R. S. Foster

EGRET Observations of the Gamma‐Ray Source 2CG 135+01

The COS B source 2CG 135+01 has been observed by the EGRET instrument on 10 different occasions during the first ~52 months of the Compton Gamma Ray Observatory mission. The source is detected in all but one of the observations. For that one, the exposure was inadequate. The only likely source that is spatially coincident with the gamma-ray position is the radio source GT 0236+610/LS I +61°303. However, there is no compelling evidence for time variations in the gamma-ray emission associated with the radio outbursts from GT 0236+610. Spectral determinations on a timescale of a few days also give no strong evidence for a spectral variation associated with the radio emission of GT 0236+610. Such fluctuations might be expected based on models involving a compact object in an elliptical binary orbit about a massive star. The search for correlations simultaneous with the 8.4 GHz radio outbursts were supported by coordinated observations with the Madrid Deep Space Network during one of the exposures and by Green Bank Interferometer observations on two others. Although there is some possible variability in the gamma-ray flux, it is not clear that it is related to the radio phasing.

Radio and X-Ray Variability of the Galactic Superluminal Source GRS 1915+105

We report results of radio and hard X-ray monitoring observations of the Galactic superluminal X-ray source GRS 1915+105 carried out with the Green Bank Interferometer and the Burst and Transient Source Experiment during the period 1994 September through 1996 March. Both the radio and the hard X-ray light curves show a complex transient behavior. The radio emission monitored at 2.25 and 8.3 GHz is correlated with episodes of enhanced hard X-ray emission. A phenomenological classification of the radio emission indicates two distinct emission modes: plateau and flaring. Plateau radio emission is in general optically thick, with a flat-topped light curve showing a rapid onset and decrease of the flux density. The radio flaring state shows large radio flares that can increase in flux density by 2 orders of magnitude in less than 18 hr, followed by an optically thin exponential decay. These observed large radio flares are consistent with external propagation of plasmoids emitting synchrotron radiation. We comment on the significance of these results, and suggest a scenario for modeling the multiwavelength behavior of GRS 1915+105.

Discovery of Correlated Behavior Between the Hard X-Ray and the Radio Bands in Cygnus X-3

Using Compton Gamma Ray Observatory BATSE hard X-ray (HXR) data and GHz radio monitoring data from the Green Bank Interferometer, we have performed a long-term study (~1800 days) of the unusual X-ray binary Cyg X-3, resulting in the discovery of a remarkable relationship between these two wavelength bands. We find that during quiescent radio states, the radio flux is strongly anticorrelated with the intensity of the HXR emission. The relationship switches to a correlation with the onset of major radio flaring activity. During major radio flaring activity, the HXR drops to a very low intensity during quenching in the radio and recovers during the radio flare. Injection of plasma into the radio jets of Cyg X-3 occurs during changes in the HXR emission and suggests that disk-related and jet-related components are responsible for the high energy emission.

A High Galactic Latitude Pulsar Survey of the Arecibo Sky

The results of a 1355 square degree survey of high galactic latitudes conducted from 1990 to 1995 with the Arecibo radio telescope are presented. The majority of the survey was performed at 430 MHz with a 32 channel 250 kHz lter-bank sampled at 4 kHz. A 7 sensitivity of 1 mJy was obtained for pulsars with periods down to 3 ms. This survey yielded 5 millisecond or recycled pulsars and 14 slow period pulsars. In addition, one millisecond pulsar and 4 slow period pulsar candidates have been identiied, but are as yet unconnrmed. One of the new slow period pulsars appears to be associated with the supernova remnant S147 located towards the galactic anti-center.

A new binary millisecond pulsar

A 4.57 ms pulsar, PSR J1713+0747, in a 67.8 day binary orbit with a low-mass (>0.28 M ○. ) companion has been detected in a systematic search of the sky at high Galactic latitudes with the Arecibo radio telescope. The new pulsar is characterized by a weak magnetic field of 1.9×10 8 G and a spin-down age of 8.9±1.0 Gyr, making it probably one of the oldest neutron stars currently observed. The distance to the pulsar is ∼0.8 kpc as indicated by its dispersion measure. Small timing residuals (∼1 μs), a relatively large flux density at GHz frequencies (S 1400 ∼3 mJy), and a narrow, sharply peaked pulse morphology, make PSR J1713+0747 a new celestial clock of extraordinary precision and a rare target for VLBI frame-tie experiments

