by Andrew B Collins
Andrew B Collins is
the author of
THE CYGNUS MYSTERY,
which features the mysteries of Cygnus X-3 and its role
as a cosmic accelerator.
Photograph of Cygnus
X-3 taken by the Chandra X-ray Observatory in November 2000.
The horizontal line is thought to be an artefact of the image(Pic:
Cygnus X-3 is a high mass X-ray binary and microquasar, with a
compact star, either a neutron star or a black hole, and a companion
star, most probably a Wolf-Rayet (WN7 or 8) with huge mass loss and
powerful stellar wind. First observed in 1966 (Giacconi, et al,
1967), Cygnus X-3 has been monitored across multiple frequencies,
from radio, infrared, optical, X-ray to gamma-rays.
It is one of the brightest galactic
X-ray sources, and is the outright brightest during the production
of bright radio flares, which can reach 20 Jy. Criticisms regarding
Cygnus X-3 being a source of cosmic rays up to PeV (Meinels,
personal communication, 11 July 2006) will necessitate a review of
findings and theories since 1985. I will review recent evidence of
Cygnus X-3's production of relativistic jets, as well as speculation
that its core might be a source of strange quark matter, producing
exotic primary particles, with the H being a possible candidate.
Cygnus X-3 (RA 307.6 dec 40.8) has been identified as a source of
ultra high energy (UHE) gamma-rays of an extremely energetic nature.
Indeed, their initial discovery in the 1970s was responsible for a
complete reassessment of particle acceleration in compact stars. As
early as 1973 the SAS-2 satellite reported gamma-radiation with a
narrow phase interval of 4.8 h, noted separately in connection with
x-ray and infrared observations of Cygnus X-3, estimated to be at 10 kiloparsecs (kpc, about 30,000 light years).
This periodicity is most likely caused
by the eclipsing of the compact star by its companion (Hillas,
1984), since jet precession is now calculated to be in the range of
5 days (Miller-Jones, et al 2004). Cygnus X-3 is also thought to be
a sporadic 12.6 ms pulsar (Chadwick, 1985) with gamma-rays produced
at or near the maximum (phase 0.6) in the 4.8 h X-ray cycle (Bowden
et al, 1992).
2 - Cygnus X-3
as a gamma-ray source
The gamma-ray emissions in association with Cygnus X-3 are known to
range between 35 and 200 MeV, although the COS-B satellite between
1975 and 1982 reported no radiation between 70 and 5000 MeV with the
point-source signature of 4.8 h (Cordova, 1986). Yet up to 1986 more
than a dozen groups had reported the detection of gamma-rays from
Cygnus X-3 with energies of at least 1011 eV. Gamma-rays in the
higher range E> 1014 ev were subsequently reported and verified by
ground-based collaborations in Germany, England, the United States,
India and Italy (Cordova, 1986, and sources quoted).
The extremely energetic gamma-rays from Cygnus X-3 were early
considered to be 'the products of interactions between even more
energetic particles within the source, mainly protons', leading to
speculation that Cygnus X-3 was 'the first astronomical object to be
identified with reasonable certainty as a source of cosmic rays',
i.e. any cosmic radiation above 108 eV (Cordova, 1986), or, indeed,
a 'cosmic accelerator' (Dar, 1986). Moreover, gamma-rays from Cygnus
X-3 indicated that 'only a very small number of sources of like
nature would be required to produce most of the observed high-energy
cosmic rays.' (Cordova, 1986).
Among the suspected method of production of gamma-rays were two
popular models. Either they were protons accelerated by the electric
field induced in the accretion disk held in the magnetic field of
the compact star, or they were accelerated by shocks in the matter
accreted on to a neutron star or black hole.
