Cygnus X-3 and the Case for Simultaneous Multifrequency

Home , Compact star, Cygnus X (star complex), EXOSAT, Quark star, Uhuru (satellite)

by France Anne-Dominic Cordova

lthough the visible radiation of Cygnus A X-3 is absorbed in a dusty spiral arm of our galaxy, its radiation in other spectral regions is observed to be extraordinary. In a recent effort to better understand the causes of that radiation, a group of astrophysicists, including the author, carried

39 Cygnus X-3

out an unprecedented experiment. For two days in October 1985 they directed toward the source a variety of instruments, located in the United States, Europe, and space, hoping to observe, for the first time simultaneously, its emissions 9 18 Gamma Rays at frequencies ranging from 10 to 10 Radiation hertz. The battery of detectors included a very-long-baseline interferometer consisting of six radio telescopes scattered across the United States and Europe; the Na- tional Radio Astronomy Observatory’s Very Large Array in New Mexico; Caltech’s millimeter-wavelength interferometer at the Owens Valley Radio Ob- servatory in California; NASA’s 3-meter infrared telescope on Mauna Kea in Ha- waii; and the x-ray monitor aboard the European Space Agency’s EXOSAT, a satellite in a highly elliptical, nearly polar orbit, whose apogee is halfway between the

earth and the moon. In addition, gamma- Wavelength (m) ray detectors on Mount Hopkins in Ari- zona, on the rim of Haleakala Crater in Fig. 1. The energy flux at the earth due to electromagnetic radiation from Cygnus X-3 as a Hawaii, and near Leeds, England, covered function of the frequency and, equivalently, energy and wavelength of the radiation. The frequencies above 102s hertz within a few data points in white are typical values for this variable source during quiet states, those in days. (The experiment is represented sche- gray are values obtained during a period of apparently increased x-ray activity, and the matically in the opening figure.) point in black is the maximum flux observed during the first radio-frequency activity of the Although not the first attempt at simul- source to be detected. As indicated by the arrows, the fluxes due to optical radiation and to taneous multi frequency Observations, the gamma rays with energies of about 109 eV are upper limits. (The energy flux is expressed in October experiment was special in two hertz times janskys (Jy), where 1 jansky = 10-26 watt - meter-2. hertz-1.) respects: it covered a wider range of frequencies than any previous effort, and its focus was an object that has tentatively standing of Cygnus X-3 has been pieced tablishment of national astronomical fa- been identified as a source of extremely together from studies of its emissions in cilities accessible to scientists from any energetic gamma rays and thus of ex- widely different spectral regions. Observa- institution. These facilities, which include tremely energetic particles. It had previ- tion of the universe at frequencies other satellite- and ground-based instruments, ously been thought unlikely that particles than optical began with the development have benefited the fields of astronomy and could be accelerated to extreme energies in of radio telescopes in the 1950s and broad- astrophysics enormously. The accompa- stars; rather, the acceleration was thought ened with the advent of x-ray telescopes in nying insert describes briefly those avail- to be the result of events in interstellar the late 1960s. These instruments revealed able today. space, such as the passage through an in- the existence of previously unknown Access to the national astronomical fa- terstellar cloud of a shock wave from a sources, and an immediate need arose for cilities helped solve one problem, but an- supernova explosion. The possibility that information about the intensity, energy, other soon became obvious. The radiation particles could be accelerated in Cygnus and temporal variation of their optical from most astronomical sources, partic- X-3 to energies greater than those attained radiation. But acquiring such information ularly high-energy sources, varies tempo- at the most powerful of today’s ac- required many hours of observing time, rally, often rapidly and unpredictably. A celerators has attracted the attention not hours that were not easy to come by at the complete picture of a variable source re- only of astrophysicists but also of particle dedicated and relatively few optical tele- quires simultaneous multi frequency ob- physicists. scopes of the time. servations. In the past few years groups of The current, but incomplete, under- This difficulty was addressed by the es- astronomers, some including as many as

40 Spring 1986 LOS ALAMOS SCIENCE Cygnus X-3

thirty participants from as many as twenty an excellent example of the power of tion about the energy incident upon the institutions, have attempted such observa- multifrequency, although not simultane- earth from the source is displayed for ref- tions. As a testament to their importance, ous, observations; the gaps in that under- erence in Fig. 1. the European Space Agency devotes at standing are ample evidence of the need least 60 percent of the EXOSAT agenda to for simultaneous coverage and for better One View at a Time experiments coordinated with observa- methods of achieving it. I will review what tions at other frequency ranges. Among has been learned about the radiation Cygnus X-3 first entered the catalogue the most successful attempts at simultane- emitted by this source, first from non- of known astronomical objects in 1966 as ous observations have been those aimed at simultaneous experiments and then from but one of the bright x-ray sources dis- flare stars, active galaxies, sources of x-ray previous attempts at simultaneity, and covered in that decade. It has since bursts and transients, and novas. what has been hypothesized about the or- provided astrophysicists with consider- Today’s understanding of Cygnus X-3 is igins of those emissions. Current informa- able intellectual excitement.

