Jun-Hui Zhao

The Simultaneous Spectrum of Sagittarius A* from 20 Centimeters to 1 Millimeter and the Nature of the Millimeter Excess

We report results of a multiwavelength campaign to measure the simultaneous spectrum of the supermassive black hole candidate Sgr A* in the Galactic center from centimeter to millimeter wavelengths using the Very Large Array, the Berkeley-Illinois-Maryland Array (BIMA), the Nobeyama 45 m, and the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescopes. The observations confirm that the previously detected millimeter excess is an intrinsic feature of the spectrum of Sgr A*. The excess can be interpreted as and effect of the presence of an ultracompact component of relativistic plasma with a size of a few Schwarzschild radii near the black hole. If so, Sgr A* might offer a unique possibility to image the putative black hole against the background of this component with future millimeter VLBI experiments.

An Unambiguous Detection of Faraday Rotation in Sagittarius A

The millimeter/submillimeter wavelength polarization of Sgr A* is known to be variable in both magnitude and position angle on timescales down to a few hours. The unstable polarization has prevented measurements made at different frequencies and different epochs from yielding convincing measurements of Faraday rotation in this source. Here we present observations made with the Submillimeter Array polarimeter at 227 and 343 GHz with sufficient sensitivity to determine the rotation measure at each band without comparing position angles measured at separate epochs. We find the 10-epoch mean rotation measure to be (-5.6 ± 0.7) × 105 rad m-2; the measurements are consistent with a constant value. We conservatively assign a 3 σ upper limit of 2 × 105 rad m-2 to rotation measure changes, which limits accretion rate fluctuations to 25%. This rotation measure detection limits the accretion rate to less than 2 × 10-7 M☉ yr-1 if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2 × 10-9 M☉ yr-1 holds even for a subequipartition, disordered, or toroidal field. The mean intrinsic position angle is 167° ± 7° and we detect variations of 31 deg. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes.

Interferometric measurements of variable 340 GHz linear polarization in sagittarius A

Using the Submillimeter Array, we have made the first high angular resolution measurements of the linear polarization of Sagittarius A* at submillimeter wavelengths and the first detection of intraday variability in its linear polarization. We detected linear polarization at 340 GHz (880 μm) at several epochs. At the typical resolution of 14 × 22, the expected contamination from the surrounding (partially polarized) dust emission is negligible. We found that both the polarization fraction and the position angle are variable, with the polarization fraction dropping from 8.5% to 2.3% over 3 days. This is the first significant measurement of variability in the linear polarization fraction in this source. We also found variability in the polarization and total intensity within single nights, although the relationship between the two is not clear from these data. The simultaneous 332 and 342 GHz position angles are the same, setting a 1 σ rotation measure (RM) upper limit of 7 × 105 rad m-2. From position angle variations and comparison of quiescent position angles observed here and at 230 GHz, we infer that the RM is a few times 105 rad m-2, a factor of a few below our direct detection limit. A generalized model of the RM produced in the accretion flow suggests that the accretion rate at small radii must be low, below 10-6-10-7 M☉ yr-1 depending on the radial density and temperature profiles, but in all cases below the gas capture rate inferred from X-ray observations.

The flare activity of Sagittarius A

Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3−4 × 10 6 Mblack hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from Sgr A*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescopeand the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA �� on Mauna Kea, Hawaii, and the Very Large Array ��� in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2−8 keV luminosity of about 33 × 10 33 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered sub- millimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.

The flare activity of Sagittarius A* New coordinated mm to X-ray observations

Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3−4 × 10 6 Mblack hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from Sgr A*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescopeand the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA �� on Mauna Kea, Hawaii, and the Very Large Array ��� in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2−8 keV luminosity of about 33 × 10 33 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered sub- millimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.

Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging

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 ± 2 Schwarzschild radii at 7-millimeter wavelength. In one of eight 7-millimeter epochs, we also detected an increase in the intrinsic size of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(60_{-17}^{+25}\%\) \end{document}. These observations place a lower limit to the mass density of Sagittarius A* of 1.4 × 104 solar masses per cubic astronomical unit.

