Frederick D. Seward

Results from an extensive Einstein stellar survey

We report the preliminary results of the Einstein Observatory stellar X-ray survey. To date, 143 soft X-ray sources have been identified with stellar counterparts, leaving no doubt that stars in general constitute a pervasive class of low-luminosity galactic X-ray sources. We have detected stars along the entire main sequence, of all luminosity classes, pre-main sequence stars as well as very evolved stars. Early type OB stars have X-ray luminosities in the range approx.10/sup 31/ to approx.10/sup 34/ ergs s/sup -1/; late type stars show a somewhat lower range of X-ray emission levels, from approx.10/sup 26/ to approx.10/sup 31/ ergs s/sup -1/. Late type main-sequence stars show little dependence of X-ray emission levels upon stellar effective temperature; similarly, the observations suggest weak, if any, dependence of X-ray luminosity upon effective gravity. Instead, the data show a broad range of emission levels (approx.three orders of magnitude) throughout the main sequence later than F0. Comparison of the data with published theories of acoustically heated coronae shows that these models are inadequate to explain our results. The data are consistent with magnetically dominated coronae, as in the solar case.

Pulsars, X-ray synchrotron nebulae, and guest stars

X-ray observations of supernova remnants and radio pulsars are used to derive luminosities of neutron stars and synchrotron nebulae. Observations of known isolated pulsars are used to develop an empirical relationship between the X-ray luminosity and the rate of loss of rotational energy. This is used to derive the characteristics of pulsars hidden in remnants which show evidence for a central compact object or associated nebular emission, but no clear pulsed signal from the neutron star itself. Possible periods and period derivatives for the hidden pulsars are discussed. Some might have periods as long as 0.5 s, and period derivatives considerably higher than that of PSR 1509 - 58, currently the pulsar with the highest known period derivative. 58 references.

Discovery of a 50 millisecond pulsar in the Large Magellanic Cloud

The present investigation is concerned with the discovery of a new pulsed X-ray source in the Large Magellanic Cloud (LMC) supernova remnant 0540 - 693. The SNR 0540 - 693 is one of three suspected Crab-like remnants in the LMC. The existing X-ray, optical, and radio observations of the remnant itself are discussed, and an analysis is conducted of the implications of the period, period derivative, and X-ray pulse shape of the new source. It is concluded that the pulsed X-ray source is almost certainly a young, isolated pulsar. Many of its properties are very similar to those of the Crab pulsar.

Chandra Observations of the Crab-like Supernova Remnant G21.5–0.9

Chandra observations of the Crab-like supernova remnant G21.5-0.9 reveal a compact central core and spectral variations indicative of synchrotron burn-off of higher energy electrons in the inner nebula. The central core is slightly extended, perhaps indicating the presence of an inner wind-shock nebula surrounding the pulsar. No pulsations are observed from the central region, yielding an upper limit of approximately 40% for the pulsed fraction. A faint outer shell may be the first evidence of the expanding ejecta and blast wave formed in the initial explosion, indicating a composite nature for G21.5-0.9.

Nonthermal X-Ray Emission from CTA 1

CTA 1 is a center-filled supernova remnant (SNR) whose morphology and spectrum indicate the presence of a central pulsar, a synchrotron nebula, and a thermal component associated with the expansion of the blast wave into the interstellar medium. The centrally bright emission surrounds the position of a faint point source of X-rays observed with the ROSAT PSPC. Here we report on ASCA observations that confirm the nonthermal nature of the diffuse emission from the central regions of the remnant. We also present evidence for weak thermal emission that appears to increase in strength toward the outer boundary of the SNR. Thus, CTA 1 appears to be an X-ray composite remnant. Both the aftermath of the explosive supernova event and the energetic compact core are observable.

X-Ray Observations of the Compact Source in CTA 1

We have studied the point source RX J0007.0+7302, at the center of supernova remnant CTA 1, using the X-Ray Multi-mirror Mission (XMM-Newton). The X-ray spectrum of the source is consistent with that of a neutron star and is well described by a power law with the addition of a soft thermal component that may correspond to emission from hot polar cap regions or to cooling emission from a light-element atmosphere over the entire star. There is evidence of extended emission on small spatial scales, which may correspond to structure in the underlying synchrotron nebula. No pulsations are observed. Extrapolation of the nonthermal spectrum of RX J0007.0+7302 to gamma-ray energies yields a flux consistent with that of EGRET source 3EG J0010+7309, supporting the proposition that there is a gamma-ray-emitting pulsar at the center of CTA 1. Observations of the outer regions of CTA 1 with ASCA confirm earlier detections of thermal emission from the supernova remnant and show that the synchrotron nebula extends to the outermost reaches of the remnant.

