The stringent upper limit on jet power in the persistent soft state source 4U~1957+11

TL;DR

This paper sets very strict upper limits on radio emission from the persistent soft state black hole candidate 4U 1957+11, indicating jet suppression stronger than theoretical models predict and supporting its classification as a black hole.

Contribution

It provides the deepest radio upper limits for 4U 1957+11 and demonstrates jet suppression levels exceeding current thin disk models, reinforcing its black hole nature.

Findings

Radio emission is at least 1500 times weaker than in hard state correlations.

No Type I bursts detected, supporting black hole accretor hypothesis.

Source likely formed in the halo or received a strong natal kick.

Abstract

We present extremely deep upper limits on the radio emission from 4U~1957+11, an X-ray binary that is generally believed to be a persistently accreting black hole that is almost always in the soft state. We discuss a more comprehensive search for Type I bursts than in past work, revealing a stringent upper limit on the burst rate, bolstering the case for a black hole accretor. The lack of detection of this source at the 1.07 $\mu$Jy/beam noise level indicates jet suppression that is stronger than expected even in the most extreme thin disk models for radio jet production -- the radio power here is 1500--3700 times lower than the extrapolation of the hard state radio/X-ray correlation, with the uncertainties depending primarily on the poorly constrained source distance. We also discuss the location and velocity of the source and show that it must have either formed in the halo or with a strong asymmetric natal kick.