iShocks: X-ray binary jets with an internal shocks model

📝 Abstract

In the following paper we present an internal shocks model, iShocks, for simulating a variety of relativistic jet scenarios; these scenarios can range from a single ejection event to an almost continuous jet, and are highly user configurable. Although the primary focus in the following paper is black hole X-ray binary jets, the model is scale and source independent and could be used for supermassive black holes in active galactic nuclei or other flows such as jets from neutron stars. Discrete packets of plasma (or `shells’) are used to simulate the jet volume. A two-shell collision gives rise to an internal shock, which acts as an electron re-energization mechanism. Using a pseudo-random distribution of the shell properties, the results show how for the first time it is possible to reproduce a flat/inverted spectrum (associated with compact radio jets) in a conical jet whilst taking the adiabatic energy losses into account. Previous models have shown that electron re-acceleration is essential in order to obtain a flat spectrum from an adiabatic conical jet: multiple internal shocks prove to be efficient in providing this re-energization. We also show how the high frequency turnover/break in the spectrum is correlated with the jet power, $\nu_b \propto L_{\textrm W}^{\sim 0.6} $, and the flat-spectrum synchrotron flux is correlated with the total jet power, $F_{\nu}\propto L_{\textrm W}^{\sim 1.4} $. Both the correlations are in agreement with previous analytical predictions.

💡 Analysis

In the following paper we present an internal shocks model, iShocks, for simulating a variety of relativistic jet scenarios; these scenarios can range from a single ejection event to an almost continuous jet, and are highly user configurable. Although the primary focus in the following paper is black hole X-ray binary jets, the model is scale and source independent and could be used for supermassive black holes in active galactic nuclei or other flows such as jets from neutron stars. Discrete packets of plasma (or `shells’) are used to simulate the jet volume. A two-shell collision gives rise to an internal shock, which acts as an electron re-energization mechanism. Using a pseudo-random distribution of the shell properties, the results show how for the first time it is possible to reproduce a flat/inverted spectrum (associated with compact radio jets) in a conical jet whilst taking the adiabatic energy losses into account. Previous models have shown that electron re-acceleration is essential in order to obtain a flat spectrum from an adiabatic conical jet: multiple internal shocks prove to be efficient in providing this re-energization. We also show how the high frequency turnover/break in the spectrum is correlated with the jet power, $\nu_b \propto L_{\textrm W}^{\sim 0.6} $, and the flat-spectrum synchrotron flux is correlated with the total jet power, $F_{\nu}\propto L_{\textrm W}^{\sim 1.4} $. Both the correlations are in agreement with previous analytical predictions.

📄 Content

arXiv:0909.1309v1 [astro-ph.HE] 7 Sep 2009 Mon. Not. R. Astron. Soc. 000, 1–13 (2009) Printed 23 October 2021 (MN LATEX style file v2.2) iShocks: X-ray binary jets with an internal shocks model O. Jamil1 ⋆, R. P. Fender1 and C. R. Kaiser 1University of Southampton, U.K. Accepted 03 September 2009 ABSTRACT In the following paper we present an internal shocks model, iShocks, for simulating a variety of relativistic jet scenarios; these scenarios can range from a single ejection event to an almost continuous jet, and are highly user configurable. Although the pri- mary focus in the following paper is black hole X-ray binary jets, the model is scale and source independent and could be used for supermassive black holes in active galac- tic nuclei or other flows such as jets from neutron stars. Discrete packets of plasma (or ‘shells’) are used to simulate the jet volume. A two-shell collision gives rise to an internal shock, which acts as an electron re-energization mechanism. Using a pseudo- random distribution of the shell properties, the results show how for the first time it is possible to reproduce a flat/inverted spectrum (associated with compact radio jets) in a conical jet whilst taking the adiabatic energy losses into account. Previous models have shown that electron re-acceleration is essential in order to obtain a flat spectrum from an adiabatic conical jet: multiple internal shocks prove to be efficient in providing this re-energization. We also show how the high frequency turnover/break in the spec- trum is correlated with the jet power, νb ∝L∼0.6 W , and the flat-spectrum synchrotron flux is correlated with the total jet power, Fν ∝L∼1.4 W . Both the correlations are in agreement with previous analytical predictions. Key words: X-Ray binaries, jets, internal shocks, adiabatic losses, synchrotron spec- trum, infra-red, radio, flat/inverted spectrum 1 INTRODUCTION In recent years there has been a lot of interest in probing the disc-jet connection in a variety of astrophysical objects. The mechanisms behind the jet formation are still not fully understood, leaving many open questions: the origin of a flat spectrum (α ∼0 when Fν ∝να) is one such question. The flat spectra have been observed both in active galactic nuclei (AGN) (for a review see Cawthorne 1991) and X-ray bina- ries (XRBs) (Fender 2001); in the XRBs it has been seen to extend from Radio to near infra-red (Corbel & Fender 2002). It is thought that this spectrum originates from the jet via the partially self absorbed synchrotron emission. The Blandford & K¨onigl (1979) model attempts to ex- plain how such a flat spectrum could arise. In their model, the jet is assumed to be conical with the magnetic field per- pendicular to the jet axis and frozen in plasma. For a given frequency, an increase in the jet volume causes a decrease in the plasma optical depth. The inner, denser, parts of jets are optically thick to lower frequencies (e.g. radio): the higher the energy density of the jet volume, the higher the op- tical depth (for a given frequency). The radio frequencies ⋆E-mail: oj1@soton.ac.uk therefore peak in the outer parts of the jet, while the infra- red peak in the inner parts of the jet. The one drawback of the Blandford & K¨onigl (1979) model is the artificial re- plenishment of the adiabatic energy losses suffered by the jet plasma. Marscher (1980) showed a model that takes electron energy losses into account, but are unable to reproduce a flat spectrum; Hjellming & Johnston (1988) presented a model where it is possible to obtain a flat spectrum under slowed expansion for a conical jet. A comprehensive study by Kaiser (2006) (more recently Pe’er & Casella 2009) shows that if the adiabatic losses are not replenished then it is impossible to obtain a flat spectrum from a conical jet. Kaiser (2006) also show that using a non-conical jet volume, it is possi- ble to recover a flat spectrum even with energy losses: the jet geometry requires fine tuning to minimize the adiabatic losses normally associated with a conical jet, while allowing enough of a change in volume to drive the changes in optical depth. In the following paper, we present a model that ad- dresses the problem of electron re-energization via a large number of ‘small’ shell collisions. Although the main focus in this paper is on black hole XRB jets, the model is not re- stricted to such systems: the model is scale independent and AGN jet volumes can also be simulated. The first part of the 2 O. Jamil , R. Fender and C. Kaiser paper goes through the details of the model, outlining the physics and the techniques employed. The results section is split into different jet scenarios: single ejection, double ejec- tion (single collision/internal shock), and multiple ejections (multiple internal shocks). The results from these increas- ingly complex scenarios are used to demonstrate the model’s capabilities, in addition to exploring the internal physics of the relativistic jets. 2 THE MODEL Our model is

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