS 0528 + 134 by the Energetic Gamma Ray Experiment Telescope (EGRET). A redshift measurement, reported here, of 2.07 confirms the identification of this object as a quasar. The differential photon spectrum is well represented by a power law with an exponent of 2.56 +/- 0.09 and a photon intensity above 100 MeV of (8.4 +/- 1.0) x 10 exp -7 photons sq cm/s. There is evidence for time variability on a time scale of a few days.

Hunter, S. D.↗

NASA NTRS1993

EGRET detection of high-energy gamma radiation from the OVV quasar 3C 454.3

High-energy gamma radiation has been observed from the optically violent variable quasar 3C 454.3 (PKS 2251 + 158) by the Energetic Gamma-Ray Experiment Telescope on the Compton Observatory. During the 1992 January-February observation, the emission showed a power-law photon spectrum with an exponent of -2.18 +/- 0.08. The flux density (greater than 100 MeV) was observed to vary within the range (0.4-1.4) x 10 exp -6 photon/sq cm s on a time scale of less than a week. Lower sensitivity observations during 1992 April and May also detected emission within that range, but with lower statistical significance. An earlier low-sensitivity exposure during 1991 August gave a 95 percent confidence upper limit of 0.26 x 10 exp 6 photon/sq cm s. The similarity of the gamma-ray emission of 3C 454.3 to that of 3C 279 parallels the similarity of these two objects at lower frequencies.

Hartman, R. C.↗

NASA NTRS1993

Detection of gamma-ray emission from the quasar PKS 0208-512

High-energy gamma-ray emission has been detected from the quasar PKS 0208-512 in the energy range above 30 MeV by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory. This region of sky was observed in five different viewing periods, and evidence of time variability in the gamma-ray emission by more than a factor of 3 was found. At the maximum intensity between 1991 September 19 and October 3, the flux density above 100 MeV was (9.1 +/- 0.4) x 10 exp -7 gamma/sq cm per sec. The photon spectrum during this period may be expressed as a power law with an exponent of - 1.69 +/- 0.05 between 30 MeV and 4 GeV. This is the hardest quasar spectrum observed by EGRET up to the present time.

Bertsch, D. L.↗

NASA NTRS1993

Constraints on the cosmic rays in the Small Magellanic Cloud

Observations of the SMC using the Energetic Gamma Ray Experimental Telescope (EGRET) on the Compton Observatory are reported. The findings yield an upper limit for gamma-ray emission above 100 MeV of 0.5 x 10 exp -7 photon/sq cm s. The expected flux if the cosmic rays (CR) are universal rather than Galactic in origin is (2.4 +/- 0.5) x 10 exp -7 photon/sq cm s, only a third of which arises from cosmic ray electron interactions. Thus, the bulk of the CR energy density is almost certainly neither metagalactic nor universal, but Galactic in origin. The results add to the evidence that the SMC is in a nonequilibrium state and indicate that the LMC is most likely in quasi-stable equilibrium, with a CR energy density near the maximum that can be contained.

Sreekumar, P.↗

NASA NTRS1993

Overview of the first results from EGRET

An overview of the early results of EGRET, one of four instruments carried on the Compton Gamma Ray Observatory, is presented with regard to observations of pulsars, bursts, galactic diffuse radiation, and active galaxies. The June 11, 1992 solar flare was observed in gamma rays with energies from the threshold of detection to energies over 1 GeV. Detailed time histories and energy spectra were obtained for the Crab and Vela pulsars, with one of the Vela pulses being quite sharp, approximately 2 percent of the total period. Several gamma-ray bursts were recorded. The one on May 3, 1991 was seen in EGRET's spark chamber system, which made it possible to determine a position. The quasars 3C 279, which has a redshift of 0.538, was seen in high energy gamma rays in both June and October 1992.

Fichtel, C. E.↗