This question may be premature.
This answer will briefly review the properties and characteristics of atsrophysical jets relevant to accelerating space craft to relativistic velocities. Particularly with relevance to an astrophysical jet generated by a supermassive black hole. This is a suitable type example for this scenario. It will also review the state of art of this propulsion mechanism.
Astrophysical jets are physical conduits along which mass, momentum,
energy and magnetic flux are channeled from stellar, galactic and
extragalactic objects to the outer medium. Geometrically, these jets
are narrow (small opening angle) conical or
cylindrical/semi-cylindrical protrusions (e.g., Das 1999). Jets are an
ubiquitous phenomenon in the universe. They span a large range of
luminosity and degree of collimation, from the most powerful examples
observed to emerge from the nuclei of active galaxies (or AGNs) to the
jets associated to low-mass young stellar objects (YSOs) within our
own Galaxy. In the intermediate scales between these two extremes one
finds evidences of outflows associated to neutron stars, massive X-ray
binary systems (with SS433 being the best example of this class),
symbiotic stars, and galactic stellar mass black holes (or
As largely stressed in the literature, most of these outflows, despite
their different physical scales and power, are morphologically very
similar, suggesting a common physical origin (see below). For example,
in one extreme, AGN jets have typical sizes ³ 10^6 pc 1, nuclear
velocities ~ c (where c is the light speed), and parent sources (which
are massive black holes) with masses 10^6-9 M 2 and luminosities
~10^43-48 L 3;
1 1 pc = 1 parsec = 3.086 10^18 cm.
2 1 M = one solar mass = 1.99 > 10^33 g
3 1 L = one solar luminosity unit = 3.826 10^33 erg/s
Source: Elisabete M. de Gouveia Dal Pino, Relativistic Jests and Outflows
The scale of astrophysical jets ranges from one parsec (pc) through kiloparsecs to one million pc. For example, this anticipates accelerating spacecraft to reativistic velocities. If a jetrider spacecraft was acceelrated to relativistic velocity by travelling along a one million parsec astrophysical jet, this is a distance of approximately three million light years, it only needs to accelerate at an average acceleration of fifty (50) metres per second per annum. This acceleration phase will take six million years. Any technological civilization carrying out projects of this kind will operating on extremely long timescales and over extremely long distances.
The complexity and intensity of energetic process involving magnetic and kinetic energy, high-energy radiation, synchrotron radiation, and particles including electrons and positrons can be found in Romero et al Reltivistic jets in Active Galactic Nuclei and Microquasars.
There are major questions about the stability and variability of astrophysical jets. The environment inside an astrophysical jet is one of the most hostile known. The long-term survival of any spacecraft could only be accomplished by extremely advanced technologically societies.
The technical literature doesn't readily yield any useful estimates for momentum transfer necessary to accelerate a spacecraft. It is recommended that if the OP wishes to obtain this information that one way to do so is by contacting the authors of scientific papers on astrophysical jets and asking them what is the estimated average momentum transfer. Since there is a nuber of different types of astrophysical jets the OP should select the type of astrophysical jet needed for the worldbuilding and make inquiries. Most scientists are delighted when someon shows an interest in their work. Even if it is some thing as quirky as translating their research into a concept like spacecraft propulsion. The OP should try to obtain information about momentum transfer per unit area. This can be translated into acceleration based on the mass of the jetrider probe.
This answer suggested this question might be premature. This is a brief note by Yoshinari Minami that he is currently undertaking research into the possibility of spacecraft using astrophysical jets as propulsion. To date, no results have been published.
A Journey to the Stars: Space Propulsion Brought About by Astrophysical Phenomena Such as Accretion Disk and Astrophysical Jet
Space Propulsion Brought About by Astrophysical Phenomena Here,
astrophysical phenomena refer mainly accretion disk and astrophysical
jet around black holes. Accretion disk is rotating gaseous disk with
accretion flow, which form around gravitating object, such as white
dwarfs, neutron stars, and black holes. At the present day, owing to
the development of observational technology, it is believed that
accretion disk causes the various active phenomena in the universe:
star formation, high energy radiation, astrophysical jet, and so on.
It should be noted; these stars such as white dwarfs, neutron stars,
and black holes have a strong magnetic field (108 Tesla－1011 Tesla).
Matter falling onto an accretion disk around black hole is ejected in
narrow jet moving at close to the speed of light like an accelerator.