Long-Term Flux Monitoring of LSI +61°303 at 2.25 and 8.3 GHz

LSI +61°303 is an exotic binary system consisting of a ~10 M☉ B star and a compact object that is probably a neutron star. The system is associated with the interesting radio source GT 0236+610, which exhibits bright radio outbursts with a period of 26.5 days. We report the results of continuous daily radio interferometric observations of GT 0236+610 at 2.25 and 8.3 GHz from 1994 January to 1996 February. The observations cover 25 complete (and three partial) cycles with multiple observations each day. We detect substantial cycle-to-cycle variability of the radio emission characterized by a rapid onset of the radio flares followed by a more gradual decrease of the emission. We detect a systematic change of the radio spectral index α (defined as Sν ∝ να), which typically becomes larger than zero at the onset of the radio outbursts. This behavior is suggestive of expansion of material initially optically thick to radio frequencies, indicating either that synchrotron or inverse Compton cooling are important or that the free-free optical depth to the source is rapidly changing. After two years of observations, we see only weak evidence for the proposed 4 year periodic modulation in the peak flux of the outbursts. We observe a secular trend in the outburst phases according the best published ephemeris. This trend indicates either orbital period evolution, or a drift in outburst orbital phase in response to some other change in the system.

The Variable Gamma-Ray Source 2CG 135+01

The EGRET instrument on board the Compton Gamma Ray Observatory has detected about 150 sources above 100 MeV. Slightly less than half of these sources have been identified either as active galactic nuclei (AGNs) associated with spectrally flat radio-loud sources of the blazar type or as isolated young/intermediate-age pulsars. Except for rotational modulation, the gamma-ray flux of pulsars does not display significant variation, whereas blazars show variability on timescales as short as ~7 hr. We show here that one of the most prominent unidentified gamma-ray sources near the Galactic plane, 2CG 135+01 (first discovered by the COS B satellite), does not fit into either of these two classes. By comparing different EGRET observations, we establish with 98.8% confidence that the gamma-ray flux from 2CG 135+01 is time variable. This is inconsistent with the constant flux observed from isolated gamma-ray pulsars. No potential spectrally flat radio-loud AGN counterpart exists for this source. We show that 2CG 135+01 belongs to a new class of variable gamma-ray emitters in the Galactic plane, a class that also includes the recently discovered nonblazar gamma-ray transient GRO J1838-04.

Optical Studies of Millisecond Pulsar Companions

In observations of four binary millisecond pulsars with the Palomar 200 inch (5.1 m) telescope, we have detected white dwarf companions to PSR J1640+2224 and PSR J2145-0750 and placed upper limits of mr = 24 and mr = 25 on companions to PSR J0751+1807 and PSR J2229+2643. The temperature inferred for the companion to PSR J1640+2224 is T = 3700 ± 300 K. The luminosity inferred for a star at a distance of 1.2 kpc is consistent with that expected from a (2 ± 1) × 109 yr old helium white dwarf. For PSR J2145-0750, the measured colors are inconsistent, possibly due to a combination of emission lines in the spectrum and systematic effects introduced when subtracting a nearby star. The companion temperature derived lies in the range 4300-6800 K. For a carbon-oxygen white dwarf, the corresponding cooling age is 2-6 × 109 yr. The cooling ages of both systems with detected white dwarf companions are significantly less than the upper limits from the pulsar spin-down ages. For both pulsars, the inferred accretion rate for mass transfer during Roche lobe overflow is 2 orders of magnitude below Eddington accretion rates.

Discovery of a Nonblazar Gamma-Ray Transient Source Near the Galactic Plane: GRO J1838–04

We report the discovery of a remarkable γ-ray transient source near the Galactic plane, GRO J1838-04. This source was serendipitously discovered by EGRET in 1995 June with a peak intensity of ~(4 ± 1) × 10-6 photons cm-2 s-1 (for photon energies larger than 100 MeV) and a 5.9 σ significance. At that time, GRO J1838-04 was the second brightest γ-ray source in the sky. A subsequent EGRET pointing in 1995 late September detected the source at a flux smaller than its peak value by a factor of ~7. We determine that no radio-loud spectrally flat blazar is within the error box of GRO J1838-04. We discuss the origin of the γ-ray transient source and show that interpretations in terms of active galactic nuclei or isolated pulsars are highly problematic. GRO J1838-04 provides strong evidence for the existence of a new class of variable γ-ray sources.

A multiwavelength study of Cygnus X-3

We present a global comparison of long term observations of the hard X-ray (20–100 keV), soft X-ray (1.5–12 keV), infrared (1–2 μm) and radio (2.25, 8.3 and 15 GHz) bands for the unusual X-ray binary Cygnus X-3. Data were obtained in the hard X-ray band from CGRO/BATSE, in the soft X-ray band from RXTE/ASM, in the radio band from the Green Bank Interferometer and Ryle Telescope and in the infrared band from various ground based observatories. Radio flares, quenched radio states and quiescent radio emission can all be associated with changes in the hard and soft X-ray intensity. The injection of plasma into the radio jet is directly related to changes in the hard and soft X-ray emission. The infrared observations are examined in the context of these findings.