Characteristics of Cygnet Primaries
Between 1983 and 30 October 1985 various ground-based air shower
arrays, including Kiel (Samorski and Stamm, 1983a, 1983b) and Fly's
Eye (the latter from 1981 through till 1988) reported extensive air
showers with the direction and periodicity of Cygnus X-3 (See
Marshak et al, 1985; Cassiday et al, 1989). In Kiel's case,
particles were detected in the 1016 eV range (initially assumed to
This was later confirmed (Lloyd-Evans et
al, 1983) with the pulse being narrow (duty cycle 2%) and occurring
at a phase 0.25 after the X-ray maximum. Thus it was concluded that
Cygnus X-3 accelerated particles to at least 1016 eV, and that if
these were electrons, then protons might reach a higher level still
(Hillas, 1984). Indeed, at Kiel the EAS reached energies of > 1018
eV (Cassiday et al, 1989; Sommer and Elbert, 1990).
At the same time two underground nucleon-decay detectors set up
originally to observe proton decay, Soudon (Marshak et al, 1985) and
NUSEX (Battistoni, 1985, Baym, 1985), reported excessive
either with a time modulation of the 4.8-h period of Cygnus X-3, or
coincident to its daily transit. The flux from single-muon events
was greater than several orders than that expected from high energy
photon flux, suggesting most probably either a primary of unique
characteristics, dubbed the 'cygnet', or a new mechanism for very
efficient muon production in high energy photon-initiated air
cascades (Dar, 1986).
Production of Muon Flux Deep Underground
Excess muon quanta reported deep underground (at a depth equivalent
of 2 to 5 kilometers of water) produced an angular spread highly
suggestive of the primaries interacting in the rock overhang down to
a depth of a few hundred meters (Kolb, 1985, Ruddick, 1986).
This was confirmed by the differences in
flux between the Soudon and NUSEX detectors, with the latter's flux
being ten times less than the former, leading to the conclusion that
this effect 'can only be explained by attenuation of the cygnet beam
in the rock' (Ruddick, 1986). It also explained the zenith angles of
the muons, which were similar to the background flux produced by EAS.
Furthermore, the underground muon energy
measurements predicted a characteristic variation of the quanta
according to depth, with detectors on the surface only being able to
detect them at near the horizontal, due to the large interaction
length of the primaries. This meant 'such detectors would have to be
very large to detect a signal' (Ruddick, 1986).
4 - Cygnet
Identification of the relativistic primaries (Ruddick, 1986)
responsible for these signal events has proved extremely difficult,
highly controversial and even questionable (Thomsen, 1986). The
enhanced muon flux recorded, particularly by the Soudon I group, was
far too high for them to be gamma-rays, which produced a mere 1/300
of the muons (µ-mesons) characteristic of the reported muon excess (Baym,
Their 4.8-hour periodicity meant that
they had to have travelled in a more or less straight line at
relativistic speeds, otherwise a spread of lower velocities would
have washed out the signal. Clearly, the path of the cygnets was not
curved by the galactic magnetic field, otherwise this would have
randomized or deflected their arrival directions (Dar, Lord and
The fact that the cygnets produced excessive muon (µ-mesons)
implied that they acted to produce hadron-induced cascades. In other
words, they were strongly-interacting particles, rather than
electromagnetic particles, like gamma-rays or weak particles such as
neutrinos (Dar, Lord and Wilkes, 1986). Further evidence against
them being neutrinos was the fact that the cygnet-produced muon flux
increased when Cygnus X-3 was overhead, and faded when it was not in
view, the so-called 'horizon effect'. This is not a characteristic
of neutrinos, which do not interact in this manner.
Thus the conclusion was that cygnet primaries, either measured in
ground-based air shower arrays or in underground detectors, were
long-lived neutral particles with energies anything up to at least PeV (Kolb, 1985; Maiani, 1985; Berezinsky, Ellis and Ioffe, 1986;
Cordova, 1986; Ruddick, 1986; Cassiday et al, 1989; Czapek et al,
1990). However, the only obvious candidate was the neutron, which is
unstable to beta decay and has a half-life of approximately 10-15
minutes. Thus the only way that they could have reached the earth
was by travelling at relativistic speeds.
Quashing this possibility was the fact
that it would require neutrons with 100 times the energy of the
monitored cygnet events. Neutral atoms could be eliminated since
their electrons would have been stripped away by the 5g per cm2
interstellar hydrogen, causing their decay long before they ever
reached the Earth. This is unless they had an incredibly high energy
in the range of 1018 eV (Baym, 1986). As already noted, the Kiel
collaboration did indeed register EAS with energies >1018 eV.
