arXiv subjects
Publications and source records attributed to Donald C. Backer.
We report here BIMA array observations of linear polarization in Sagittarius A*, the compact radio source in the Galactic Center. These observations had a resolution of 20 arcsec x 5 arcsec oriented North-South. We do not detect linear or circular polarization at a level of 1.8% (1-sigma) at 112 GHz. This puts a new constraint on models that show a sharp change in linear polarization fraction at 100 GHz.
We report here on multi-frequency VLBA observations of three extragalactic sources within 1 degree of the Galactic Center. These sources have been used as astrometric reference sources for VLA and VLBA determinations of the proper motion of Sagittarius A*, the compact nonthermal radio source in the Galactic Center. Each source has a main component with a brightness temperature in excess of 10^{7.5} K, confirming that the sources are active galactic nuclei. The sources have simple structure that can be characterized by one or two Gaussian components. The frequency dependence of the structure indicates that the positions of Sgr A* determined by the VLA astrometry of Backer & Sramek (1999) at 4.8 and 8.4 GHz should have an offset of \sim 2 mas. This offset is in the same direction as the 5 mas shift measured by Backer & Sramek (1999). The structure is unlikely to bias the 43 GHz VLBA results of Reid et al (1999). Motions of components in the calibrator sources could lead to errors in the proper motion of Sgr A* on the order of a few km s^{-1}. All three sources show frequency dependent structure consistent with scattering which is significantly stronger than that of the Galactic scattering model of Taylor & Cordes (1993) but significantly weaker than that of the hyperstrong Galactic Center scattering. Combined with other observations, this suggests the existence of a new component of Galactic scattering located several kpc from the Galactic Center.
We report on X-ray observations of the solitary 4.8 ms pulsar PSR J0030+0451. The pulsar was one of the last targets observed in DEC-98 by the ROSAT PSPC. X-ray pulses are detected on a $4.5σ$ level and make the source the $11^{th}$ millisecond pulsar detected in the X-ray domain. The pulsed fraction is found to be $69\pm18%$. The X-ray pulse profile is characterized by two narrow peaks which match the gross pulse profile observed at 1.4 GHz. Assuming a Crab-like spectrum the X-ray flux is in the range $f_x= 2-3\times 10^{-13}$ erg s$^{-1}$ cm$^{-2} $ ($0.1-2.4$ keV), implying an X-ray efficiency of $L_x/\dot{E}\sim 0.5-5 \times 10^{-3} (d/0.23 {kpc})^2$.
We report the discovery of pulsars J0030+0451, J0711+0931, and J1313+0931 that were found in a search of 470 square degrees at 430 MHz using the 305m Arecibo telescope. The search has an estimated sensitivity for long period, low dispersion measure, low zenith angle, and high Galactic latitude pulsars of ~1 mJy, comparable to previous Arecibo searches. Spin and astrometric parameters for the three pulsars are presented along with polarimetry at 430 MHz. PSR J0030+0451, a nearby pulsar with a period of 4.8 ms, belongs to the less common category of isolated millisecond pulsars. We have measured significant polarization in PSR J0030+0451 over more than 50% of the period, and use these data for a detailed discussion of its magnetospheric geometry. Scintillation observations of PSR J0030+0451 provide an estimate of the plasma turbulence level along the line of sight through the local interstellar medium.
We present a search for linear polarization at 22 GHz, 43 GHz and 86 GHz from the nearest super massive black hole candidate, Sagittarius A*. We find upper limits to the linear polarization of 0.2%, 0.4% and 1%, respectively. These results strongly support the conclusion of our centimeter wavelength spectro-polarimetry that Sgr A* is not depolarized by the interstellar medium but is in fact intrinsically depolarized.
We report here the detection of circular polarization in the Galactic Center black hole candidate, Sagittarius A*. The detection was made at 4.8 GHz and 8.4 GHz with the Very Large Array. We find that the fractional circular polarization at 4.8 GHz is $m_c=-0.36 \pm 0.05%$ and that the spectral index of the circular polarization is $α=-0.6 \pm 0.3$ ($m_c \propto ν^α$). The systematic error in $m_c$ is less than 0.04% at both frequencies. In light of our recent lower limits on the linear polarization in Sgr A*, this detection is difficult to interpret with standard models. We consider briefly whether scattering mechanisms could produce the observed polarization. Detailed modeling of the source and the scattering medium is necessary. We propose a simple model in which low energy electrons reduce linear polarization through Faraday depolarization and convert linear polarization into circular polarization. Circular polarization may represent a significant new parameter for studying the obscured centimeter wavelength radio source in Sgr A*.
Synchrotron radiation from active galactic nuclei (AGN) is often highly polarized. We present a search for linear polarization with the Very Large Array (VLA) at 4.8 GHz and 8.4 GHz from the nearest AGN, Sagittarius A*. As a part of this study we used spectro-polarimetric data that were sensitive to a rotation measure (RM) as large as 3.5 x 10^6 rad m^-2 at 4.8 GHz and 1.5 x 10^7 rad m^-2 at 8.4 GHz. The upper limit to the linear polarization of Sgr A* over a broad range of RM is 0.2% at both frequencies. We also present continuum observations with the VLA at 4.8 GHz which give an upper limit of 0.1% for RMs less than 10^4 rad m^-2. We conclude that depolarization is unlikely to occur in the Galacter Center scattering medium. However, it is possible for depolarization to occur in the accretion region of Sgr A* if the outer scale of turbulence is small enough. We also consider the implications of a very low intrinsic polarization for Sgr A*.
We present here space-based VLBI observations with VSOP and a southern hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 GHz and 5 GHz. The brightness temperature of the core at 1.6 GHz is $5 \times 10^{11}$ K. The size is near the minimum observable value in the direction of NRAO~530 due to interstellar scattering. The 5 GHz data show a single component with a brightness temperature of $\sim 3 \times 10^{12}$ K, significantly in excess of the inverse Compton limit and of the equipartition brightness temperature limit (Readhead 1994). This is strong evidence for relativistic motion in a jet requiring model-dependent Doppler boosting factors in the range 6 to 60. We show that a simple homogeneous sphere probably does not model the emission region accurately. We favor instead an inhomogeneous jet model with a Doppler boosting factor of 15.
We present 7 mm lambda VLBA observations of the compact nonthermal radio source in the Galactic Center, Sgr A*. These observations confirm the hypothesis that the image of Sgr A* is a resolved elliptical Gaussian caused by the scattering of an intervening thermal plasma. The measured major axis of Sgr A* is 0.76 \pm 0.04 mas, consistent with the predicted scattering size of 0.67 \pm 0.03. We find an axial ratio of 0.73 \pm 0.10 and a position angle of 77.0 \pm 7.4 deg. These results are fully consistent with VLBI observations at longer wavelengths and at 3 mm lambda. We find no evidence for any additional compact structure to a limit of 35 mJy. The underlying radio source must be smaller than 4.1 AU for a galactocentric distance of 8.5 kpc. This result is consistent with the conclusion that the radio emission from Sgr A* results from synchrotron or cyclo-synchrotron radiation of gas in the vicinity of a black hole with a mass near 10^6 M_sun.