# Stars/Quasars

ULAS J1120+0641 is a very distant quasar powered apparently by a black hole with a mass two billion times that of the Sun.
The power radiated by quasars is enormous: the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way.
High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some host galaxies are strongly interacting or merging.
The peak epoch of quasar activity was approximately 10 billion years ago.
As of 2017, the most distant known quasar is ULAS J1342+0928 at redshift z = 7.54; light observed from this quasar was emitted when the Universe was only 690 million years old. The supermassive black hole in this quasar, estimated at 800 million solar masses, is the most distant black hole identified to date.
The spectral lines of these objects, which identify the chemical elements of which the object is composed, were also extremely strange: some of them changed their luminosity very rapidly in the optical range and even more rapidly in the X-ray range, suggesting an upper limit on their size, perhaps no larger than our own Solar System. This implies an extremely high power density.

## Theoretical quasars

Def. "extragalactic objects, starlike in appearance and having spectra with characteristically large redshifts, that [are (putatively)] the most distant and most luminous objects in the universe" are called quasars.

## Sources

"The quasar 3C 380 (B1828+487) [at right] has a complicated, convoluted structure on kiloparsec scales which is consistent with it being a moderate-sized classical double source seen approximately end on [...] 6 cm images from 1982.9 to 1993.4 (the 1990.8 image appears [at right] reveal a highly complex, filamentary, structure which exhibits rapid local brightness changes over its entire ~ 100 pc length. Motion [occurs] in three regions of the jet
* C12, at a distance [...] from the core C appears to move outwards with a velocity of [0.85 c,]
* The bright component A, [...] moves with an apparent velocity [of 4.4±0.5 c, and]
* the peak of emission in the region F [...] appears to move with an apparent velocity of [6.0±0.3 c.]"
For "most of the period between 1982.9 and 1993.4 A moved outwards from C along P.A. 330±1° with little change in the speed or the direction of the apparent velocity vector. In 1988.4 however, A had doubled in brightness and apparently became dissociated from the underlying jet pattern appearing edge-brightened towards the East. This change was accompanied by an apparent deceleration by almost 50%. Between 1988.4 and 1990.8 A apparently accelerated again but its brightness barely changed and by 1990.8 its brightness peak had shifted back to its "standard" P.A. and continued in this direction through 1992.7 and 1993.4 with no significant changes in the velocity and only a slight decrease in flux density. The apparent acceleration from ≃ [0.85 c] at a few pc to ≃ [6.0 c] at [100 pc] is similar to that seen in the best-studied superluminal source 3C 345".
There "are gross changes in the brightness structure of the jet taking place very quickly. For example in three epochs (1988.2, 1990.8 and 1993.4) the jet appears to be bifurcated or edge-brightened in regions B and D while in others it is center-brightened. [...] the rapid brightness changes may be due to phase effects at the intersection of these shocks".

## Meteors

"[S]uperluminal motion for each of [two] knots, [in the BL Lacertae object OJ 287 is suggested] at an angular speed of 0.28 mas yr$^{-1}$, corresponding to β$_{app}$ = v$_{app}$/c ≃ 3.3h$^{-1}$ (for z = 0.306, H$_0$ = 100h km s$^{-1}$ Mpc$^{-1}$, and q$_0$ = 0.5)." "Superluminal motion for each knot, with an apparent velocity ~3.3h$^{-1}$c, is suggested by the polarization data. The polarizations of C [the core] and K2 [knot two] changed markedly over the year between observations."
Subsequent VLBI "observations of the total intensity structure of the BL Lacertae object OJ 287 have been made with an angular resolution of 7 x 1 mas at λ6 cm. The source consists of a core and three knots in a VLBI jet at position angle θ ≃ -100°. Previously suspected superluminal motion in the outer two knots at β$_{app}$h ≃ 3 ... has been confirmed."
For the speeds in units of c, β = v/c, "[i]n the usual interpretation of superluminal motion, the apparent velocity is given by
    $\beta _{app}={\beta _{jet}\sin \phi \over 1-\beta _{jet}\cos \phi },$
where β$_{jet}$c is the jet velocity, and the jet makes an angle Φ to the line of sight."
In April 2010, radio astronomers working at the Jodrell Bank Observatory of the University of Manchester reported an unknown object in M82. The object has started sending out radio waves, and the emission does not look like anything seen anywhere in the universe before. There have been several theories about the nature of this unknown object, but currently no theory entirely fits the observed data. It has been suggested that the object could be a "micro quasar", having very high radio luminosity yet low X-ray luminosity, and being fairly stable. However, all known microquasars produce large quantities of X-rays, whereas the object's X-ray flux is below the measurement threshold. The object is located at several arcseconds from the center of M82. It has an apparent superluminal motion of 4 times the speed of light relative to the galaxy center.