Initial findings strongly indicated that the cygnet primary bore the
1) no electric charge
2) no magnetic charge
3) a rest mass estimated
to be between zero and 1/20 of a proton mass, and less than
its energy by a factor of around 104 (Baym, 1986; Ruddick,
4) it was strongly
interacting, in that it was hadron-inducing
5) it possessed a
half-life relative to its assumed passage at relativistic
speed. Protons, neutrons, nuclei, atoms, and micrograins of
ordinary matter could all be ruled out
The cygnets were not charged cosmic
particles since they are affected by the galactic magnetic field
which randomizes their directional flow and ruins any chances of
ascertaining their astronomical source, which can only be determined
if they correlate with activity in other frequencies that might
contain a known periodicity or direction.
Since neutral primaries arrive directly from source without being
affected by the galactic magnetic field, they are crucial to
determining the original trajectory of any cosmic ray. Gamma-rays
are neutral, and so can also arrive directly from source, why we can
trace the astronomical source of GRBs.
Thus in order to determine point sources of cosmic rays it is better
to examine neutral particles, which retain their primary trajectory
across the galaxy and when travelling at relativistic speeds will
also retain their unique signature, which has been the case with
Cygnus X-3 and Hercules X-1.
Indeed, as long ago as 1983 it was suggested that 'since the
galactic magnetic field seems sufficient to randomize all charged
particles during their long flights through space, pristine cosmic
rays may not be charged particles at all.' (See Hecht and Torrey,
A study of the five strongest recorded
UHE cosmic ray events (E>1020 eV) by Farrar and Torrey (1998) led to
the conclusion that their trajectory pointed back to extra-galactic
QSOs (quasar stellar objects) with a margin of error of 0.005. In
order that the primaries do not violate the Greisen-Zatsepin-Kuzmin
(GZK) cutoff for distance travel of a photon or nuclei, they saw
them as long-living, neutral hadrons of a possible exotic nature
(Farrar, et al, 1998).
5 - Cygnets
and Strange Quark Matter
As far back as the mid 1980s it was proposed that cygnets were
exotic hadrons resulting from strange quarks produced in strange
quark matter in the core of Cygnus X-3. Maiani (1985), for instance,
noted the observation of 'very energetic particles' arriving from
Cygnus X-3, with estimated energies up to 104 TeV, as well as the
observation underground of high energy muons correlated with a 4.8h
He also accepted that poorer statistics
might have been behind why other collaborations failed to register
these increased muon fluxes underground, such as FREJUS and HPW. The
primaries, he suggested, could be photons, which produce high energy
showers in the atmosphere, and might well explain underground muon
fluxes like those observed by Soudon and NUSEX. Yet results from
these collaborations showed an increased muon flux too high for
photons to be the simple answer (Dar, 1986).
In contrast to Kolb and Ruddick,
saw an anticorrelation in the reported muon flux versus the depth of
the traversed rock, and the fact that NUSEX results were less than
Soudon I. This was evidence, he felt, merely of the 'absorption
effect' (Maiani, 1985).
6 - Strange
Kolb (1985) asserted that quark nuggets might lead to an enhancement
in muon production over normal nuclear matter, yet even then only by
a factor of two. He additionally considered the R-odd particle from
super-symmetry and also the H-particle, after Jaffe. This last cited
he saw as having a lifetime long enough to reach the earth, because
of its double beta decay. Moreover, it bears four of the main
characteristics of the proposed cygnet, although whether it can
produce the enhanced muon flux depended upon its method of
Despite this, the mass of the H, might
not be low enough, something that only experimentation could
determine. Even if the mass was close to that of the cygnet, the
muon flux would be smaller than that reported. Moreover, the H
cannot account for the angular spread of the underground muon flux.
For instance, the NUSEX signal was seen coming from a 10 degree by
10 degree window in celestial coordinates, larger than the 0.5
degrees expected for an angular resolution.
