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Register a new userLinearly Polarized Millimeter and Submillimeter Continuum Emission of Sgr A* Constrained by ALMA
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Hauyu Baobab Liu Mr.
Publication date
2016
fields
Physics
and research's language is
English
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Our aim is to characterize the polarized continuum emission properties including intensity, polarization position angle, and polarization percentage of Sgr A* at $sim$100 (3.0 mm), $sim$230 (1.3 mm), $sim$345 (0.87 mm), $sim$500 (0.6 mm), and $sim$700 GHz (0.43 mm). 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. We successfully detect polarized continuum emission from all observed frequency bands. We observed lower Stokes I intensity at $sim$700 GHz than that at $sim$500 GHz, which suggests that emission at $gtrsim$500 GHz is from optically thin part of a synchrotron emission spectrum. Both the Stokes I intensity and the polarization position angle at our highest observing frequency of $sim$700 GHz, may be varying with time. However, we do not yet detect variation in the polarization percentage at $>$500 GHz. The polarization percentage at $sim$700 GHz is likely lower than that at $sim$500 GHz. By comparing the $sim$500 GHz and $sim$700 GHz observations with the observations at lower frequency bands, we suggest that the intrinsic polarization position angle of Sgr A* is varying with time. This paper also reports the measurable polarization properties from the observed calibration quasars. The future simultaneous multi-frequency polarization observations are required for clarifying the time and frequency variation of polarization position angle and polarization percentage.
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We report linearly polarized continuum emission properties of Sgr A* at $sim$492 GHz, based on the Atacama Large Millimeter Array (ALMA) observations. We used the observations of the likely unpolarized continuum emission of Titan, and the observations of Ctextsc{i} line emission, to gauge the degree of spurious polarization. The Stokes I flux of 3.6$pm$0.72 Jy during our run is consistent with extrapolations from the previous, lower frequency observations. We found that the continuum emission of Sgr A* at $sim$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 may be explained by a constant polarization position angle of $sim$158$^{circ}$$pm$3$^{circ}$. The fitted polarization percentage of Sgr A* during our observational period is 14%$pm$1.2%. The calibrator quasar J1744-3116 we observed at the same night can be fitted to Stokes I = 252 mJy, with 7.9%$pm$0.9% polarization in position angle P.A. = 4.1$^{circ}$$pm$4.2$^{circ}$. The observed polarization percentage and polarization position angle in the present work appear consistent with those expected from longer wavelength observations in the period of 1999-2005. In particular, the polarization position angle at 492 GHz, expected from the previously fitted 167$^{circ}$$pm$7$^{circ}$ intrinsic polarization position angle and (-5.6$pm$0.7)$times$10$^{5}$ rotation measure, is 155$^{+9}_{-8}$, which is consistent with our new measurement of polarization position angle within 1$sigma$. The polarization percentage and the polarization position angle may be varying over the period of our ALMA 12m Array observations, which demands further investigation with future polarization observations.
We present ALMA 97.5 GHz total intensity and linear polarization observations of the mm-band afterglow of GRB 190114C spanning 2.2 to 5.2 hours after the burst. We detect linear polarization at the $approx 5,sigma$ level, decreasing from $Pi=(0.87pm0.13)%$ to $(0.60pm0.19)%$, and evolving in polarization position angle from $(10pm5)^circ$ to $(-44pm12)^circ$ during the course of the observations. This represents the first detection of polarized millimeter emission in a $gamma$-ray burst. We show that the optical and X-ray observations between $0.03$ days and $sim0.3$ days are consistent with a fast cooling forward shock expanding into a wind environment. However, the optical observations at $lesssim0.03$ days, as well as the radio and millimeter observations arise from a separate component, which we interpret as emission from the reverse-shocked ejecta. Using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the ejecta to an angular size of $theta_{rm B} approx10^{-3}$ radian, while the rotation of the polarization angle rules out the presence of large scale, ordered axisymmetric magnetic fields, and in particular a large scale toroidal field, in the jet.
