Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign

Abstract

Sagittarius A* (Srg A*), the supermassive black hole at the center of the Milky Way, provides a unique laboratory to study accretion dynamics and plasma processes near the event horizon. We investigated the variability and polarization properties of Srg A* using ALMA observations during the 2018 Event Horizon Telescope campaign. We analyzed high-cadence full-polarization light curves from ALMA at millimeter wavelengths, performed time-series analysis, and investigated the temporal behavior during an X-ray flare observed by Chandra on 2018 April 24. The variability characteristics are compared with expectations from standard accretion flow models. We find low variability in total intensity ($\sigma /\mu <10\%$), but significantly higher variability in linear and circular polarization (~ 30% and ~ 50%, respectively). A time-series analysis reveals red-noise variability, with power spectral densities between -2 and -3 across all Stokes parameters. Polarized intensity shows stable intra-day timescales, while total intensity exhibits more variable timescales, suggesting distinct emission regions, with polarization likely arising from a coherent structure. On April 24, a statistically significant inter-band delay in polarized intensity coincides with a near-simultaneous X-ray and millimeter peak that deviates from the typical delayed flare scenario. This event also features enhanced millimeter variability and coherent polarization loop evolution. The observed simultaneity challenges standard models of transient synchrotron emission with cooling delays, favoring instead a scenario of continuous energy injection in an optically thin region. Our results offer new constraints on the physical mechanisms driving variability in Srg A*, and provide key observational input for refining theoretical models of accretion and plasma behavior in the vicinity of supermassive black holes.

References in corpus (69)

• First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
• Hot Accretion Flows Around Black Holes
• Efficient Generation of Jets from Magnetically Arrested Accretion on a Rapidly Spinning Black Hole
• CASA, the Common Astronomy Software Applications for Radio Astronomy
• HARM: A Numerical Scheme for General Relativistic Magnetohydrodynamics
• Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A*
• Faraday Rotation Measure Synthesis
• Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre
• First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole
• Relativistic redshift of the star S0-2 orbiting the Galactic center supermassive black hole
• Detection of orbital motions near the last stable circular orbit of the massive black hole SgrA*
• First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multi-wavelength Observations, Data Processing, and Calibration
• General relativistic magnetohydrodynamical simulations of the jet in M87
• An Unambiguous Detection of Faraday Rotation in Sagittarius A*
• The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits
• Black hole flares: ejection of accreted magnetic flux through 3D plasmoid-mediated reconnection
• Revisiting the radio interferometer measurement equation. I. A full-sky Jones formalism
• Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz
• Resolved Magnetic-Field Structure and Variability Near the Event Horizon of Sagittarius A*
• An X-ray, IR, and Submillimeter Flare of Sagittarius A*
• Interferometric Measurements of Variable 340 GHz Linear Polarization in Sagittarius A*
• First-Principles Plasma Simulations of Black-Hole Jet Launching
• Polarimetry of near-infrared flares from Sagittarius A*
• UVMULTIFIT: A versatile tool for fitting astronomical radio interferometric data
• Constraining the Accretion Rate Onto Sagittarius A* Using Linear Polarization
• First Simultaneous NIR/X-ray Detection of a Flare from SgrA*
• Sagittarius A* Accretion Flow and Black Hole Parameters from General Relativistic Dynamical and Polarized Radiative Modeling
• X-ray flares reveal mass and angular momentum of the Galactic Center black hole
• The role of electron heating physics in images and variability of the Galactic Centre black hole Sagittarius A*
• The 1.28 GHz MeerKAT Galactic Center Mosaic
• Radiative, two-temperature simulations of low luminosity black hole accretion flows in general relativity
• Flares in the Galactic center I: orbiting flux tubes in Magnetically Arrested Black Hole Accretion Disks
• Radio and Millimeter Monitoring of Sgr A*: Spectrum, Variability, and Constraints on the G2 Encounter
• Are Particles in Advection-Dominated Accretion Flows Thermal?
• Ab Initio Horizon-Scale Simulations of Magnetically Arrested Accretion in Sagittarius A* Fed by Stellar Winds
• Sgr A* near-infrared flares from reconnection events in a magnetically arrested disc
• Simultaneous Chandra, CSO and VLA Observations of Sgr A*: The Nature of Flaring Activity
• Millimeter light curves of Sagittarius A* observed during the 2017 Event Horizon Telescope campaign
• Polarimetric properties of Event Horizon Telescope targets from ALMA
• Orbital motion near Sagittarius A* -- Constraints from polarimetric ALMA observations
• Polarimetry and Astrometry of NIR Flares as Event Horizon Scale, Dynamical Probes for the Mass of Sgr A*
• Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow
• The Rotation Measure and 3.5mm Polarization of Sgr A*
• ALMA Polarimetry of Sgr A*: Probing the Accretion Flow from the Event Horizon to the Bondi Radius
• Rapid Variability of Sgr A* across the Electromagnetic Spectrum
• Dynamically important magnetic fields near the event horizon of Sgr A*
• An Extended Magnetohydrodynamics Model for Relativistic Weakly Collisional Plasmas
• The ALMA Phasing System: A Beamforming Capability for Ultra-High-Resolution Science at (Sub)Millimeter Wavelengths
• Calibration of ALMA as a phased array: ALMA observations during the 2017 VLBI campaign
• Collisionless accretion onto black holes: dynamics and flares
• Calibration of mixed-polarization interferometric observations
• EHT-HOPS pipeline for millimeter VLBI data reduction
• Simultaneous X-ray and Infrared Observations of Sagittarius A*'s Variability
• Images of magnetospheric reconnection-powered radiation around supermassive black holes
• Polarimetric signatures of hot spots in black hole accretion flows
• Multiwavelength Light Curves of Two Remarkable Sagittarius A* Flares
• Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA
• The internal Faraday screen of Sagittarius A*
• Second Scale Submillimeter Variability of Sagittarius A* during flaring activity of 2019: On the Origin of Bright Near Infrared Flares
• Fitting the light curves of Sagittarius A* with a hot-spot model
• Simultaneous Monitoring of X-ray and Radio Variability in Sagittarius A*
• A Characterization of the ALMA Phasing System at 345 GHz
• Hot-spots around Sagittarius A*: Joint fits to astrometry and polarimetry
• Adiabatic Index in Fluid Models of Collisionless Black Hole Accretion
• Continuation of the X-ray monitoring of Sgr A*: the increase in bright flaring rate confirmed
• A first search of transients in the Galactic Center from 230 GHz ALMA observations
• First polarization study of the M87 jet and active galactic nuclei at submillimeter wavelengths with ALMA
• The impact of resistivity on the variability of black hole accretion flows
• High-pass Filter Periodogram: An Improved Power Spectral Density Estimator for Unevenly Sampled Data