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GRS1915+105: The Fastest Fireball in the Galaxy

by John G. Cramer

Alternate View Column AV-71
Keywords: quasar superluminal jet relativistic illusion red blue shift
Published in the April-1995 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 9/20/94 and is copyrighted 1994 by John G. Cramer.
All rights reserved. No part may be reproduced in any form withoutthe explicit permission of the author.

Our galaxy contains a remarkable "faster-than-light" object discovered only a few months ago by radio astronomers. The object, a blob of hot radio-emitting matter thrown off from gamma ray source GRS1915+105, was observed to separate from its larger parent object at an apparent speed of 1.25 times the velocity of light. The operant word here is "apparent". The faster-than-light separation speed (separation distance divided by time) is believed to be a consequence of the same relativistic illusion previously observed for radio objects ejected by quasars. We will start with some facts about quasars.

Quasars are the brightest and most distant objects in the visible universe. Their light output is enormous, and shows fast time-variations indicating that they are relatively small. They emit radio waves as well as light. The technique of long-baseline interferometry with radio waves, using many radio telescopes on several continents, has permitted radio astronomers to "dissect" quasars, observing them with much better angular resolution than is possible with visible light.

These fine-grain observations produced a result that sent shock waves through the astrophysics community of the 1970's: the quasar 3C345 showed up in the high-resolution observations as two distinct radio "hot spots" that, in observation made between 1969 and 1976, moved apart as a function of time with a separation velocity of 8 times the velocity of light! This fast radio object was given the label "superluminal". The same superluminal phenomenon has since been observed for over half of all known quasars.

The orthodox explanation of superluminal objects, now widely accepted by the astrophysics community, is that the large velocity is an illusion produced by relativity when the emitting source travels at near light-speed at a small angle with the line-of-sight to the observer.

To understand the superluminal illusion we will need to examine its geometry and algebra. Suppose the blob leaves the source in a direction that makes an angle q to the observer's line of sight, with a velocity bc, where c is the speed of light and b is the object's velocity as a fraction of c. Assume the blob emits one burst of light at time zero in the observer's time reference frame, and a second burst of light at a later time t (after having traveled a distance bct). We will assume that the second light flash is emitted at the instant when the emitting blob is a distance D from the observer, in the observer's reference frame.

The light from the first light burst must travel a distance bct Cosq +D, and so reaches the observer at time T1 = bt Cosq + D/c. Light from the second light burst reaches the observer at time T2 = t + D/c. The difference in time of arrival will be dT = (T2 - T1) = t (1 - b Cosq). The observer sees the projected separation of the two light bursts as dX = bct SinA, so the apparent velocity of the light-emitting source is vapp = dX/dT = c [b Sinq/(1 - b Cosq)]. If the object is traveling near the speed of light (b1) and q is very small, then using the small angle approximation, vapp 2c/q.  If the direction of the object's velocity is close to that of the line of sight, then the angle  will be much less than 1 radian (~57o), and vapp can be many times larger than c. The superluminal velocity is an illusion that occurs because the space separation dX of the flashes depends on q, while their time difference dT depends on q2. The apparent velocity, the ratio of these quantities, depends on 1/q and becomes very large when q is small.

The light from an approaching source will also be shifted to higher frequencies because of the Doppler effect. The approaching source will also appear brighter because Lorentz contraction produces a forward "beaming" of the light from the source, a relativistic effect. In other words, the apparent faster-than-light velocities of superluminal objects can be explained by relativity and geometry, provided we assume that quasars have a high probability of spitting out energetic blobs of hot gas in our direction. However, quasars are denizens of the early universe and are separated from our galaxy by an appreciable fraction of the diameter of the universe. Their superluminal fireworks show is very far from us in time and space.

Within our own galaxy, the record for the fastest speed of travel by matter has been held by the object called SS433, a famous galactic source of radio jets. A study of the spectral lines in the visible light from SS433 showed that it had three light components, a normal component, a red-shifted component, and a blue-shifted component. This, along with the elongated radio image of SS433, led to a model of the object as a central massive core that was emitting twin jets of hot matter, one moving rapidly away from us and producing red-shifted light and the other moving toward us and produced blue-shifted light. The whole system rotates with a period of 164 days, and this modulates the Doppler shifts of light from the jets. The velocities of the hot gas in the jets of SS433 with respect to their parent object is calculated to be 26% of c. Up to now, this has been the galactic speed record for intrinsic (non-cosmological) velocity. The jet velocity of SS433, while high by astrophysics standards, is not really relativistic. The relativistic mass-increase factor g, which is a good measure of most relativistic effects, is only 1.04, so relativity gives the jet matter a small 4% increase in mass.

