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| Black Holes & Stellar Remnants | ||||||||||||||||||||||||||||||||||
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Abstract Using the JVLA, we explored the Galactic center (GC) with a resolution of 0.05" at 33.0 and 44.6 GHz. We detected 64 hyper-compact radio sources (HCRs) in the central parsec. The dense group of HCRs can be divided into three spectral types: 38 steep-spectrum (α ≤ −0.5) sources; 10 flat-spectrum (−0.5 < α ≤ 0.2) sources; and 17 inverted-spectrum sources having α > 0.2, assuming S ∝ να. The steep-spectrum HCRs are likely represent a population of massive stellar remnants associated with nonthermal compact radio sources powered by neutron stars and stellar black holes. The surface-density distribution of the HCRs as function of radial distance (R) from Sgr A* can be described as a steep power-law Σ(R) ∝ R−Γ, with Γ = 1.6 ∓ 0.2, along with presence of a localized order-of-magnitude enhancement in the range 0.1 - 0.3 pc. The steeper profile of the HCRs relative to that of the central cluster might result from the concentration massive stellar remnants by mass segregation at the GC. The GC magnetar SGR J1745-2900 belongs to the inverted-spectrum sub-sample. We find that two spectral components present in the averaged radio spectrum of SGR J1745-2900, separated at ν ∼ 30 GHz. The centimeter-component is fitted to a power-law with αcm = −1.5 ∓ 0.6. The enhanced millimeter-component shows a rising spectrum αmm = 1.1 ∓ 0.2. Based on the ALMA observations at 225 GHz, we find that the GC magnetar is highly variable on a day-to-day time scale, showing variations up to a factor of 6. Further JVLA and ALMA observations of the variability, spectrum, and polarization of the HCRs are critical for determining whether they are associated with stellar remnants.
Abstract Deep observations achieving a sensitivity of a few μJy beam-1 rms are enabled at radiomillimeter- submillimeter telescopes by improvements in both hardware and software for wideband capability. Radio detections now become possible of some stellar sources at the Galactic center, such as X-ray binaries and bright pulsars. We have recently detected a large population of compact radio sources at the Galactic center with the Very Large Array (VLA) in the flux density range from a few tens of μJy to a few tens of mJy, revealing a population of radio sources that are plausibly associated with neutron stars and stellar black holes produced at the ends of stellar evolution. In this talk, we will present our updated results based on radio observations, showing their radio morphology, spatial and luminosity distributions, and cross-correlation with X-ray surveys. We will discuss the possible nature of these Galactic center compact radio sources (GCCRs). The prospects for study of the GCCRs at millimeter-submillimeter wavelengths will be discussed as well.
Abstract Using the Very Large Array, we have investigated a non-thermal radio filament (NTF) recently found very near the Galactic black hole and its radio counterpart, SgrA*. While this NTF -- the Sgr A West Filament (SgrAWF) -- shares many characteristics with the population of NTFs occupying the central few hundred parsecs of the Galaxy, the SgrAWF has the distinction of having an orientation and sky location that suggest an intimate physical connection to SgrA*. We present 3.3 and 5.5 cm images constructed using an innovative methodology that yields a very high dynamic range, providing an unprecedentedly clear picture of the SgrAWF. While the physical association of the SgrAWF with SgrA* is not unambiguous, the images decidedly evoke this interesting possibility. Assuming that the SgrAWF bears a physical relationship to SgrA*, we examine the potential implications. One is that SgrA* is a source of relativistic particles constrained to diffuse along ordered local field lines. The relativistic particles could also be fed into the local field by a collimated outflow from SgrA*, perhaps driven by the Poynting flux accompanying the black hole spin in the presence of a magnetic field threading the event horizon. Second, we consider the possibility that the SgrAWF is the manifestation of a low-mass-density cosmic string that has become anchored to the black hole. The simplest form of these hypotheses would predict that the filament be bi-directional, whereas the SgrAWF is only seen on one side of SgrA*, perhaps because of the dynamics of the local medium.
Abstract Aims. Our aim is to characterize the polarized continuum emission properties including intensity, polarization position angle, and polarization percentage of Sgr A* at ~100 (3.0 mm), ~230 (1.3 mm), ~345 (0.87 mm), ~500 (0.6 mm), and ~700 GHz (0.43 mm). Methods. We report continuum emission properties of Sgr A* at the above frequency bands, based on the Atacama Large Millimeter Array (ALMA) observations. We measured flux densities of Sgr A* from ALMA single pointing and mosaic observations. We performed sinusoidal fittings to the observed (XX-YY)/I intensity ratios, to derive the polarization position angles and polarization percentages. Results. We successfully detect polarized continuum emission from all observed frequency bands. We observed lower Stokes I intensity at ~700 GHz than that at ~500 GHz, which suggests that emission at ≳500 GHz is from the optically thin part of a synchrotron emission spectrum. Both the Stokes I intensity and the polarization position angle at our highest observing frequency of ~700 GHz, may vary with time. However, as yet we do not detect variation in the polarization percentage at >500 GHz. The polarization percentage at ~700 GHz is likely lower than that at ~500 GHz. By comparing the ~500 GHz and ~700 GHz observations with the observations at lower frequency bands, we suggest that the intrinsic polarization position angle of Sgr A* varies with time. This paper also reports the measurable polarization properties from the observed calibration quasars. Conclusions. Future simultaneous multi-frequency polarization observations are required to clarify the time and frequency variation of the polarization position angle and polarization percentage.