Einstein observations of Vela X and the Vela pulsar

The imaging detectors of the Einstein X-ray observatory have been used to study the Vela pulsar and its surrounding region. Although the pulsar is a relatively strong source of X-rays, there are not detectable pulsations at the pulsar period; the pulsed X-ray luminosity is less than approximately 10 to the 31st ergs/s in the 0.1-4.5 keV band. Nor is there evidence for other types of variability in the pulsars X-ray emission. The images reveal structure on four spatial scales: (1) a pointlike object coincident with the pulsar; (2) a relatively bright, approximately 4-arcmin nebula of diffuse emission about the pulsar; (3) an approximately 1-degree nebula of hard emission that lies between the pulsar and the radio feature Vela X; and (4) thermal emission from the entire approximately 5-degree Vela supernova remnant. Spectral data from the small nebula are well fitted by a synchrotron model. Under the assumption that emission from the pulsar itself is thermal blackbody radiation, the surface temperature of the neutron star is calculated to be 1 million K. 47 references.

A further study of the molecular cloud associated with the supernova remnant G109.1-1.0

The region of the semicircular SNR G109.1-1.0 is studied on the basis of CO observations with a 45-m radio telescope and X-ray data from the archive of the Einstein Observatory. By observing the J = 1-0 transition of CO at 115 GHz, the distribution of the molecular cloud associated with the remnant is investigated in detail. The resolution of the CO mapping observations is 0.6-1.2 pc and the number of the CO spectra obtained is about 2000. The molecular ridge (CO arm), which was known to show an apparent anticorrelation with the curled x-ray jetlike feature of the remnant, is resolved into two CO filaments. The hardness of the X-ray spectrum toward the CO arm is consistent with the column densities of the two CO filaments. However, the overall appearance of the remnant will not be affected very much by the absorption, because the X-ray absorption is found to be a minor effect. 30 refs.

X-RAY-SPECTRUM OF CAPELLA AND ITS RELATION TO CORONAL STRUCTURE AND ULTRAVIOLET EMISSION

The binary system Capella has been observed on 1979 March 15 and on 1980 March 15-17, with the objective grating spectrometer on board the Einstein Observatory. The spectrum measured with the 1000 1/mm grating covers the range 5-30 A with a resolution less than 1 A. The spectra show evidence for a bimodal temperature distribution of emission measure in an optically thin plasma with one component about 5,000,000 K and the other one about 10,000,000 K. Spectral features can be identified with line emissions from O VIII, Fe XVII, Fe XVIII, Fe XXIV, and Ne X ions. Good spectral fits have been obtained assuming standard cosmic abundances. The data are interpreted in terms of emission from hot static coronal loops rather similar to the magnetic arch structures found on the sun. It is shown that the conditions for such a model can exist on Capella. Typical values of loop base pressure and half-length are derived for both temperature components and discussed in relation to UV observations.

Chandra Observations of the W51C Supernova Remnant

We present the results of Chandra ACIS observations of the supernova remnant (SNR) W51C. The remnant is located in the cross section of the Sagittarius arm, and the Chandra images present a superb X-ray view of this complex region including the massive star-forming region W51B. The properties of thermal diffuse SNR emission are consistent with the results of previous studies except that an enhanced abundance of sulfur is required. It is found that the ASCA hard X-ray source CXO J192318.5+1403035, proposed to be a pulsar wind nebula, is composed of a relatively bright core surrounded by an extended (~7.0 × 2.5 pc) diffuse envelope. The core contains a compact (<0.1 pc) source at the center. The X-ray spectrum of CXO J192318.5+1403035 can be fitted by a power-law emission model with Γ = 1.82, and the implied X-ray (0.2-4.0 keV) luminosity is 5 × 1033 ergs s-1. The core contributes ~20% of the total luminosity. The core-envelope structure and its spectral properties support CXO J192318.5+140305 as a pulsar-wind nebula associated with the SNR W51C. There is also enhanced radio emission coincident with the source. It is conceivable that the central compact source might harbor a pulsar. However, the large extent and the hardening of X-rays at the extreme boundaries need to be explained. We have searched for the Brγ line from the ionized gas associated with the source, but no emission was detected.