Baym (1986) likewise proposed that the cygnet primary was the
theorized H particle. Should its mass be less than that of the
lambda (?) (1.116 GeV) plus that of the neutron (0.938 GeV), then
it was possible that the lifetime of the H could be sufficiently
long for it to be the cygnet primary, since it would not undergo the
rapid decay into a single lambda or neutron.
Wilk and Wlodarczyk (1996) acknowledged 'anomalous cosmic ray bursts
from Cygnus X-3' as the result of strange quark matter existing in
its presumed neutron star (Wilk and Wlodarczyk, 1996). This
supposition was explored further by Rybczynski, Wlodarczyk and Wilk
In similar to Kolb and Baym, Weber (2005) saw further support for
the existence of strange matter in Cygnus X-3, which he speculates
produces cosmic rays that to arrive as point-source signal events
means that they have to be 'electrically neutral', like the cygnet
primaries. Acknowledging their main characteristics, he confirms
also that to survive the trip from Cygnus X-3 such particles are
'long-lived'. In his opinion, the 'only known particle which can
quantitatively satisfy this constraint is the photon'. This is
despite the fact that, as he states, they would only produce air
showers with a 'small muon component'.
Weber goes on to predict that the 'natural candidate' for the cygnet
is the H particle. Their potentially long lifetime means that they
'may be present as components of existing neutral particle beams'
(Weber, 2005). Yet in order to give it long life, it would need to
have 'mass below single weak decay stability'. Furthermore, in order
to generate enough H particles, the source would have to be a
Weber admits that the problem with
Cygnus X-3 is that,
'it is accreting mass and thus has a crust, such
that there is no exposed strange matter surface where small strangelets could be produced and subsequently accelerated
electrodynamically to high energies into the atmosphere of the
companion star where H particles were created via spallation
Yet other evidence of strangelets in
balloon-borne counter experiments, air-shower arrays and large
emulsion chambers has convinced Weber that,
'some primary cosmic rays
may contain non-nucleus components which generate extended air
showers that contain both a large number of muons as well as very
high energetic photons', with Cygnus X-3 being a unique candidate.
7 - Cygnets,
Neutrons and Relativistic Flow
The idea that cygnet primaries are the result of exotic nuclei
within Cygnus X-3's compact star being accelerated towards the earth
is based on the view that they interact as hadrons to induce
cascades uncommon to the normal production of muons in the
atmosphere. However, should it be shown that the particles travel at
relativistic speeds and thus contain considerably higher energies
than previously reported, then they might be explainable in more
Soudon reported that the muon excess
from Cygnus X-3 was coincident to major radio flares (from 0.1 mJy
up to 20Jy), which have themselves occurred coincident with X-ray
and infrared observations (Baym, 1986). Moreover, gamma-rays with
the 4.8 h periodicity of Cygnus X-3 monitored by ground-based air
arrays have also coincided with considerable excess muon flux
underground as mentioned earlier.
Sommers and Elbert (1989) examined the evidence for EeV neutrons
and/or photons from Cygnus X-3, based on the Fly's Eye data, and
stated that 'because of synchroton radiation losses, EeV particle
acceleration cannot occur gradually while a particle orbits in a
strong magnetic field.' As a consequence, Sommers and Elbert
suspected that 'if particles are accelerated in a neutron star's
magnetosphere, some type of linear accelerator must be responsible
Accepting that the cosmic rays from Cygnus X-3 are neutral, since
charged particles would be dispersed by the galactic magnetic field
at EeV energies, the question remains of how neutral particles might
be produced by accelerated charged particles. According to Sommers
and Elbert, the range of possible models for the production of EeV
neutral particles 'is greater than the range considered for the
production of 1015 eV neutral particles from Cyg X-3. This is partly
because the EeV neutral particles can be neutrons as well as
gamma-rays' (Sommers and Elbert, 1989).