We present 44 and 226 GHz observations of the Galactic center within 20$$ of Sgr A*. Millimeter continuum emission at 226 GHz is detected from eight stars that have previously been identified at near-IR and radio wavelengths. We also detect a 5.8 mJy source at 226 GHz coincident with the magnetar SGR~J1745-29 located 2.39$$ SE of Sgr A* and identify a new 2.5$times1.5$ halo of mm emission centered on Sgr A*. The X-ray emission from this halo has been detected previously and is interpreted in terms of a radiatively inefficient accretion flow. The mm halo surrounds an EW linear feature which appears to arise from Sgr A* and coincides with the diffuse X-ray emission and a minimum in the near-IR extinction. We argue that the millimeter emission is produced by synchrotron emission from relativistic electrons in equipartition with a $sim 1.5$mG magnetic field. The origin of these is unclear but its coexistence with hot gas supports scenarios in which the gas is produced by the interaction of winds either from the fast moving S-stars, the photo-evaporation of low-mass YSO disks or by a jet-driven outflow from Sgr A*. The spatial anti-correlation of the X-ray, radio and mm emission from the halo and the low near-IR extinction provides compelling evidence for an outflow sweeping up the interstellar material, creating a dust cavity within 2$$ of Sgr A*. Finally, the radio and mm counterparts to eight near-IR identified stars within $sim$10arcs of Sgr A* provide accurate astrometry to determine the positional shift between the peak emission at 44 and 226 GHz.
We study the environment of Sgr A* using spectral and continuum observations with the ALMA and VLA. Our analysis of sub-arcsecond H30alpha, H39alpha, H52alpha and H56alpha line emission towards Sgr A* confirm the recently published broad peak ~500 km/s~spectrum toward Sgr~A*. We also detect emission at more extreme radial velocities peaking near -2500 and 4000 km/s, within 0.2. We then present broad band radio continuum images at multiple frequencies on scales from arcseconds to arcminutes. A number of elongated continuum structures lie parallel to the Galactic plane, extending from ~0.4 to 10. We note a nonthermal elongated structure on an arcminute scale emanating from Sgr A* at low frequencies between 1 and 1.4 GHz where thermal emission from the mini-spiral is depressed by optical depth effects. The position angle of this elongated structure and the sense of motion of ionized features with respect to Sgr A* suggest a symmetric, collimated jet emerging from Sgr A* with an opening angle of ~30deg and a position angle of ~60deg punching through the medium before accelerating a significant fraction of the orbiting ionized gas to high velocities. The jet with estimated mass flow rate ~1.4x10^{-5} solar mass/yr emerges perpendicular to the equatorial plane of the accretion flow near the event horizon of Sgr A* and runs along the Galactic plane. To explain a number of east-west features near Sgr A*, we also consider the possibility of an outflow component with a wider-angle launched from the accretion flow at larger radii.
We present relativistic magnetohydrodynamic (RMHD) simulations of stationary overpressured magnetized relativistic jets which are characterized by their dominant type of energy, namely internal, kinetic, or magnetic. Each model is threaded by a helical magnetic field with a pitch angle of $45^circ$ and features a series of recollimation shocks produced by the initial pressure mismatch, whose strength and number varies as a function of the dominant type of energy. We perform a study of the polarization signatures from these models by integrating the radiative transfer equations for synchrotron radiation using as inputs the RMHD solutions. These simulations show a top-down emission asymmetry produced by the helical magnetic field and a progressive confinement of the emission into a jet spine as the magnetization increases and the internal energy of the non-thermal population is considered to be a constant fraction of the thermal one. Bright stationary components associated with the recollimation shocks appear presenting a relative intensity modulated by the Doppler boosting ratio between the pre-shock and post-shock states. Small viewing angles show a roughly bimodal distribution in the polarization angle due to the helical structure of the magnetic field, which is also responsible for the highly stratified degree of linear polarization across the jet width. In addition, small variations of the order of $26^circ$ are observed in the polarization angle of the stationary components, which can be used to identify recollimation shocks in astrophysical jets.
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