arXiv subjects

Geoffrey C. Bower

Publications and source records attributed to Geoffrey C. Bower.

At least 145 records · Page 8Linked to original sources

arXiv2004

Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow

We report the discovery of variability in the linear polarization from the Galactic Center black hole source, Sagittarius A*. New polarimetry obtained with the Berkeley-Illinois-Maryland Association array at a wavelength of 1.3 mm shows a position angle that differs by 28 +/- 5 degrees from observations 6 months prior and then remains stable for 15 months. This difference may be due to a change in the source emission region on a scale of 10 Schwarzschild radii or due to a change of 3 x 10^5 rad m^-2 in the rotation measure. We consider a change in the source physics unlikely, however, since we see no corresponding change in the total intensity or polarized intensity fraction. On the other hand, turbulence in the accretion region at a radius ~ 10 to 1000 R_s could readily account for the magnitude and time scale of the position angle change.

astro-ph↗

arXiv2004

Detection of 21 Centimeter HI Absorption at z = 0.78 in a Survey of Radio Continuum Sources

We report the detection of a deep broad HI 21 cm absorption system at z = 0.78 toward the radio source [HB89] 2351+456 (4C+45.51) at z = 1.992. The HI absorption was identified in a blind spectral line survey conducted at the Green Bank Telescope spanning 0.63 < z < 1.10 toward a large sample of radio continuum sources. The HI column density is N(HI) = 2.35 x 10^19 (T_s/f) cm^-2, where T_s is the spin temperature and f is the continuum covering factor of the absorbing gas. For T_s/f > 8.5 K, this system is by definition a damped Ly alpha absorption system (N(HI) >= 2 x 10^20 cm^-2). The line is unusually broad, with a FWHM of 53 km/s and a full span of 163 km/s, suggesting a physically extended HI gas structure. Radio surveys identify damped Ly alpha systems in a manner that bypasses many of the selection effects present in optical/UV surveys, including dust extinction and the atmospheric cutoff for z < 1.65. The smooth broad profile of this HI 21 cm absorption system is similar to the z = 0.89 HI absorption toward PKS 1830-211, which suggests that the absorber toward [HB89] 2351+456 is also a gravitational lens and a molecular absorption system. However, very long baseline interferometry and Hubble Space Telescope observations show little evidence for gravitational lensing, and BIMA millimeter observations show no HCO+ (1-2) or HCN (1-2) absorption down to tau = 0.15 (3 sigma) in 5 km/s channels. Although this radio damped Ly alpha selection technique would include dusty, molecule-rich systems, [HB89] 2351+456 appears to be a ``vanilla'' HI 21 cm absorber.

astro-ph↗

arXiv2004

Detection of the Intrinsic Size of Sagittarius A* through Closure Amplitude Imaging (includes Online Supporting Material)

We have detected the intrinsic size of Sagittarius A*, the Galactic Center radio source associated with a supermassive black hole, showing that the short-wavelength radio emission arises from very near the event horizon of the black hole. Radio observations with the Very Long Baseline Array show that the source has a size of $24 \pm 2$ Schwarzschild radii at 7 mm wavelength. In one of eight 7-mm epochs we also detect an increase in the intrinsic size of $60^{+25}_{-17}$%. These observations place a lower limit to the mass density of Sgr A* of $1.4\times10^{4}$ solar masses per cubic astronomical unit.

astro-ph↗

arXiv2004

Modeling the Counts of Faint Radio Loud Quasars: Constraints on the Supermassive Black Hole Population and Predictions for High Redshift