## Radios

"Historically, the connection between radio loud [RLQs] and radio quiet quasars [RQQs] has been unclear, but new studies are pointing toward a similarity in the physical mechanisms of core radio emission in the two classes. Both classes appear to have similar core spectral index distributions, including or inverted cores in RQQs, both have compact VLBI cores, and both appear to be about equally time-variable. This suggests that RQQs possess beamed, relativistic jets like those in RLQs. If RQQs are really lower power cousins of RLQs, the jets in RQQs should show superluminal motions. With the advent of very high sensitivity VLBI using the High Sensitivity Array, it should now be possible to detect the parsec-scale jets in RQQs and measure component speeds."

## Superluminals

"Apparent superluminal motion is observed in many radio galaxies, blazars, quasars and recently also in microquasars. The effect was [apparently] predicted before it was observed by Martin Rees ... and can be explained as an optical illusion caused by the object partly moving in the direction of the observer, when the speed calculations assume it does not. The phenomenon does not contradict the theory of special relativity. Interestingly, corrected calculations show these objects have velocities close to the speed of light (relative to our reference frame). They are the first examples of large amounts of mass moving at close to the speed of light. Earth-bound laboratories have only been able to accelerate small numbers of elementary particles to such speeds.
“Maps of the radio structure of the quasar 3C273 provide evidence of a superluminal expansion during the period 1977-1980. The superluminal expansion might be attributed to the movement of a single knot away from the nucleus along the jet. The apparent constant velocity of 10 times the speed of light is an important constraint on theories of apparent superluminal expansion.”
"Superluminal motion in quasar 3C279 is shown [at right] in a "movie" mosaic of five radio images made over seven years. The stationary core is the bright red spot to the left of each image. The observed location of the rightmost blue-green blob moved about 25 light years from 1991 to 1998, hence the changes appear to an observer to be faster than the speed of light or "superluminal". The motion is not really faster than light, the measured speed is due to light-travel-time effects for a source moving near the speed of light almost directly toward the observer. The blue-green blob is part of a jet pointing within 2 degrees to our line of sight, and moving at a true speed of 0.997 times the speed of light. These five images are part of a larger set of twenty-eight images made with the VLBA and other radio telescopes from 1991 to 1997 to study the detailed properties of this energetic quasar." The images are in the K band, 1.2 cm, 22 GHz.
"A time ordered sequence of images of the microquasar GRO J1655-10, with earlier times at the top and later times underneath. This sequence covers a month in the life of the microquasar, as imaged by the VLBA. GRO J1655-40 is a binary system in which the outer envelope of the normal star is overflowing onto its black hole companion. The accreting matter swirls into a rapidly rotating disk as it falls towards the black hole; near the center, some of this matter is "squirted out" perpendicular to the disk, at relativistic speeds (i.e, speeds approaching the speed of light). The radio emission associated with these relativistic jets is what we see here. The intrinsic speed of these jets is about 90% of the speed of light. Because the jets are moving almost as fast as the radiation they emit, the approaching jet appears to move even faster, and the apparent motions range up to 1.3 times the speed of light."
"VLBA images [are] at 1.6 GHz. Images made from data collected in August and September, 1994. Top image from Aug 18-19; next one from Aug 22-23; next Aug 25-26; next Aug 25-26; next Sep 1-2; next Sep 8-9; next Sep 12-13; and last Sep 20-21."
"It is generally accepted that the blazar phenomenon is due to the anisotropic boosting of the radiation along the direction of motion which gives rise to an apparent enhanced luminosity at all wavelengths if the observer is located close to the direction of motion. There are many observations which support this interpretation including the one sided appearance of blazar jets and the rapid flux density variability observed at many wavelengths. However, the only direct observations of relativistic motion are at radio wavelengths when motion close to the line of sight produces a compression of the time frame resulting in apparent superluminal motion. High resolution interferometric radio images are able to measure such motions which are typically less than one milliarcsecond per year."
The "details of the kinematics [of superluminal source motions] have remained elusive. One of the problems is, that contrary to indications of early observations (e.g., Cohen et al. 1977), the radio jets often do not contain simple well defined moving components. Instead, the jets may show a complex brightness distribution with regions of enhanced intensity that may brighten and fade with time. Some features appear to move; others are stationary, or may break up into two or more separate features, and it is often unclear how these moving features are related to the actual underlying relativistic flow."
The third image at right "shows the distribution of apparent linear velocity for the 157 components contained in our full sample that have well-determined motions. This includes 104 quasar components, 31 BL Lac components, and 22 components associated with the nucleus of an active galaxy."
This image "is in marked contrast to early discussions of superluminal motion, which indicated typical values of γ in the range 5 to 10 (Cohen et al. 1977, Porcas 1987). [...] Most of the sources in our sample are quasars and their velocity distribution is peaked near low values of v/c between zero and ten, but there is a tail extending out to v/c ~ 34. Features associated with the active nuclei of galaxies all appear to have motions in the range 0 < v/c < 8, while the BL Lac objects appear more uniformly distributed over the entire range from 0 to 35."