Intrinsic Size of Sagittarius A*: 72 Schwarzschild Radii

Recent proper motion studies of stars at the very center of the Galaxy strongly suggest that Sagittarius A*, the compact nonthermal radio source at the Galactic center, is a 2.5×106 M☉ black hole. By means of near-simultaneous multiwavelength Very Long Baseline Array measurements, we determine for the first time the intrinsic size and shape of Sgr A* to be 72 Rsc() by less than 20 Rsc(), with the major axis oriented essentially north-south, where Rsc() (≡ 7.5×1011 cm) is the Schwarzschild radius for a 2.5×106 M☉ black hole. Contrary to previous expectation that the intrinsic structure of Sgr A* is observable only at λ≤1 mm, we can discern the intrinsic source size at λ=7 mm because (1) the scattering size along the minor axis is half that along the major axis and (2) the near-simultaneous multiwavelength mapping of Sgr A* with the same interferometer makes it possible to extrapolate precisely the minor axis scattering angle at λ=7 mm. The intrinsic size and shape place direct constraints on the various emission models for Sgr A*. In particular, the advection-dominated accretion flow model may have to incorporate a radio jet in order to account for the structure of Sgr A*.

Molecular Superbubbles in the Starburst Galaxy NGC 253

The central 2; 1 kpc of the starburst galaxy NGC 253 have been imaged using the Submillimeter Array at a 60 pc resolution in the J = 2 - 1 transitions of (12)CO, (13)CO, and C(18)O, as well as in the 1.3 mm continuum. Molecular gas and dust are distributed mainly in a circumnuclear disk of similar to 500 pc radius, with warm (similar to 40 K) and high area filling factor gas in its central part. Two gas shells or cavities have been discovered in the circumnuclear disk. They have similar to 100 pc diameters and have large velocity widths of 80 - 100 km s(-1), suggestive of expansion at similar to 50 km s(-1) . Modeled as an expanding bubble, each shell has an age of similar to 0.5 Myr and needed kinetic energy of similar to 1; 10(46) J, as well as mean mechanical luminosity of similar to 1; 10(33) W, for its formation. The large energy allows each to be called a superbubble. A similar to 10(6) M circle dot super star cluster can provide the luminosity and could be a building block of the nuclear starburst in NGC 253. Alternatively, a hypernova can also be the main source of energy for each superbubble, not only because it can provide the mechanical energy and luminosity but also because the estimated rate of superbubble formation and that of hypernova explosions are comparable. Our observations indicate that the circumnuclear molecular disk harboring the starburst is highly disturbed on 100 pc or smaller scales, presumably by individual young clusters and stellar explosions, in addition to being globally disturbed in the form of the well-known superwind.

The Linear Polarization of Sagittarius A*. I. VLA Spectropolarimetry at 4.8 and 8.4 GHz

Synchrotron radiation from active galactic nuclei (AGNs) is often highly polarized. We present a search for linear polarization with the Very Large Array (VLA) at 4.8 and 8.4 GHz from the nearest AGN, Sagittarius A*. As a part of this study we used spectropolarimetric data that were sensitive to a rotation measure (RM) as large as 3.5 × 106 rad m-2 at 4.8 GHz and 1.5 × 107 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 that give an upper limit of 0.1% for RMs less than 104 rad m-2. We conclude that depolarization is unlikely to occur in the Galactic 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*.

Radio Variability of Sagittarius A*—a 106 Day Cycle

We report the presence of a 106 day cycle in the radio variability of Sagittarius A* based on an analysis of data observed with the Very Large Array over the past 20 years. The pulsed signal is most clearly seen at 1.3 cm with a ratio of cycle frequency to frequency width . The periodic signal is also clearly observed at f/Df p 2.2 5 0.3 2 cm. At 3.6 cm the detection of a periodic signal is marginal. No significant periodicity is detected at both 6 and 20 cm. Since the sampling function is irregular, we performed a number of tests to ensure that the observed periodicity is not the result of noise. Similar results were found for a maximum entropy method and a periodogram with a CLEAN method. The probability of false detection for several different noise distributions is less than 5% based on Monte Carlo tests. The radio properties of the pulsed component at 1.3 cm are a spectral index a » (for ), an amplitude , and a characteristic timescale days. a 1.0 5 0.1 S / nD S p 0.42 5 0.04 Jy Dt … 25 5 5 FWHM The lack of a VLBI detection of a secondary component suggests that the variability occurs within Sgr A* on a scale of »5 AU, suggesting an instability of the accretion disk. Subject headings: accretion, accretion disks — black hole physics — galaxies: active — Galaxy: center — radio continuum: galaxies On-line material: color figures