Crucially, Sommers and Elbert go on to state that 'although free
neutrons decay with a mean proper lifetime of 898 seconds', time
dilation allows some neutrons at these energies to travel the
distance from Cyg X-3. On this basis, the energy threshold (0.5 EeV)
for the data used in the Fly's Eye analysis suggests that the
reported increased muon flux could be neutrons, even though the
collaboration was at the time unable to distinguish between a
neutron-initiated shower and a gamma-ray shower (Sommers and Elbert,
In final conclusion, they stated that 'Cyg
X-3 is a strong source of EeV cosmic rays'.
The significance of Sommers and Elbert's proposal is that with a
relativistic linear acceleration through jet production, the view
that cygnets are exotic strange quark particles becomes unnecessary.
The neutral particles might indeed be neutrons, reliant on a new
model based upon synchrotron radiation loss through relativistic
Cygnus X-3 during its
major radio flare in September 2001 (after Miller Jones, et al,
A one-side relativistic jet was observed in association with radio
flare activity in Cygnus X-3 by Mioduszewski, et al (2001) using the
VLBA in February 1997. It was estimated to have an opening angle of
12 degrees, and a small inclination angle of > 12 degrees towards
the Earth, leading to the conclusion that it constituted the
galaxy's first blazar.
A precessional phase of 30 days with an
anticlockwise movement was also noted in association with jet
production. These parameters were comparable with those obtained
during the observation in September 2001 of a separate major radio
flare by Miller-Jones, et al. (2004) using the VLBA over a period of
six days following a peak outburst. The southern jet was estimated
to be moving within 10.5 degrees to the line of sight, with a
precessional phase in a clockwise motion of 5.3 days. The northern
jet was weakly observed.
Clearly, the implication was that both
the precession cycle and the direction of the jets had shifted
between 1997 and 2001. Through extrapolation of the jet motion back
to source Miller-Jones estimated that the jets were ejected about
2.5 days after the radio brightness of Cygnus X-3 began to increase.
Overall the parameters of the southern jet have been found to be
consistent with what Mioduszewski et al. had previously observed.
Bipolar jets were also observed in October-November 2000 using the
VLA and examined by the NRAO (Marti, et al, 2002), although whether
or not these were a separate mechanism to the observed north-south
orientated jets observed in 2001 has still to be decided.
The speed of Cygnus X-3's suspected one-sided jet was originally
estimated at 0.35c (Cordova, 1986). More recent assessments of the
relativistic jets following the 1997 VLBA observations by
Mioduszewski, et al (2001), provided a revised speed up to 0.81c,
while Miller Jones, et al, following the 2001 observations concluded
that the rate was 0.63c. Yet they accepted that faster speeds could
precede the observable appearance of the series of bead-like knots
marking the whereabouts of the jets; see also Hannikainen, et al
(2003, poster) for further discussion on this subject.
Such speeds might be enough to enable short-lived neutrons to reach
the Earth, indicating a realistic process for the arrival of
low-mass neutral particles, and the possible production of increased
air showers and underground muon quanta from Cygnus X-3.
It has been pointed out that an observed velocity of Cygnus X-3s jet
at a maximum of 0.81c would be too slow to compress time so that
neutrons might have time to decay into protons (Meinel, private
communication, 11 July 2006).
The 0.81c for the speed of Cygnus X-3s southern jet for the February
1997 observations (Mioduszewski et al, 2001) is based on estimates
of jet motion in radio flaring, and does not necessarily relate to
the initial velocity of ejecta on all frequencies. Moreover, it is
clear that the speed of the jets change, as is seen in the 2001
observations, where a velocity of just 0.63c was deduced
Earlier estimates of jet speed were even
lower. Moreover, the bipolar jets monitored by the NRAO using the
VLA in 2000 (Marti, et al, 2001) determined that they had an
infrared speed of just 0.48c, which is much slower than the higher
speed radio flares. Thus there is no reason why UHE and HE cosmic
rays and gamma-rays from Cygnus X-3, or indeed any point source,
might not exceed the velocity speed of radio flares.
Signal events from Cygnus X-3 which feature the arrival of GeV
gamma-ray emissions and hadron-like neutral particles have coincided
with intense bursts of energies across multiple frequencies during
the production of jets. For instance, this occurred in October 1985
when an increased muon flux at PeV levels coincided with intense
bursts of radio emissions (Berenzinsky, 1988).