About 4 months ago at this writing (September, 1994), in the course of a general study of the radio-astronomy images of strong galactic gamma-ray sources, radio astronomers I. F. Mirabel of France, and L. F. Rodriguez of Mexico, using the Very Large Array (VLA) radio telescope in Socorro, New Mexico made a remarkable observation, which began a few months after the source GRS1915+105 had produced strong outbursts of radio-wave emission. Between March 27, 1994 and April 30, 1994, VLA observations showed twin blobs of radio-wave emitting matter emerging from the central object and traveling in opposite directions. The brighter of the two objects, appeared to be about 8 times brighter and to move away from the central object with about twice the speed of its twin.

Mirabel and Rodriguez assumed that what they were observing was an active parent object, a neutron star or black hole, that was emitting identical objects in opposite directions at identical velocities  relative to the parent. Combining the observations of relative brightness and speed of angular separation, they were able to extract the emission angle A, the distance D, and the velocity b of the event. They calculated that A=70o and the distance to the object is D=12.5 kiloparsecs (4.08 104 light years), placing it well within our galaxy. They also calculated that the velocity of the emitted objects is b=0.92 (i.e., 92% of c). This gives a mass-increase factor of g=2.55, indicating that the objects have a kinetic energy with is 1.5 times greater than their rest-mass energy, so these are truly relativistic objects.

The value of the distance to the object, when combined with the observed brightness of the radio-wave images, permits an estimate of the mass of the emitted objects. Mirabel and Rodriguez estimate that each emitted object has a mass of 2 1022 kilograms, about 1/3 the mass of the Moon. The kinetic energy of such a massive object moving with 92% of c is extremely large, about 3 1039 joules. Mirabel and Rodriguez estimate that the object was accelerated to this velocity in about 3 days, which would require a power of 2.1 1034 watts, about 50 million times the power output of our Sun and about 400 times the estimated steady-state power output of the highly energetic parent object, GRS1915+105.

We don't understand the details of an acceleration process that can accelerate a lunar-size object to relativistic speeds in a few days. If the process involves acceleration of mass by emitted radiation, the power present in the radiation would have to be many times larger than the fraction diverted into acceleration of the massive object. The source GRS1915+105 is suspected of producing the repeating bursts of soft gamma radiation that have been observed in its region of the sky, but the power implicit in the observed gamma bursts is only about 10% of that required for the acceleration, even if all available energy went into the acceleration of mass. Therefore, the acceleration mechanism remains a mystery.

What is this object that at times is the brightest gamma ray source in our galaxy? The evidence from gamma rays and X-rays indicates that it is the massive remnant of an exploded star, perhaps a neutron star but more probably a black hole, that is converting the mass-energy of nearby mass, perhaps supplied by a companion star, into a wide spectrum of electromagnetic radiation and into the ejection of mass.

From the point of view of astrophysics, this new discovery is of high importance for two reasons. First, it represents a quasar-like object that is in our own galaxy and is likely to produce a series of outbursts of radiation emission and mass ejection that can be studied in far more detail than quasar events at the edge of the visible universe. But second, it is the first known representative of a class of objects that may provide independent measurements of distance in our galactic neighborhood. This is because the mass-ejection event provides a distance estimate that is independent of the usual distance measures, relative brightness and red shift. It therefore represents a completely new way of measuring interstellar distances and offers the opportunity of comparing one technique with another.

Unfortunately, this comparison would require optical observations as well as those in the radio and gamma ray regions, and GRS1915+105 lies so close to the galactic plane that its light intensity is estimated to be attenuated by 20 magnitudes by interstellar dust. Therefore, there is little hope of obtaining its visible-light image, even with our best telescopes.

From the point of view of science fiction, GRS1915+105 offers other opportunities. How could the almost unimaginable energies of this natural engine of destruction be harnessed? What would it be like to ride on a fireball accelerated in three days to nearly the speed of light? What would it be like to live on a planet that had this massive source of energy and radiation in the immediate stellar neighborhood? What would it be like to discover that 3 1039 joules of energy was headed directly for your solar system? Watch these pages for further developments ...


Superluminal Source: 
I. F. Mirabel and L. F. Rodriguez, Nature 371, 46 (September 1, 1994); 
Galen Gisler, Nature 371, 18 (September 1, 1994).

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