## Interstellar medium

"In far-distant quasars and galaxies, millions or even billions of light-years away, the gravitational energy of supermassive black holes is capable of accelerating "jets" of subatomic particles to speeds approaching that of light. The VLA has observed such jets for many years. In some of these jets, blobs of material have been seen to move at apparent speeds greater than that of light -- a phenomenon called superluminal motion. The apparent faster-than-light motion actually is an illusion seen when a jet of material is travelling close to -- but below -- the speed of light and directed toward Earth."
"In the Spring of 1994, Felix Mirabel from Saclay, France, and Luis Rodriguez, from the National Autonomous University in Mexico City, were observing an X-ray emitting object called GRS 1915+105, which had just shown an outburst of radio emission. This object was known to be about 40,000 light-years away, within our own Milky Way Galaxy -- in our own cosmic neighborhood. Their time series of VLA observations, seen in this image, showed that a pair of objects ejected from GRS 1915+105 were moving apart at an apparently superluminal speed. This was the first time that superluminal motion had been detected in our own Galaxy."
"This surprising result showed that the supermassive black holes at the centers of galaxies -- black holes millions of times more massive than the Sun -- have smaller counterparts capable of producing similar jet ejections. GRS 1915+105 is thought to be a double-star system in which one of the components is a black hole or neutron star only a few times the mass of the Sun. The more-massive object is pulling material from its stellar companion. The material circles the massive object in an accretion disk before being pulled into it. Friction in the accretion disk creates temperatures hot enough that the material emits X-rays, and magnetic processes are believed to accelerate the material in the jets."
"Since Mirabel and Rodriguez discovered the superluminal motion in GRS 1915+105, several other Galactic "microquasars" have been discovered and studied with the VLA and the VLBA. In 1999, NRAO astronomer Robert Hjellming turned the VLA toward a bursting microquasar within 24 hours of a reported X-ray outburst. Working with X-ray observers Donald Smith and Ronald Remillard of MIT, Hellming found that this object is a microquasar only 1,600 light-years away, making it the closest black hole to Earth yet discovered."
"Microquasars within our own Galaxy, because they are closer and thus easier to study, have become invaluable "laboratories" for revealing the physical processes that produce superfast jets of material. For discovering this new class of celestial object, Mirabel and Rodriguez received the prestigous Bruno Rossi Prize of the American Astronomical Society in 1997."
"On the far side of our Galaxy, gas clouds explode away from a small black hole. This might seem peculiar, as black holes are supposed to attract matter. But material falling toward a black hole collides and heats up, creating an environment similar to a quasar that is far from stable. In the [at second right] time-lapse sequence, micro-quasar GRS1915 expels bubbles of hot gas in spectacular jets. These computer enhanced radio images show one plasma bubble coming almost directly toward us at 90 percent the speed of light, and another moving away. Each of the four frames marks the passage of one day. Originally detected on October 29th, these bubbles have now faded from view."