Entity of the astrophysical jet is a jet of plasma gas from the active
galactic nucleus (accretion disk in there). It is said that such
astrophysical jet is held together by strong magnetic field tendrils,
while the jet's light is created by particles revolving around these
wispthin magnetic field lines. Furthermore, since the system of black
hole and accretion disk is like a gravitational power plants, the
energy of the heat and the light are produced by the release of
Although the system of accretion disk and astrophysical jet around
black holes are currently left many unresolved issues, the elucidation
of these mechanisms and principles that are common to the entire
universe may provide a new space propulsion principle. Especially, the
breaking of magnetic field lines and magnetic field reconnection are
possible to produce many kinds of charged particles such as electron
positron pairs. Generally, in a high-temperature plasma, electron -
positron pairs are readily formed by collisions between the high
energy protons, electrons, photons. Since the dynamics of the
accretion disk has been decided by a magnetic field, it is important
that solving the dynamics of the magnetic field.
The application of mechanism of accretion disk and astrophysical jet
around black holes will lead to the concrete system design of
propulsion engine and power source installed in space drive propulsion
system [1,2,5]. Author is now investigating above-stated research in
Spacecraft accelerated by an astrophysical jet are effectively riding an overpowered particle-beam. Geoffrey Landis' paper Interstellar Flight by Particle Beam provides useful information about this technology. The advantages of particle-beam propulsion include: –
A particle-beam pushed sail has many advantages as a propulsion system
for interstellar flight:
1. Light weight. Since the sail reflecting the beam is not a physical object, it can be made extremely light.
2. Large target. The actual reflecting area of a mini-magnetosphere sail is much larger than the magnetic loop itself. Therefore, the sail
can be quite large. This makes the aiming, beam stabilization, and
beam divergence problem much lower.
3. High acceleration. The limit on the acceleration of a lightsail is set by the temperature limit of the sail material (Landis, 1997,
2000). Since the active part of a mini-magnetosphere magnetic sail is,
in fact, a magnetic field, there is not a significant thermal limit.
The field is sustained by a plasma, which likewise is not subject to
melting. Only the physical magnet itself is thermally limited, and
this magnet is extremely small compared to the sail area. By
increasing the acceleration, cruise velocity can be achieved in a
shorter distance, again decreasing the requirements for beam
divergence and the aiming and stabilization difficulty.
4. Higher momentum/energy ratio. A particle beam has a much more momentum per unit energy than a photon (e.g., laser) beam, and hence
transfers force to a sail with better energy efficiency. The
(relativistically correct) relation between momentum (p) and energy
(E) for both particle beams and photon beams is:
p^2 = E^2/c^2 + moE (1)
(note that energy here refers only to the applied kinetic energy of
the beam, not including the rest-energy mc^2 of the particles). For a
photon beam, the rest mass mo is zero. It is clear that the ratio of
momentum to energy increases directly as the rest mass of the
particles composing the beam increases. Since the ratio of momentum to
energy equals force produced on the sail per unit of beam power, a
particle-beam pushed sail has a higher energy efficiency.
The proposed mechanism for particle-beam propulsion is the use of a mini-magnetosphere.
the invention of the mini-magnetosphere plasma propulsion, or "M2P2"
(Winglee et al. 2000, 2001), has brought the idea of a particle-beam
pushed sail closer to reality. The particle beam is reflected by a
magnetic field. In the mini-magnetosphere, the magnetic field is
inflated to large areas by the injection of a plasma, and hence large
magnetic field areas are possible with only a small physical
One deficiency of a mini-magnetosphere is that the plasma will slowly leak away. One way of replenishing this will be capturing some of the plasma in the astrophysical jet itself.
Many astrophysical jets themselves have velocities close that of lightspeed. This suggests the kinetic energy and momentum that can be transferred to an accelerating spacecraft will be high. This makes the possibility a jetrider vehicle attaining relativistic velocities is itself high. Any propulsion technology that advantage of this will need to be extremely robust.
This suggests that if the advanced technological society launching probes via astrophysical jets is sufficently advanced it construct a pusher plate made of nuclear dense materials. Effectively, a thin sheet of neutronium. This will certainly survive relativistic plasma, high-energy radiation, and even electron-positron plasma. This does assume extremely highly advanced technology, but the aliens didn't possess suitable technology they wouldn't attempt using astrophysical jetriding vehicles of any kind.
While this add considerable mass to any spacecraft if that probe is riding a one megaparsec astrophysical jet even low rates of acceleration will eventually result in relativistic velocity.
Many question of the issues the OP wanted to find answers are not readily available without more direct inquiry, research into space vehicles propelled by astrophysical jets is only beginning, and the level of technology required for viable spacecraft operating in the environment of astrophysical jets is so far beyond any conception of our current state of knowledge that it is simpler to assume the aliens with jetrider technology can do it without any explanation.