A Temporal Analysis Indicates a Mildly Relativistic Compact Jet in GRS~1915+105

TL;DR

This study estimates the bulk velocity of the compact jet in GRS~1915+105 using temporal and spatial analysis during high plateau states, suggesting mildly relativistic speeds around 0.17 to 0.43 times the speed of light.

Contribution

It provides the first combined temporal and spatial analysis to constrain the jet velocity in GRS~1915+105, narrowing down previous uncertain estimates.

Findings

Jet speed estimated between 0.17c and 0.43c

Consistent with previous radio image estimates

Supports the mildly relativistic nature of the jet

Abstract

Most of our knowledge of the radio morphology and kinematics of X-ray binary partially synchrotron self-absorbed compact jets (hereafter compact jets) is based on the observations of GRS~1915+105 which has the most prominent compact jet. Yet, the compact jet bulk velocity, $v$, is poorly constrained in the literature, $0.07 < v/c < 0.98$. In spite of this uncertainty, compact jets are often unified with relativistic jets in active galactic nuclei. We have estimated $v$ as part of a temporal analysis of GRS~1915+105 jets in "high plateau states" (HPS). We define the HPS as a state showing a hard X-ray spectrum and low level of long-term ($>10$s) X-ray activity associated with 15 GHz flux density $>70$ mJy for $>7$ consecutive days. The radio emission is associated with compact jet emission. Two HPS were monitored at 15 GHz during their termination with e-folding times of 3.8 hrs and 8.6 hrs. We combine this time scale with the scale of spatial variation of the linear source of a VLBA image preceding the fade of one of these HPS in order to estimate the jet speed. Our assumption that the reduction in radio emissivity propagates as an approximate discontinuity down the HPS jet (leaving a weak jet in its wake) indicates $0.17 <v/c< 0.43$. This agrees closely with the only other existing $v$ estimates that are derived directly from radio images, jet asymmetry produced by Doppler enhancement.