Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz

Abstract

We measured the linear polarization of Sagittarius A* to be 7.2 ± 0.6% at 230 GHz using the BIMA array with a resolution of 3.6×0.9 arcsec. This confirms the previously reported detection with the JCMT 14-m antenna. Our high resolution observations demonstrate that the polarization does not arise from dust but from a synchrotron source associated with Sgr A*. We see no change in the polarization position angle and only a small change in the polarization fraction in four observations distributed over 60 days. We find a position angle 139 ± 4 degrees that differs substantially from what was found in earlier JCMT observations at the same frequency. Polarized dust emission cannot account for this discrepancy leaving variability and observational error as the only explanations. The BIMA observations alone place an upper limit on the magnitude of the rotation measure of 2 × 10 6 rad m −2 . These new observations when combined with the JCMT observations at 150, 375 and 400 GHz suggest RM = −4.3 ± 0.1 × 10 5 rad m −2 . This RM may be caused by an external Faraday screen. Barring a special geometry or a high number of field reversals, this RM rules out accretion rates greater than ∼ 10 −7 M ⊙ y −1 . This measurement is inconsistent with high accretion rates necessary in standard advection dominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for low accretion rates as a major factor in the overall faintness of Sgr A*.

Stellar winds near massive black holes – the case of the S-stars

“…The accretion rate we find in Figure 5 is marginally consistent with the one observed, being close to the upper limits derived from observations (grey shaded area Baganoff et al 2003;Bower et al 2003;Nayakshin 2005). This can be caused by overestimating the mass loss rate of the stars or not treating feedback and accretion onto the black hole properly.…”

“…This is in contrast with constraints from linear polarization measurements at 150 GHz and above which limit the accretion rate near the event horizon to ∼ 10 −7 − 10 −8 M⊙ year −1 (e.g. Bower et al 2003). However, the latter estimates strongly depend on the assumption of equipartition between particle and magnetic field energy of a uniform magnetic field.…”

X-ray flares from Sgr A*: Star-disk interactions?

“…Several years ago it became clear that these hot flows also drive powerful thermal winds (Blandford & Begelman 1999), so the accretion rate onto the black hole is significantly reduced. This theoretical picture seems to be confirmed in its gross features by the observations (e.g., Narayan et al 1995;Bower et al 2003;Quataert 2003), although radio jets may be an important additional ingredient of the model (Falcke & Markoff 2000;Yuan et al 2002). There are also alternative ideas that may explain the low luminosity of Sgr A * (e.g., Melia 1994;Coker & Melia 2000;Proga & Begelman 2003).…”

Variable Linear Polarization from Sagittarius A*: Evidence of a Hot Turbulent Accretion Flow

“…The mean of all BIMA observations is . 0.6% 7.5% ‫ע‬ 0.5% If we exclude the last observation from Bower et al (2003), which appears to be an outlier, then the mean polarization fraction is . (0.8 ‫ע‬ 1.6) # 10 rad m 27 and 2004 January 5 results, we find an RM of (2.9 ‫ע‬ .…”