An upper-limit on the linear polarization fraction of the GW170817 radio continuum
Alessandra Corsi, Gregg W. Hallinan, Davide Lazzati, Kunal P. Mooley, Eric J. Murphy, Dale A. Frail, Dario Carbone, David L. Kaplan, Tara Murphy, Shrinivas R. K… 展开作者
Alessandra Corsi, Gregg W. Hallinan, Davide Lazzati, Kunal P. Mooley, Eric J. Murphy, Dale A. Frail, Dario Carbone, David L. Kaplan, Tara Murphy, Shrinivas R. Kulkarni, Kenta Hotokezaka
We present late-time radio observations of GW170817, the first binary neutron star merger discovered through gravitational waves by the advanced LIGO and Virgo detectors. Our observations, carried out with the Karl G. Jansky Very Large Array, were optimized to detect polarized radio emission, and thus to constrain the linear polarization fraction of GW170817. At an epoch of ~244 days after the merger, we rule out linearly polarized emission above a fraction of ~12% at a frequency of 2.8 GHz (99% confidence). Within the structured jet scenario (a.k.a. successful jet plus cocoon system) for GW170817, the derived upper-limit on the radio continuum linear polarization fraction strongly constrains the magnetic field configuration in the shocked ejecta. We show that our results for GW170817 are compatible with the low level of linear polarization found in afterglows of cosmological long gamma-ray bursts. Finally, we discuss our findings in the context of future expectations for the study of radio counterparts of binary neutron star mergers identified by ground-based gravitational-wave detectors.
Continuum Foreground Polarization and Na~I Absorption in Type Ia SNe
我们提出了一个研究的连续偏振超过400- 600 nm范围内的19 Ia型SNe获得的FORS在VLT。我们把它们分成两类，一类是在宿主速度下显示Na I D线的，另一类是不显示Na I D线的。钠样品在最大光附近的连续偏振显示了很宽的值范围，从SN 2006 X这样的极端偏振情况到SN 2011 ae这样的几乎未偏振的情况。非钠样品通常显示出较小的极化值。钠样品在400- 600 nm范围内的连续偏振与波长呈线性关系，可用平均偏振度（P $_{\rm{mean}}$）表征。它的值跨越很宽的范围，并显示出与颜色，颜色过剩和消光在视觉波段的线性相关性。较大的色散相关性被发现与Na I D和Ca II H & K线的等效宽度，以及P $_{\rm{mean}}$和$R_{V}$之间的噪声关系，总的选择性消光比。红色SNe显示更强的连续偏振，具有更大的颜色过量和重叠。我们还证实，高连续极化与小值的$R_{V}$。消光和偏振--以及偏振角--之间的相关性表明，尘埃偏振的主要部分是在宿主星系的星际区域中留下的印记。我们发现，Na I D线的前景物质在SN主机通常与非星系ISM，挑战典型的假设前景星际极化模型。