## BL Lacertae objects

A BL Lacertae object or BL Lac object is a type of active galaxy with an active galactic nucleus (AGN) and is named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large-amplitude flux variability and significant optical polarization.
All known BL Lacs are associated with core dominated radio sources, many of them exhibiting superluminal motion.
QSO B0323+022 is a BL Lacertae object. The image at right is taken with the ESO NTT using the R filter.

## Relativistic jets

In the images at right are the effects of charged particles apparently moving six times the speed of light.
"We see almost a dozen clouds which appear to be moving out from the galaxy's center at between four and six times the speed of light. These are all located in a narrow [relativistic] jet of gas streaming out from the region of the black hole at the galaxy's center".
"We believe this apparent speed translates into an actual velocity just slightly below that of light itself."
"The speeds reported are two to three times faster than the fastest motions previously recorded in M87, the only nearby galaxy to show evidence for superluminal motion."
"This discovery goes a long way towards confirming that radio galaxies, quasars and exotic BL Lac objects are basically the same beast, powered by super massive black holes, and differ only in orientation with respect to the observer".
"Here we have, for the first time, a fairly normal radio galaxy with both excellent evidence for a super-massive black hole, as well as superluminal jet speeds similar to those seen in distant quasars and BL Lac objects."
"This is the first time superluminal motion has been seen with any optical telescope, and this discovery was made possible by the extremely fine resolution obtained by Hubble".
"The structure of relativistic jets in [active galactic nuclei] AGN on scales of light days reveals how energy propagates through jets, a process that is fundamental to galaxy evolution."
Their lengths can reach several thousand or even hundreds of thousands of light years. The hypothesis is that the twisting of magnetic fields in the accretion disk collimates the outflow along the rotation axis of the central object, so that when conditions are suitable, a jet will emerge from each face of the accretion disk. If the jet is oriented along the line of sight to Earth, relativistic beaming will change its apparent brightness. The mechanics behind both the creation of the jets and the composition of the jets are still a matter of much debate in the scientific community; it is hypothesized that the jets are composed of an electrically neutral mixture of electrons, positrons, and protons in some proportion.
A relativistic jet emitted from the AGN of M87 is traveling at speeds between four and six times the speed of light.
"The term 'superluminal motion' is something of a misnomer. While it accurately describes the speeds measured, scientists still believe the actual speed falls just below the speed of light."
"It's an illusion created by the finite speed of light and rapid motion".
"Our present understanding is that this 'superluminal motion' occurs when these clouds move towards Earth at speeds very close to that of light, in this case, more than 98 percent of the speed of light. At these speeds the clouds nearly keep pace with the light they emit as they move towards Earth, so when the light finally reaches us, the motion appears much more rapid than the speed of light. Since the moving clouds travel slightly slower than the speed of light, they do not actually violate Einstein's theory of relativity which sets light as the speed limit."