Abstract Aims. Our aim is to characterize the polarized continuum emission properties including intensity, polarization position angle, and polarization percentage of Sgr A* at ~492 GHz. This frequency, well into the submillimeter bump where the emission is supposed to become optically thin, allows us to see down to the event horizon. Hence the reported observations contain potentially vital information on black hole properties. We have compared our measurements with previous, lower frequency observations, which provides information in the time domain. Methods. We report continuum emission properties of Sgr A* at ~492 GHz, based on Atacama Large Millimeter Array (ALMA) observations. We measured flux densities of Sgr A* from the central fields of our ALMA mosaic observations. We used calibration observations of the likely unpolarized continuum emission of Titan and the observations of Ci line emission, to gauge the degree of spurious polarization. Results. The flux density of 3.6 ± 0.72 Jy which we measured during our run is consistent with extrapolations from previous, lower frequency observations. We found that the continuum emission of Sgr A* at ~492 GHz shows large amplitude differences between the XX and the YY correlations. The observed intensity ratio between the XX and YY correlations as a function of parallactic angle can be explained by a constant polarization position angle of ~158°± 3°. The fitted polarization percentage of Sgr A* during our observational period is 14% ± 1.2%. The calibrator quasar J1744-3116 we observed on the same night can be fitted to Stokes I = 252 mJy, with 7.9% ± 0.9% polarization at position angle PA = 14°± 4.2°. Conclusions. The observed polarization percentage and polarization position angle in the present work appear consistent with those expected from longer wavelength observations in the period of 1999−2005. In particular, the polarization position angle at 492 GHz expected from the previously fitted 167°± 7° intrinsic polarization position angle and (−5.6 ± 0.7) × 105 rotation measure is 155°, which is consistent with our new measurement of polarization position angle within 1σ. The polarization percentage and the polarization position angle may be varying over the period of our ALMA 12 m Array observations, which demands further investigation with future polarization observations.
Abstract We report the results from recent observations of Sgr A* at short-/submillimeter wavelengths made with the partially finished Submillimeter Array (SMA) on Mauna Kea. A total of 25 epochs of observations were carried out over the past 15 months from 2001 March to 2002 May. Noticeable variations in flux density at 1.3 mm were observed showing three "flares." The SMA observations suggest that Sgr A* highly increases toward submillimeter wavelengths during a flare, suggesting the presence of a break wavelength in spectral index around 3 mm. A cross-correlation of the SMA data at 1 mm with the Very Large Array data at 1 cm shows a global delay of t_delay > 3 days, suggesting that submillimeter wavelengths tend to peak first. Only marginal day-to-day variations in flux density (2-3 sigma) have been detected at 1.3 mm. No significant flares on a short timescale (~1 hr) have been observed at 1.3 mm. We also failed to detect significant periodic signals at a level of 5% (3 sigma) from Sgr A* in a periodic searching window ranging from 10 minutes to 2.5 hr. The flares observed at the wavelengths between short-centimeter and submillimeter might be a result of collective mass ejections associated with X-ray flares that originate from the inner region of the accretion disk near the supermassive black hole.
Abstract 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) x 105 rad m-2 the measurements are consistent with a constant value. We conservatively assign a 3 sigma upper limit of 2 x 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 x 10-7 Msun yr-1 if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2 x 10-9 Msun yr-1 holds even for a subequipartition, disordered, or toroidal field. The mean intrinsic position angle is 167+/-7deg and we detect variations of 31+18-9 deg. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes.
Abstract We observed Sgr A* using the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT) at multiple centimeter and millimeter wavelengths on 2003 June 17. The measured flux densities of Sgr A*, together with those obtained from the Submillimeter Array (SMA) and the Keck II 10 m telescope on the same date, are used to construct a simultaneous spectrum of Sgr A* from 90 cm to 3.8 micron. The simultaneous spectrum shows a spectral break at about 3.6 cm, a possible signature of synchrotron self-absorption of the strong radio outburst that occurred near epoch 2003 July 17. At 90 cm, the flux density of Sgr A* is 0.22+/-0.06 Jy, suggesting a sharp decrease in flux density at wavelengths longer than 47 cm. The spectrum at long cm wavelengths appears to be consistent with free-free absorption by a screen of ionized gas with a cutoff ~100 cm. This cutoff wavelength appears to be three times longer than that of ~30 cm suggested by Davies, Walsh, & Booth based on observations in 1974 and 1975. Our analysis suggests that the flux densities of Sgr A* at wavelengths longer than 30 cm could be attenuated and modulated by stellar winds from massive stars close to Sgr A*.
Abstract By means of near-simultaneous multi-wavelength VLBA measurements, we determine for the first time the intrinsic size of Sgr A* to be 3.6 AU by < 1 AU or 72 Rsc(star) by < 20 Rsc(star), with the major axis oriented essentially north-south, where Rsc(star) ~7.5 times 1011 cm is the Schwarzschild radius for a 2.5 times 106 Msun black hole. Contrary to previous expectation that the intrinsic structure of Sgr A* is observable only at wavelengths shorter than 1 mm, we can discern the intrinsic source size at 7 mm because (1) the scattering size along the minor axis is half that along the major axis, and (2) the near simultaneous multi-wavelength mapping of Sgr A* makes it possible to extrapolate precisely the minor axis scattering angle at 7 mm. The intrinsic size and shape place direct constraints on the various theoretical models for Sgr A*.
Abstract A new high-intensity radio source that appeared in late December 1990 near the center of the Galaxy is described. Following its first detection, the flux density of this Galactic-center transient (GCT) increased rapidly to a maximum a month later, and then declined gradually during the next 3 months. However, throughout its rising and decay phases, the GCT maintained a steep radio spectrum. Neutral-hydrogen-absorption and OH observations indicate that the GCT is either embedded in, or located behind, a molecular cloud moving with the velocity of +20 km/sec with respect to the local standard of rest. It is suggested that the GCT represents a transient radio emission from synchrotron-radiating plasma associated with an X-ray binary system.
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