We use a physically motivated semi-analytic model, based on the mass function of dark matter halos, to predict the number of radio-loud quasars as a function of redshift and luminosity. Simple models in which the central BH mass scales with the velocity dispersion of its host halo as M(bh) sigma(halo)^5 have been previously found to be consistent with a number of observations, including the optical and X-ray quasar luminosity functions. We find that similar models, when augmented with an empirical prescription for radio emission, overpredict the number of faint (10 micro-Jy) radio sources by 1-2 orders of magnitude. This translates into a more stringent constraint on the low-mass end of the quasar black hole mass function than is available from the Hubble and Chandra Deep Fields. We interpret this discrepancy as evidence that black holes with masses below 10^7 Msun are either rare or are not as radio-loud as their more massive counterparts. Models that exclude BHs with masses below 10^7 Msun are in agreement with the deepest existing radio observations, but still produce a significant tail of high-redshift objects. In the 1-10GHz bands, at the sensitivity of 10 micro-Jy, we find surface densities of 100, 10, and 0.3 deg^-2 for sources located at z>6, 10, and 15, respectively. The discovery of these sources with instruments such as the Allen Telescope Array (ATA), Extended Very Large Array (EVLA), and the Square Kilometer Array (SKA) would open a new window for the study of supermassive BHs at high redshift. We also find surface densities of 0.1 deg^-2 at z > 6 for mJy sources that can be used to study 21 cm absorption from the epoch of reionization. We suggest that, although not yet optically identified, the FIRST survey may have already detected 10^3-10^4 such sources.

astro-ph↗

arXiv2003

A Giant Outburst at Millimeter Wavelengths in the Orion Nebula

BIMA observations of the Orion nebula discovered a giant flare from a young star previously undetected at millimeter wavelengths. The star briefly became the brightest compact object in the nebula at 86 GHz. Its flux density increased by more than a factor of 5 on a timescale of hours, to a peak of 160 mJy. This is one of the most luminous stellar radio flares ever observed. Remarkably, the Chandra X-ray observatory was in the midst of a deep integration of the Orion nebula at the time of the BIMA discovery; the source's X-ray flux increased by a factor of 10 approximately 2 days before the radio detection. Follow-up radio observations with the VLA and BIMA showed that the source decayed on a timescale of days, then flared again several times over the next 70 days, although never as brightly as during the discovery. Circular polarization was detected at 15, 22, and 43 GHz, indicating that the emission mechanism was cyclotron. VLBA observations 9 days after the initial flare yield a brightness temperature Tb > 5 x 10^7 K at 15 GHz. Infrared spectroscopy indicates the source is a K5V star with faint Br gamma emission, suggesting that it is a weak-line T Tauri object. Zeeman splitting measurements in the infrared spectrum find B ~ 2.6 +/- 1.0 kG. The flare is an extreme example of magnetic activity associated with a young stellar object. These data suggest that short observations obtained with ALMA will uncover hundreds of flaring young stellar objects in the Orion region.

astro-ph↗

arXiv2003

Linear and Circular Polarization from Sagittarius A* and M81*

We describe recent results for Sgr A*, M81* and other low luminosity active galactic nuclei. We have conducted linear and circular polarimetry over a frequency range of 1.4 to 230 GHz and detected a variety of phenomena. The polarization properties of the studied sources are substantially different from higher powered AGN. In the case of Sgr A*, we are able to eliminate ADAFs and Bondi-Hoyle flows as possible models based on mm $λ$ polarimetry. Our success with Sgr A* demonstrates that we can learn about the nature of accretion and outflow in these sources with unprecedented detail. We may also develop probes of general relativity in the strong-field limit.

astro-ph↗

arXiv2003

Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz

We measured the linear polarization of Sagittarius A* to be 7.2 +/- 0.6 % at 230 GHzusing the BIMA array with a resolution of 3.6 x 0.9 arcsec. This confirms the previously reported detection with the JCMT 14-m antenna. Our high resolution observations demonstrate that the polarization does not arise from dust but from a synchrotron source associated with Sgr A*. We see no change in the polarization position angle and only a small change in the polarization fraction in four observations distributed over 60 days. We find a position angle 139 +/- 4 degrees that differs substantially from what was found in earlier JCMT observations at the same frequency. Polarized dust emission cannot account for this discrepancy leaving variability and observational error as the only explanations. The BIMA observations alone place an upper limit on the magnitude of the rotation measure of 2 x 10^6 rad m^-2. These new observations when combined with the JCMT observations at 150, 375 and 400 GHz suggest RM =-4.3 +/- 0.1 x 10^5 rad m^-2. This RM may be caused by an external Faraday screen. Barring a special geometry or a high number of field reversals, this RM rules out accretion rates greater than ~ 10^-7 M_sun y^-1. This measurement is inconsistent with high accretion rates necessary in standard advection dominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for low accretion rates as a major factor in the overall faintness of Sgr A*.

