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Sources of Relativistic Jets in the Galaxy

I. Felix Mirabel, Luis F. Rodriguez

arXiv:astro-ph/9902062v1astro-ph

TL;DR

The paper examines how relativistic jets from Galactic compact-object binaries can be identified and understood through coordinated X-ray and radio observations. It uses microquasars and two-sided ejecta to connect accretion-disk behavior with relativistic outflows and to constrain jet kinematics. The resulting picture supports relativistic bulk motions and suggests future distance and black-hole-spin applications.

  • Problem

    The paper addresses how Galactic compact-object binaries produce relativistic jets and how their nearby, observable behavior can illuminate jets and accretion around more distant black holes.

  • Method

    It combines hard-X-ray, radio, infrared, and other multiwavelength observations with special- and general-relativistic interpretations of jet motions and accretion behavior.

  • Results

    Two-sided microquasar ejecta are inferred to consist mainly of matter moving with relativistic bulk motions, while multiwavelength observations connect accretion-disk oscillations with plasma-cloud ejections.

  • Takeaways & Limitations

    Microquasars provide nearby laboratories for studying accretion disks and relativistic jets and may enable distance measurements and investigations of black-hole spin.

  • Takeaways & Limitations

    The fraction of inner accretion-disk mass that disappears through a black-hole horizon remains unclear because mass estimates are uncertain.

Abstract

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Black holes of stellar mass and neutron stars in binary systems are first detected as hard X-ray sources using high-energy space telescopes. Relativistic jets in some of these compact sources are found by means of multiwavelength observations with ground-based telescopes. The X-ray emission probes the inner accretion disk and immediate surroundings of the compact object, whereas the synchrotron emission from the jets is observed in the radio and infrared bands, and in the future could be detected at even shorter wavelengths. Black-hole X-ray binaries with relativistic jets mimic, on a much smaller scale, many of the phenomena seen in quasars and are thus called microquasars. Because of their proximity, their study opens the way for a better understanding of the relativistic jets seen elsewhere in the Universe. From the observation of two-sided moving jets it is inferred that the ejecta in microquasars move with relativistic speeds similar to those believed to be present in quasars. The simultaneous multiwavelength approach to microquasars reveals in short timescales the close connection between instabilities in the accretion disk seen in the X-rays, and the ejection of relativistic clouds of plasma observed as synchrotron emission at longer wavelengths. Besides contributing to a deeper comprehension of accretion disks and jets, microquasars may serve in the future to determine the distances of jet sources using constraints from special relativity, and the spin of black holes using general relativity.

1. JETS IN ASTROPHYSICS

Relativistic synchrotron jets arise in compact-object X-ray binaries and provide Galactic, small-scale analogues of quasar jets. Multiwavelength observations identify microquasars and connect their accretion disks, high-energy emission, and relativistic outflows.

  • 1. JETS IN ASTROPHYSICS: SS433 provided the first confirmed Galactic radio jets when time-dependent imaging verified a model of two precessing, collimated outflows moving at 0.26c.The system established that binary stellar systems can produce jet-like ejecta on much smaller scales than galactic nuclei.
  • 1. JETS IN ASTROPHYSICS: About 200 Galactic X-ray binaries are known, roughly 10 percent are radio-loud, and nine have shown evidence of relativistic synchrotron jets.The review focuses on this subset of radio-emitting X-ray binaries.
  • 1. JETS IN ASTROPHYSICS: Relativistic synchrotron jets in this review have velocities v ≥0.1c and occur in X-ray binaries containing neutron stars or black holes.This distinguishes them from stellar jets with non-relativistic velocities and predominantly thermal emission.
  • 2. MICROQUASARS: The discovery of microquasars followed hard-X-ray and radio observations, revealing sources whose compact cores and two-sided jets resemble distant radio galaxies.Examples near the Galactic center include 1E1740.7-2942 and GRS 1758-258, persistent X-ray and relativistic-jet sources with weak extended lobes.
  • 2. MICROQUASARS: Microquasars are stellar-mass black-hole binaries whose hard X-rays trace accretion flows while radio and other synchrotron emission traces relativistic jets.Their jets and accreting black holes reproduce key quasar ingredients on much smaller spatial and temporal scales.
  • 2. MICROQUASARS: Microquasars extend quasar-like accretion and jet physics across enormous mass, length, and time scales while allowing nearby observations of both components.Their proximity motivates using them to study accreting black holes and relativistic jets more directly than distant systems.

3. SUPERLUMINAL SOURCES

Galactic X-ray binaries provide two-sided views of apparently superluminal ejecta, resolving whether the motions represent waves or relativistic bulk plasma. Observations find relativistic speeds, source-dependent asymmetries, and interactions with surrounding material.