In addition to this, there was a
correlation between the excess muon flux recorded by the Soudon II
deep underground experiment between 1991 and 2000 and Cygnus X-3's
production of large or intermediate radio flares (Thomson, 1991;
Marshak, 2000; Allison, 1999).
Thus there is every reason to
conclude that the production of gamma-rays and long-lived neutral
particles in Cygnus X-3 might well be the result of narrow,
magnetically driven relativistic jets within a small angle of the
NRAO image showing
the production of bipolar jets by Cygnus X-3 in October-November
2000 (pic: NRAO/AUI) .
8 - Some Criticisms
of Cygnus X-3 as a Cosmic Accelerator
It has been suggested that the cone angle of Cygnus X-3 poses a
problem regarding any working model for it being a cosmic
accelerator in its role as a blazar. In order to send cosmic rays in
the Earth's direction with the solar region being significantly
inside the limit to the probability distribution has been estimated
to be about 0.5 radian (Meinel, private communication, 11 July
Work has yet to be undertaken on this level, and the recorded data
from both particle physics and astrophysics needs careful evaluation
in this respect. Much of the findings cited from the 1980s of long
lasting neutral particles from Cygnus X-3 has largely been ignored.
This is a shame, for it clearly suggests that Cygnus X-3 possesses
an extraordinary acceleration mechanism for the production of cosmic
rays. As we have seen , their appearance correlates well with radio
flaring and hard X-ray outbursts. Nothing has so far been published
on any possible correlations between cygnets and major radio flaring
during the years 1997, 2000, 2001 and 2006.
The question with regards Cygnus X-3 is not whether it can
accelerate out cosmic rays and UHE gamma-rays, but how exactly they
might be produced. In my opinion, it is the production of
relativistic jets, or shock waves in association with their
production, that remain the best mechanism for their production,
something predicted by Sommers and Elbert as far back as 1989.
What is important about this and other
similar conclusions during the 1980s is that there was no hard data
available then suggesting that Cygnus X-3 might be a blazar. This
came only through the February 1997 observations (see Mioduszewski,
et al, 2001), and confirmed during the 2001 observations
(Miller-Jones, et al, 2004). In other words, working models for the
acceleration process of cosmic rays from Cygnus X-3 were shown to be
correct, making the evidence of long-lasting neutral particles
originating from here an extremely likely possibility.
What is more, they have continued to be
reported. Soudon II, announced in 1999 that in its first ten years
of operation the collaboration had regularly tracked excess muon
events in the Tev range or above from the direction of Cygnus X-3,
and again in 2000 (Marshak, 2000). There is every likelihood that
cygnets are accelerated from source during jet production, and that
at such energies only stable, neutral particles can travel the 10
kpc distance from Cygnus X-3 to the earth 'along trajectories which
point back to the source' (Allison, 1999).
Moreover, the Soudon II collaboration
conclude that since known stable neutral particles - photons and
neutrinos - have only small probabilities of producing detectable
muons 'Tev muons associated with Cygnus X-3 requires either exotic
interactions of known primaries, exotic primaries or very large
fluxes of neutrinos or photons' (Allison, 1999).
Despite the Soudon and NUSEX observations of an increased muon flux
underground during the 1980s remaining controversial, there exists
good evidence for the existence of long-lived, low-mass,
strongly-interacting neutral primaries from Cygnus X-3.
Exotic primaries have been proposed to explain the reported EAS and
increased muon flux underground, with strange quark matter from a
strange matter compact star, most likely a neutron star, being
currently the most popular model. However, with the introduction of
a relativistic flow for the acceleration of the neutral primaries,
it is possible that cygnet primaries are simply neutrons.
Regardless of this, the idea of them
being produced within neutron stars by strange matter remains an
attractive theory, and exploration into this area of astrophysics is
to be encouraged. It is ironic that the absence of a well-defined
mechanism of production of these neutral primaries, along with their
erratic data, has diminished the impact of these extremely important
findings, which arguably hold the key to determining the first
confirmed point source of galactic cosmic rays.
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