## Active galactic nuclei

For active galactic nuclei (AGNs) "bright jet features typically exhibit apparent superluminal speeds and accelerated motions."
AGN "jets with the fastest superluminal speeds all tend to have high Doppler boosted radio luminosities. [...] there is a correlation between intrinsic jet speed and intrinsic (de-beamed) luminosity".
The AGN showing superluminal motion in B1950 are
* 0003+380 S4 0003+38 J0006.1+3821 z=0.229 quasar
* 0003-066 NRAO 005 ... z=0.3467 BL Lac
* 0007+106 III Zw 2 ... z=0.0893 radio galaxy
* 0010+405 4C +40.01 ... z=0.256 quasar
* 0015-054 PMN J0017-0512 J0017.6-0510 z=0.226 quasar
* 0016+731 S5 0016+73 ··· z=1.781 quasar
* 0048-097 PKS 0048-09 J0050.6-0929 z=0.635 BL Lac
* 0055+300 NGC 315 ··· z=0.0165 radio galaxy
* 0059+581 TXS 0059+581 J0102.7+5827 z=0.644 quasar
* 0106+013 4C +01.02 J0108.6+0135 z=2.099 quasar
* 0109+224 S2 0109+22 J0112.1+2245 z=0.265 BL Lac
* 0109+351 B2 0109+35 ··· z=0.450 quasar
* 0110+318 4C +31.03 J0112.8+3208 z=0.603 quasar
* 0111+021 UGC 00773 ··· z=0.047 BL Lac
* 0116-219 OC -228 J0118.8-2142 z=1.165 quasar.
At right is a radio image of quasar S4 0003+38. This object was image on March 9 and December 1, 2006, March 28 and August 24, 2007, May 1 and July 17, 2008, March 25, 2009, and July 12, 2010. From its movement as it was leaving its source in µas y$^{-1}$, S4 0003+38 before it left its source was moving at 2.62±0.84c, left its source at a back projected date of 2003.01±0.24, continued accelerating to 4.63±0.32c, then began to decelerate successively at each observation epoch from 0.67±0.20, 0.36±0.32, to 0.16±0.26.
The second image at right is of apparent superluminal motion in NGC 315. In both of these images the apparent motion is rectilinear or close to it. NGC 315 is a low-luminosity radio galaxy.
The third image at right shows the approximate radial motion of the jet component versus the core.
"Inward motions are rare (2% of all features), are slow (< 0.1 mas per y), are more prevalent in BL Lac jets, and are typically found within 1 mas of the unresolved core feature. [...] Considering only the AGN with a known redshift, the inward components of 1458+718 [fourth at right] are the only ones which appear significantly superluminal, ranging from 1.4 c to 4.6 c. With the exception of 1458+718, 2021+614, and 2230+114, the inward motions all occur within ∼ 1 mas of the core, in typically the innermost component. In particular, the innermost two jet components of two TeV-emitting BL Lacs in our sample: 0219+428 (3C 66A) [at fifth right] and 1219+285 (W Comae) are both inward-moving. The small velocities and core separations of these moving components may indicate that the core is not a stable reference point in these two jets."
The line in the image for 1458+718 connects the two components. The "the apparent inward motion [...] of [the] two component [is] in a complex emission region located ∼ 25 mas south of the core in this compact steep spectrum quasar. [There is] one additional component in this complex [...] that is also moving inward, in a non-radial direction."
"The moving features are generally non-ballistic, with 70% of the well-sampled features showing either significant accelerations or non-radial motions."
"A substantial number of components showed no significant acceleration, but had non-radial motion vectors. [...] This is in stark contrast to the kinematics of features in stellar (Herbig-Haro) jets, which are well described by ballistic models [...] Of the 739 components with statistically significant (≥ 3σ) speeds, 38% exhibited significant non-radial motion, implying non-ballistic trajectories.".
The "acceleration [is resolvable into] terms μ$_⊥$ and μ$_ǁ$ in directions perpendicular and parallel, respectively, to the mean angular velocity direction φ."
"[S]ignificant parallel accelerations [are] seen in roughly one third of our sample, and significant perpendicular accelerations in about one fifth of our sample."
"VLBA images of the jet in the broad-line radio galaxy 3C 111. The picture shows the variable parsec-scale structure of the jet in this active galactic nucleus. The features observed correspond to ejected plasma regions traveling at relativistic speeds. Those appear to be larger than the speed of light due to projection effects. The sixteen images are spaced by their relative time intervals. The images show that a major radio flux-density outburst in 1996 was followed by a particularly bright plasma ejection associated with a superluminal jet component. This major event was followed by trailing features in its evolution. A similar event is seen after mid 2001. The jet dynamics in this source is revealed: a plasma injection into the jet beam leads to the formation of multiple shocks that travel at different speeds downstream (ranging from 3c to 6c) and interact with each other and with the ambient medium. This is in agreement with numerical relativistic magnetohydrodynamic structural and emission simulations of jets."
"Images were taken at 15 GHz with the full Very Long Baseline Array as part of the 2cm Survey/MOJAVE collaboration. The observing runs usually last 8 hr and the total observing time on source is approximately 50 minutes. The typical dynamic range in the images is of 1000:1 (the lowest shown flux density is typically of 1-2 mJy/beam). The images are convolved with a common restoring beam of 0.5x1.0 milliarcseconds (P.A. of 0 deg). The image alignment is (arbitrary) to the brightness peak. The superluminal speeds of the features in the jet were determined from a detailed analysis of multiple Gaussian model fits to the observed visibilities."