  • 3. SUPERLUMINAL SOURCES: Two-sided Galactic ejecta show that apparent superluminal motions can reflect relativistic bulk motion rather than waves propagating through slowly moving jets.The bilateral geometry removes an ambiguity that persists for one-sided extragalactic jets.
  • 3. SUPERLUMINAL SOURCES: GRS 1915+105’s binary nature remains difficult to establish optically because heavy dust extinction prevents optical study and leaves competing companion interpretations.The source is nevertheless likely to harbor a black hole based on its X-ray properties and mass-related luminosity arguments.
  • 3. SUPERLUMINAL SOURCES: GRS 1915+105 repeatedly produces collimated, Doppler-boosted ejecta whose proper-motion and brightness asymmetries support anti-parallel twin clouds moving relativistically.The jets are already collimated at about 10 AU, while discrete ejecta appear near 500 AU and can move ballistically.
  • 3. SUPERLUMINAL SOURCES: GRO J1655–40 has intrinsic velocities greater than 0.9c, but its alternating brightness asymmetries cannot be explained by simple relativistic Doppler boosting.Its jets appear intrinsically asymmetric, with the asymmetry changing between events.
  • 3. SUPERLUMINAL SOURCES: XTE J1748-288 provides a Galactic case in which relativistic jets collide with environmental material, decelerate, and brighten at their leading edge.At an assumed 8 kpc distance, the observed motions imply apparent speeds of 0.9c and 1.5c and intrinsic velocities above 0.9c.

4. SPECIAL RELATIVITY EFFECTS

Special-relativistic relations connect the proper motions, flux asymmetries, and frequency shifts of twin ejecta to their speed, orientation, and distance. These constraints explain observed brightness differences and can yield source distances or cosmological tests.

  • 4.1 Parameters of the Ejection: The ejection model represents twin condensations moving at speed βc along an axis inclined by θ, with µa and µr describing their apparent proper motions.These relations provide the kinematic inputs for determining intrinsic speed, orientation, and distance.
  • 4.1 Parameters of the Ejection: The proper motions of approaching and receding ejecta constrain the distance through their relativistic geometry, with proper motions alone providing an upper limit.For GRS 1915+105, the resulting upper limit D ≤13.7 kpc agrees with its measured distance of 12.5±1.5 kpc.
  • 4.2 A Relativistic Distance Determination: Doppler factors are ratios of observed to emitted frequency, and measuring either approaching or receding frequency shift together with β cos θ determines β, θ, and the distance.A known spectral line from either condensation would therefore permit a precise distance determination.
  • 4.2 A Relativistic Distance Determination: The same relativistic relations can be applied to distant ejecta pairs using angular-size distance and redshift corrections, potentially testing different cosmological models.The Galactic formulation replaces physical distance with angular-size distance for cosmological sources.

5. ACCRETION DISK INSTABILITIES AND JET FORMATION

Multiwavelength observations connect accretion-disk instabilities in X-rays with synchrotron-emitting plasma ejections, while showing that hard-X-ray activity alone does not guarantee jet formation.

  • Disk–jet connection: Hard-X-ray activity is necessary but not sufficient for collimated synchrotron jets, because some unusual activity and sudden flux drops lack associated radio ejections.GRO J1655-40 also showed hard-X-ray outbursts without following radio flare/ejection events.
  • Accretion-flow interpretation: The inner-disk instability may advect most dissipated energy into a black hole, whereas a neutron-star surface reradiates deposited thermal energy as a slower-decaying X-ray signal.In the advection-dominated mode, ion–electron energy transfer is slower than infall, so energy is stored and transported inward.
  • Multifrequency timing: Infrared emission precedes radio flares, while twin radio peaks show wavelength-independent delays of 70±20 minutes and no Doppler boosting.These observations support expanding clouds moving in opposite directions with non-relativistic bulk motions.
  • Disk–jet connection: X-ray dips and recoveries in GRS 1915+105 are consistently associated with the emergence of relativistic plasma clouds and subsequent infrared and radio flares.The ejection appears during replenishment of the inner disk after the dip, with particle injection lasting up to tens of minutes.
  • Mass budget: The estimated minimum cloud mass is ∼10^19 g versus ∼10^21 g removed from the inner disk per cycle, leaving the fraction crossing the horizon uncertain.The authors consider it plausible that most disappearing inner-disk material is advected into the black hole and only some is propelled into synchrotron-emitting clouds.
  • Energetics: Synchrotron luminosity in the oscillations reaches at least 10^36 erg s−1, making the jet emission non-negligible compared with the thermal X-ray luminosity.The results support infrared synchrotron jets extending to distances of a few thousand AU from GRS 1915+105.

