Dynamically important magnetic fields near the event horizon of Sgr A*

Abstract

We study the time-variable linear polarisation of Sgr A* during a bright near-infrared flare observed with the GRAVITY instrument on July 28, 2018. Motivated by the time evolution of both the observed astrometric and polarimetric signatures, we interpret the data in terms of the polarised emission of a compact region (“hotspot”) orbiting a black hole in a fixed, background magnetic field geometry. We calculated a grid of general relativistic ray-tracing models, created mock observations by simulating the instrumental response, and compared predicted polarimetric quantities directly to the measurements. We take into account an improved instrument calibration that now includes the instrument’s response as a function of time, and we explore a variety of idealised magnetic field configurations. We find that the linear polarisation angle rotates during the flare, which is consistent with previous results. The hotspot model can explain the observed evolution of the linear polarisation. In order to match the astrometric period of this flare, the near horizon magnetic field is required to have a significant poloidal component, which is associated with strong and dynamically important fields. The observed linear polarisation fraction of ≃30% is smaller than the one predicted by our model (≃50%). The emission is likely beam depolarised, indicating that the flaring emission region resolves the magnetic field structure close to the black hole.

Electromagnetic signatures of strong-field gravity from accreting black holes

“…The above-mentioned behaviour of the polarization angle at certain radius close to the black hole horizon is a signature of strong gravity. Although this provides a specific evidence which in future can help us to measure the model parameters, so far its practical use has been attempted only in a few preliminary cases with tentative results, such as the SMBH in Sagittarius A*, where the near-infrared polarimetry has been achieved from ground-based observations (Meyer et al, 2006;Shahzamanian et al, 2015;Gravity Collab. et al, 2020b), and M87 nucleus with the EHT in mm domain (Kravchenko et al, 2020).…”

Polarized Image of Equatorial Emission in the Kerr Geometry

“…An important EHT observing target is the black hole Sgr A* at the center of our own galaxy. The polarization of Sgr A* has significant time variability in both submillimeter [52][53][54] and near-infrared [55][56][57][58] observations, with particularly rapid variability in near-infrared flares. The flares are likely born out of various plasma and MHD effects such as magnetic reconnection [59,60].…”

“…In particular, using infrared interferometry, the GRAVITY collaboration has recently reported the first resolved centroid motion and polarization during flares of Sgr A* [57]. Both the centroid and polarization traced loops over time, which were interpreted using a model of an orbiting equatorial hotspot [57,58]. In simulations of motion of a small Gaussian hotspot, [57] (see Appendix D therein) found that the presence of a single polarization loop in Q, U , in which the orbital period in polarization is the same as the orbital period of the hotspot, is a signature of magnetic fields perpendicular to the orbital plane (i.e.…”

“…Follow-up work in [58] considered the July 28 flare in further detail, and compared observations to simulations of a Gaussian hotspot as well as a simplified non-relativistic analytic model of a point emitter, finding that a single loop in Q, U arises only from fields with a nonzero vertical component, with the best fit to the July 28 flare having a vertical plus azimuthal field. We substantiate the same claims in [57,58] about single (double) loops arising from vertical (equatorial) magnetic fields using analytic results in the Kerr geometry, providing additional physical insight into GRAVITY observations and the general observational distinction between vertical and equatorial fields.…”

“…Section V considers the observed EVPA for black holes viewed on-axis, quantifying the effects of spin on EVPA. Section VI considers the application of our model to the polarized images of orbiting hotspots and provides additional details of comparisons with [57,58]. Section VII gives a brief summary of our conclusions and directions for future investigation.…”