astro-ph↗

arXiv2002

A Radio Survey for Linear and Circular Polarization in Low Luminosity Active Galactic Nuclei

We conducted a Very Large Array survey of eleven low luminosity active galactic nuclei for linear and circular polarization at 8.4 GHz. We detected circular polarization in one source (M81*) and linear polarization in 3 sources. Sensitivity limits were ~0.1% for both modes of polarization in 9 of 11 sources. The detections confirm the importance of nonthermal emission in LLAGN. However, detection rates for circular and linear polarization are lower for these sources than for more powerful AGN. Fractional linear polarization in detected sources is also lower than in more powerful AGN. The weak linear polarization in the survey sources indicates their overall similarity to Sgr A*. Confusion with thermal sources, depolarization and weaker, less extended jets may contribute to these differences. We detect a rotation measure >~ 7 x 10^4 rad m^-2 for NGC 4579. This may arise from magnetized plasma in the accretion, outflow or interstellar regions. Inverted spectra are present in both M81* and Sagittarius A* and absent from all sources in which circular polarization is not detected. This suggests that optical depth effects are important in the creation of circular polarization.

astro-ph↗

arXiv2002

The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz

We report here multi-epoch, multi-frequency observations of the circular polarization in Sagittarius A*, the compact radio source in the Galactic Center. Data taken from the VLA archive indicate that the fractional circular polarization at 4.8 GHz was -0.31% with an rms scatter of 0.13% from 1981 to 1998, in spite of a factor of 2 change in the total intensity. The sign remained negative over the entire time range, indicating a stable magnetic field polarity. In the Summer of 1999 we obtained 13 epochs of VLA A-array observations at 1.4, 4.8, 8.4 and 15 GHz. In May, September and October of 1999 we obtained 11 epochs of Australia Telescope Compact Array observations at 4.8 and 8.5 GHz. In all three of the data sets, we find no evidence for linear polarization greater than 0.1% in spite of strong circular polarization detections. Both VLA and ATCA data sets support three conclusions regarding the fractional circular polarization: the average spectrum is inverted with a spectral index ~0.5 +/- 0.2; the degree of variability is roughly constant on timescales of days to years; and, the degree of variability increases with frequency. We also observed that the largest increase in fractional circular polarization was coincident with the brightest flare in total intensity. Significant variability in the total intensity and fractional circular polarization on a timescale of 1 hour was observed during this flare, indicating an upper limit to the size of 70 AU at 15 GHz. The fractional circular polarization at 15 GHz reached -1.1% and the spectral index is strongly inverted during this flare. We conclude that the spectrum has two components that match the high and low frequency total intensity components. (abridged)