6. JET FORMATION

Jet models combine magnetohydrodynamic acceleration and collimation, but observations and simulations indicate unresolved stability and formation-mechanism problems.

  • MHD acceleration and collimation: MHD models accelerate plasma centrifugally and collimate it through magnetic hoop stress as wound-up toroidal fields exert an inward force.The outflow begins with a wide-angle equatorial component and becomes predominantly poloidal on larger scales.
  • Collimation limits: The toroidal field traditionally invoked for collimation may be unstable, prompting proposals that the poloidal magnetic field supplies the collimating action.The instability is described as preventing effective collimation by the traditional mechanism.
  • Relativistic simulations: General-relativistic MHD simulations produce a two-layer jet with an inner fast gas-pressure-driven component and an outer slow magnetically driven component.The inner component arises from shocks and strong pressure increases in rapidly advecting flows inside the last stable orbit, a feature absent from non-relativistic calculations.
  • Jet velocities: Measured jet velocities appear bimodal, with some sources near 0.3c and others at or above 0.9c, although the sample is small.Proposed explanations relate terminal speed either to the Keplerian velocity at the footpoint or to a magnetic switch.
  • Transient ejections: Steady-state MHD models may explain continuous relativistic jets, but disk disappearance followed by sudden condensate ejection may require a different mechanism.The observations motivate new jet-formation models that incorporate the transient behavior of stellar jets.
  • Particle production: Most X-ray binaries are radio-quiet, implying that relativistic electrons or magnetic fields are not always sufficiently present for synchrotron emission.Jet models must therefore account not only for acceleration and collimation but also for producing relativistic emitting particles.

7. SYNCHROTRON EMISSION

Synchrotron emission from expanding jet clouds can be modeled through their spectral and temporal evolution, while observations require constrained expansion, continued injection, and substantial energetics.

  • Expanding-cloud models: The simple van der Laan model gives Sν ∝ν−0.7 t−4.8 for p=2.4 and fits the flux decrease of several radio-emitting X-ray binaries.The model assumes an optically thin, spherical cloud expanding linearly with time.
  • Constrained expansion: Milliarcsecond maps show that GRS 1915+105 clouds expand mainly in one direction at hundreds of AU, supporting constrained rather than three-dimensional expansion.Constrained expansion can produce shallower flux declines than the simplest spherical model.
  • Observed temporal evolution: In SS 433 and GRS 1915+105, the flux declines as t−1.3 near the source and t−2.6 beyond ∼2 × 10^17 cm, indicating a break in the expansion evolution.The change is modeled as initially slowed expansion followed by free expansion in two dimensions.
  • Particle injection: Shallower flux declines may also reflect continued particle or magnetic-field injection, including in-situ acceleration as shocked gas entrains ambient material.The optically thick rise occurs rapidly and has not yet been observed in sufficient detail for direct comparison with theory.
  • Energetics: The 1994 March 19 GRS 1915+105 event had an estimated magnetic field of about 50 mGauss, electron energy of about 4 × 10^43 ergs, and minimum mechanical power of ∼5 × 10^38 erg s−1.The required power was comparable to the source’s maximum observed steady photon luminosity.
  • Source comparison: GRS 1915+105 injects roughly 10^23 g per year in 0.92c–0.98c outflows with Lmech ∼10^3L⊙, whereas SS 433 reaches Lmech ∼10^5L⊙ through more continuous flow.Thus GRS 1915+105 produces more energetic but more sporadic bursts.

8. POSSIBLE LABORATORIES FOR GENERAL RELATIVITY

Microquasar timing and spectroscopy provide potential probes of black-hole mass, spin, and strong-field gravity, although interpretations of the relevant frequencies remain uncertain.

  • High-frequency QPOs: The maximum stable QPO frequencies are 67 Hz in GRS 1915+105 and 300 Hz in GRO J1655-40, and they are absent during strong radio flares or jet injection.These frequencies are believed to depend on fundamental black-hole properties such as mass and spin.
  • Spin inference: If the maximum stable frequency corresponds to the last stable circular orbit, it can constrain spin when the black-hole mass is independently known.For GRO J1655-40, with a mass of 4–7 solar masses, this interpretation yields a Kerr black hole rotating at ≥70% of maximum spin.
  • Interpretive uncertainty: Alternative interpretations invoke disk precession or relativistic disk seismology, so theoretical work is needed before QPOs can robustly determine black-hole spin.The two sporadic superluminal-jet sources are inferred to contain black holes spinning close to the maximum limit under one interpretation.
  • Strong-field tests: X-ray iron-line profiles and high-frequency oscillations may probe relativistic effects in the innermost accretion flow and near the black-hole horizon.The interpretation of the maximum stable frequency remains uncertain, but its origin is believed to be close to the horizon.

