[Paper Review] A jet model for the broadband spectrum of XTE J1118+480: Synchrotron emission from radio to X-rays in the Low/Hard spectral state

Sera Markoff, H. Falcke+1 more|arXiv (Cornell University)|Oct 27, 2000

Astrophysical Phenomena and Observations17 references304 citations

TL;DR

This paper proposes that the broadband spectrum of XTE J1118+480 in the Low/Hard state is primarily explained by synchrotron emission from a relativistic, adiabatically expanding jet, with the X-ray power-law component arising from optically thin synchrotron emission in a shock acceleration region. The model successfully fits the radio to X-ray spectrum using physically reasonable jet parameters, suggesting a canonical spectral cutoff near 100 keV determined by synchrotron cooling losses.

ABSTRACT

Observations have revealed strong evidence for powerful jets in the Low/Hard states of black hole candidate X-ray binaries. Correlations, both temporal and spectral, between the radio -- infrared and X-ray bands suggest that jet synchrotron as well as inverse Compton emission could also be significantly contributing at higher frequencies. We show here that, for reasonable assumptions about the jet physical parameters, the broadband spectrum from radio through X-rays can be almost entirely fit by synchrotron emission. We explore a relatively simple model for a relativistic, adiabatically expanding jet combined with a truncated thermal disk conjoined by an ADAF, in the context of the recently discovered black hole binary XTE J1118+480. In particular, the X-ray power-law emission can be explained as optically thin synchrotron emission from a shock acceleration region in the innermost part of the jet, with a cutoff determined by cooling losses. For synchrotron cooling-limited particle acceleration, the spectral cutoff is a function only of dimensionless plasma parameters and thus should be around a ``canonical'' value for sources with similar plasma properties. It is therefore possible that non-thermal jet emission is important for XTE J1118+480 and possibly other X-ray binaries in the Low/Hard state.

Motivation & Objective

• To explain the broadband radio-to-X-ray spectrum of XTE J1118+480 in the Low/Hard state without relying solely on inverse Compton emission.
• To test whether synchrotron emission from a relativistic jet can account for the observed flat radio spectrum and X-ray power-law component.
• To investigate the role of synchrotron cooling in setting the spectral cutoff in the X-ray band for jet-dominated emission.
• To assess the viability of a jet model in place of or alongside the standard thermal disk plus coronal inverse Compton model for low-luminosity black hole XRBs.
• To determine whether the observed X-ray cutoff at ~100 keV is a natural outcome of plasma parameters like the cooling timescale and shock velocity.

Proposed method

• Model a relativistic, adiabatically expanding jet coupled with a truncated thermal disk and ADAF-like inner flow, using physical parameters derived from observations of XTE J1118+480.
• Assume that the X-ray power-law originates from optically thin synchrotron emission in a shock acceleration region at the jet base, with the spectral shape governed by electron energy distribution and cooling.
• Use synchrotron cooling losses as the dominant energy loss mechanism for electrons, determining the high-energy cutoff of the spectrum via the dimensionless parameter ξ, which depends on magnetic field and electron energy.
• Fit the observed broadband spectrum (radio to X-rays) using a combination of optically thick synchrotron (radio to IR) and optically thin synchrotron (X-rays) components, with seed photons from the inner disk.
• Fix key parameters such as black hole mass (6 M☉), distance (1.8 kpc), disk temperature (1.5×10⁵ K), and jet luminosity (2.6×10³⁶ erg s⁻¹), based on observational constraints.
• Compare model predictions with multi-wavelength data, including EUV, radio, and X-ray observations, and assess the role of inverse Compton emission, which is found to be negligible under the assumed conditions.

Experimental results

Research questions

• RQ1 Can synchrotron emission from a jet reproduce the broadband radio-to-X-ray spectrum of XTE J1118+480 in the Low/Hard state without invoking inverse Compton emission?
• RQ2 What physical conditions in the jet lead to a spectral cutoff near 100 keV, and is this cutoff naturally explained by synchrotron cooling?
• RQ3 How does the presence of a weak thermal disk affect the electron cooling timescale and the dominance of synchrotron over inverse Compton emission in the X-ray band?
• RQ4 Is the observed X-ray power-law in XTE J1118+480 consistent with being optically thin synchrotron emission from a shock-accelerated electron population?
• RQ5 Can the observed spectral turnover in the radio band be used to constrain the jet length and shock location?

Key findings

• The broadband spectrum of XTE J1118+480 from radio to X-rays is well-fitted by a jet model dominated by synchrotron emission, with the X-ray power-law arising from optically thin synchrotron emission in a shock acceleration region.
• The X-ray spectral cutoff at ~100 keV is naturally explained by synchrotron cooling losses, with the cutoff energy determined by the dimensionless plasma parameter ξ, which is ~100 for a canonical cutoff.
• Inverse Compton emission is negligible in the model due to low external photon density from the weak thermal disk, making synchrotron cooling the dominant energy loss mechanism.
• The flat radio spectrum is attributed to optically thick synchrotron emission from the jet at larger radii, while the X-ray power-law is produced by a shock at z ≈ 45 rₛ.
• The model predicts a spectral turnover in the radio band at ~2–15 GHz, consistent with a jet length of ~4×10¹³ cm (~1.5 mas at 1.8 kpc), supporting the jet origin of the radio emission.
• The model is scalable and likely applicable to other X-ray binaries in the Low/Hard state with flat radio spectra, suggesting that jet-dominated synchrotron emission may be a common feature in such systems.