astro-ph↗

arXiv2001

The Power of Jets: New Clues from Radio Circular Polarization and X-rays

Jets are ubiquitous in accreting black holes. Often ignored, they may be a major contributor to the emitted spectral energy distribution for sub-Eddington black holes. For example, recent observations of radio-to-X-ray correlations and broad band spectra of X-ray binaries in the low/hard state can be explained by a significant synchrotron contribution from jets also to their IR-to-X-ray spectrum as proposed by Markoff, Falcke, Fender 2001. This model can also explain state-transitions from low/hard to high/soft states. Relativistic beaming of the jet X-ray emission could lead to the appearance of seemingly Super-Eddington X-rays sources in other galaxies. We show that a simple population synthesis model of X-ray binaries with relativistic beaming can well explain the currently found distribution of off-nucleus X-ray point sources in nearby galaxies. Specifically we suggest that the so-called ultra-luminous X-ray sources (ULXs, also IXOs) could well be relativistically beamed microblazars. The same model that can be used to explain X-ray binaries also fits Low-Luminosity AGN (LLAGN) and especially Sgr A* in the Galactic Center. The recent detection of significant circular polarization in AGN radio cores, ranging from bright quasars down to low-luminosity AGN like M81*, Sgr A* and even X-ray binaries, now places additional new constraints on the matter contents of such jets. The emerging picture are powerful jets with a mix of hot and cold matter, a net magnetic flux, and a stable magnetic north pole.

astro-ph↗

arXiv2001

BIMA Observations of Linear Polarization in Sagittarius A* at 112 GHz

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.

astro-ph↗

arXiv2001

VLBA Observations of Astrometric Reference Sources in the Galactic Center

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.

astro-ph↗

arXiv2000

Polarization in Sagittarius A*

We summarize the current state of polarization observations of Sagittarius A*, the compact radio source and supermassive black hole candidate in the Galactic Center. These observations are providing new tools for understanding accretion disks, jets and their environments. Linear polarization observations have shown that Sgr A* is unpolarized at frequencies as high as 86 GHz. However, recent single-dish observations indicate that Sgr A* may have strong linear polarization at frequencies higher than 150 GHz. Circular polarization, which was recently discovered in Sgr A*, is detectable to frequencies as high as 43 GHz. It is strongly variable on a 10-day timescale, with the degree of variability increasing with frequency. At low frequencies, fractional circular polarization is stable over nearly 20 years. We discuss some of the possible models for the origin of the circular polarization.

astro-ph↗

arXiv2000

III Zw 2, the first superluminal jet in a Seyfert galaxy

So far all relativistically boosted jets with superluminal motion have only been detected in typical radio galaxies with early type host galaxies. We have now discovered superluminal motion in the Seyfert I galaxy III Zw 2, classified as a spiral. Superluminal motion was first inferred from the spectral evolution of the source and then confirmed by VLBI observations. The lower limit for the apparent expansion speed is 1.25 +/- 0.09 c. The fact that the spectral and spatial evolution are closely linked demonstrates that we are dealing with real physical expansion. Prior to this rapid expansion we have seen a period of virtually no expansion with an expansion speed less than 0.04 c. Since III Zw 2 is also part of a sample of so called radio-intermediate quasars (RIQ), it confirms earlier predictions of superluminal motion for this source, based on the argument that RIQs could be relativistically boosted jets in radio-weak quasars and Seyfert galaxies.

astro-ph↗

arXiv1999

The millimeter VLBI Properties of EGRET Blazars

We give a progress report on a snapshot 86 GHz-VLBI survey of the EGRET blazars with the observatories of the CMVA. A high fraction (17/18) of the EGRET blazars were detected on the Pico Veleta-Onsala baseline with a baseline length on the order of 500 $Mλ$. The detection threshold on the Pico Veleta-Onsala baseline was $\sim 0.2$ Jy. Six of these sources were not previously detected with 3-millimeter VLBI. We also present the detection of three new non-EGRET sources. The high detection rate for EGRET sources indicates that gamma-ray flux is a robust predictor of millimeter wavelength intensity. Future more sensitive high-energy gamma-ray experiments should find a larger class of objects detectable with millimeter wavelength VLBI.

astro-ph↗

arXiv1999

The Linear Polarization of Sagittarius A* II. VLA and BIMA Polarimetry at 22, 43 and 86 GHz

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.

astro-ph↗

arXiv1999

Detection of Circular Polarization in the Galactic Center Black Hole Candidate Sagittarius A*

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*.

astro-ph↗

arXiv1999

The Linear Polarization of Sagittarius A* I. VLA Spectro-polarimetry at 4.8 and 8.4 GHz

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*.

astro-ph↗