9. OTHER SOURCES OF RELATIVISTIC JETS IN THE GALAXY

Galactic X-ray binaries show a broad range of jet behavior, from persistent faint radio structures to transient relativistic ejecta. Proper-motion measurements indicate faster jets from black-hole binaries than from neutron-star systems, while observational difficulties complicate source identification and monitoring.

  • Persistent X-ray sources generally have persistent radio emission, whereas transient X-ray sources produce sporadic radio outbursts and ejecta.
  • Jet studies are difficult because extended radio features may not be physically associated with the X-ray source, and transient ejecta can evolve too rapidly for follow-up.
  • Proper motions have been measured accurately in GRS 1915+105, GRO J1655-40, XTE J1748-288, and SS 433, with less accurate measurements in several additional sources.
  • Proper motions show velocities above 0.9c for several black-hole binaries, compared with velocities no greater than 0.3c for four neutron-star binaries.
  • Jet velocities may help distinguish neutron stars from black holes if near-light-speed jets are confirmed to occur only in black-hole binaries.
  • Sgr A* may host a jet, with 7-mm VLBA observations suggesting an elongated radio source extending 72 Schwarzschild radii.

10. INTERACTION OF RELATIVISTIC JETS WITH THE ENVIRONMENT

Relativistic jets transfer substantial energy to their surroundings through shocks and outflows, producing structures from compact ejecta to large radio lobes and X-ray jets. The clearest large-scale example is SS 433, while associations in other systems often remain uncertain.

  • Jet–environment interactions include deceleration and brightening in XTE J1748-288 and interaction of CI Cam jets with an HII and dust shell.
  • SS 433 jets produce X-ray structures extending about 30 arcmin and radio lobes reaching up to 1° through interaction with the interstellar medium.The sub-arcsec jets carry kinetic power of about 10^39 erg s^-1.
  • The kinetic energy transferred by SS 433 into the W50 nebula is about 2 10^51 ergs, making the jets an important part of its energy budget.
  • Evidence for large-scale associations is mixed: aligned features near GRS 1915+105 and GRO J1655-40 were not conclusively linked to their compact sources.
  • Faster but more sporadic jets in GRS 1915+105 and GRO J1655-40 may explain their lack of obvious lobes compared with SS 433.

11. MICROBLAZARS AND GAMMA-RAY BURSTS

Microblazars are expected when relativistic jets point close to the line of sight, producing strongly shortened timescales and boosted fluxes. Their study may inform gamma-ray-burst physics, although the two source classes differ substantially.

  • Known sources generally have large viewing angles, including θ ≃79° for SS 433, 66°–70° for GRS 1915+105, and 85° for GRO J1655-40.
  • For viewing angles θ ≤10°, observed timescales shorten by 2γ and flux densities are boosted by 8γ^3 relative to the condensation rest frame.
  • At v = 0.98c with γ = 5, the timescale shortens by about 10 and flux density increases by about 10^3.
  • Microblazars may be difficult to detect because small viewing angles are unlikely and their flux declines rapidly.
  • Microquasar afterglows may arise from internal shocks or from impacts between jets and interstellar matter.
  • Gamma-ray bursts differ from known microquasars because they are nonrepeating catastrophic events with much larger super-Eddington luminosities, so the mass-scaling analogy does not apply.

12. CONCLUSIONS AND PERSPECTIVES

Microquasars provide nearby laboratories for studying relativistic jets, accretion-flow instabilities, and strong-field gravity. Their two-sided ejecta establish relativistic bulk motion and support prospective distance, spin, and jet-formation diagnostics, subject to unresolved theoretical interpretations.

  • Two-sided moving jets show that microquasar ejecta have relativistic bulk motions with Lorentz factors comparable to those believed common in quasars.
  • Short microquasar timescales allow links between X-ray accretion-flow instabilities and radio, infrared, or optical synchrotron ejecta to be studied directly.
  • Proper motions combined with Doppler information can constrain jet-source distances using special relativity, as demonstrated by an upper limit of D ≤13.7 kpc.
  • Multiwavelength observations of GRS 1915+105 suggest that plasma clouds are ejected during replenishment of the inner disk after its sudden disappearance.
  • Maximum stable QPO frequencies may constrain stellar-mass black-hole spin when the mass is independently known, but competing general-relativistic interpretations require theoretical discrimination.
  • Microquasars may provide future tests of general relativity in the strong-field region near black-hole horizons.
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