## Double-lobed quasars

The "detection of superluminal motion in the central component of the double-lobed quasar 3C 245 [is shown] [...] This object has a strong nucleus (S = 0.91 Jy at 5 GHz; [...] [an] extended, steep-spectrum emission [and] is intermediate between lobe-dominated (R < 1) and core-dominated (R > 1) objects. [Where R is the ratio of compact to extended flux density at an emitted frequency ν = 5 GHz.]" The object was observed at five successive epochs: 1981.76, 1983.11, 1983.78, 1984.95, and 1986.17.
"The apparent separation of the centroids of [each lobe] changes from 0.33 mas to 0.46 mas between epochs 4 and 5; [...] The milliarcsecond-scale structure of 3C 245 may be interpreted as a typical "core-jet" source, [...] The jet components [...] are aligned, within the errors, with a one-sided jet on a scale of ~ 10-30 mas, which in turn is aligned with the inner knots of a curved arcsecond jet [...] z = 1.029 [...] the curvature (~ 20°) in the arcsecond-scale jet is moderate for extragalactic radio sources. [...] moderate apparent curvature is expected in a source with small intrinsic curvature aligned at a fairly small angle to the line of sight, θ."
The "strong, rapid variability provides evidence for bulk relativistic motion. [...] The proper motion of 0.11 ± 0.05 mas yr$^{-1}$ [...] translates into an apparent transverse velocity [of] (3.1 ± 1.4) [c]. The apparent velocity in 3C 245 lies between that of the other of two known steep-spectrum, double-lobed superluminal quasars (4.5 [c] for 3C 179, [and] 1.3 [c] for 3C 263 [...] The average apparent velocity in the three steep-spectrum, double-lobed quasars is half that in the seven best-studied, strongly core-dominated superluminal quasars"
A "linear-correlation coefficient test using these 10 quasars shows a correlation of β$_{app}$ with R significant at the 96% level".

## Recent history

The recent history period dates from around 1,000 b2k to present.
"VLBI (very-long-baseline interferometry) observations between 1971 and 1983 have been used to determine the positions of the 'core' of the quasar 3C345 relative to the more distant compact quasar NRAO512 with a fractional uncertainty as small as two parts in a hundred million. The core of 3C345 appears stationary in right ascension to within 20 arc microsec/yr, a subluminal bound corresponding to 0.7c. The apparent velocities of the jets are superluminal, up to 14c in magnitude."
The first quasars (3C 48 and 3C 273) were discovered in the late 1950s, as radio sources in all-sky radio surveys. With small telescopes and the Lovell Telescope as an interferometer, they were shown to have a very small angular size.
Measurements taken during one of the occultations by the Moon using the Parkes Radio Telescope allowed a visible counterpart to the radio source to be found and an optical spectrum using the 200-inch Hale Telescope on Mount Palomar to be obtained which revealed the same strange emission lines that were likely ordinary spectral lines of hydrogen redshifted by 15.8 percent - an extreme redshift never seen in astronomy before such that if this was due to the physical motion of the "star", then 3C 273 was receding at an enormous velocity, around 47,000 km/s, far beyond the speed of any known star and defying any obvious explanation.