National Astronomical Facilities

mong the first of the national The most widely used guest facility is ing water vapor, welcomed guest ob- astronomical facilities was Kitt one located in space—an ultraviolet servers to another region of the spectrum. APeak National Observatory near telescope aboard the International Ultra- And in 1981 the Very Large Array of Tucson, Arizona. Its 4-meter Mayan violet Explorer satellite launched by twenty-seven radio telescopes at the Na- telescope, the third largest optical NASA in 1978 and still operating today. tional Radio Astronomy Observatory near telescope in the United States, was opened The telescope, a collaborative effort by Socorro, New Mexico, provided observers to guest observers in 1973, and many x-ray NASA, the European Space Agency, and with high-resolution images of radio-fre- astronomers, in particular, soon began to the United Kingdom’s Science and Engi- quency sources. study for themselves the optical counter- neering Research Council, has been used Scientists from Los Alamos National parts of x-ray sources. Kitt Peak National to study almost every known type of Laboratory’s Space Astronomy and Astro- Observatory, which today provides six astronomical source. physics Group have used all of these facili- other optical telescopes, Cerro Tololo In- In 1978 NASA also launched Einstein, ties in their explorations of accreting com- ter-American Observatory in northern the second of its High Energy Astrophysi- pact stars in binary systems and of pulsing, Chile, which offers a view of the southern cal Observatory satellites and the first to collapsed remnants of supernovas. sky through a twin of Kitt Peak’s Mayan carry a focusing x-ray telescope. This fa- The techniques for collecting and telescope, and the National Solar Ob- cility caused the migration of astronomers analyzing astronomical data differ dra- servatory, which includes solar telescopes to take a new turn as optical astronomers matically from one spectral region to an- on Kitt Peak and Sacramento Peak in New studied the x rays emitted by stars that other, but the national facilities offer how- Mexico, compose the National Optical As- radiate primarily in the visible region. to assistance and computer software in tronomy Observatories. NOAO is oper- Einstein became inoperative in 1981; its addition to observing time. Such user- ated for the National Science Foundation role as a guest x-ray facility is filled today friendliness encourages astronomers to by AURA, the Association of Universities by the European Space Agency’s EXOSAT venture outside a narrow specialty and for Research in Astronomy. Sixty percent satellite launched in 1983. explore many spectral regions, each con- of the time available on the NOAO In 1979 NASA’s Infrared Telescope Fa- taining different but complementary in- telescopes is allocated to guest observers cility on 13,800-foot Mauna Kea, far formation about the physics of a on the merit of their proposed research. above much of the earth’s infrared-absorb-

LOS ALAMOS SCIENCE Spring 1986 41 Cygnus X-3

BINARY SYSTEM Point Point Source Source with Scattering Structure

Observer Observer Observer DEDUCED LIGHT CURVE

Fig. 2. Modulation by orbital motion of the Suppose that the system includes in addi- metric variation in height of the opaque intensity of radiation from a binary system. tion a structure whose extent above and structure results in a modulation that is (a) Consider a binary system consisting of a below the plane of the orbit is azimuthally asymmetric about the minimum; the phase pointlike, nonvariable source of radiation symmetric about the source. If this struc- of the minimum (relative to the minimum and a larger, nonradiating, opaque, spheri- ture is composed of matter that scatters the due to the scattering structure) depends on cal companion rotating about each other in radiation, the source appears extended, and the location of the opaque structure. The a circular orbit. (The system is shown in the periodic decreases in intensity are grad- light curve sketched corresponds to an cross section through the orbital plane.) ual with a width at half-minimum approx- opaque structure located on an arc of a The intensity of the radiation from the imately equal to that in (a). If, as il- circle centered on the source; the height of system, observed at a far distance along a lustrated, the radius of the scattering struc- the structure varies linearly from some line of sight perpendicular to the axis of ture is less than that of the companion, the maximum at its leading edge to zero at its rotation, undergoes an abrupt decrease to minimum intensity is zero. However, if the trailing edge. The structures in (b) and (c), zero intensity once per orbital period as the radius of the structure is greater than that although seemingly contrived, are simpli- source is eclipsed by the companion. In the of the companion (or if the axis of rotation fied versions of structures invoked to ex- light curve for the system (graph of in- of the binary and the line of sight are not plain the x-ray light curves of x-ray tensity versus time during an orbital perpendicular), the minimum intensity is binaries in which the point source is a period), the decrease appears as a negative nonzero. (c) Suppose further that the sys- compact star (such as a neutron star) and square pulse lasting for a fraction of the tem includes a structure whose extent above the companion is a more or less normal, period P equal to the angle in radians and below the plane of the orbit is azi- low-mass star. The scattering structure ex- subtended by the companion at the source. muthally asymmetric about the source. If emplifies an optically thick corona within (In all the light curves shown t = O cor- this structure is opaque to the radiation, an accretion disk (see Fig. 3), and the responds to the position of the source then an additional periodic decrease in in- opaque structure exemplifies a bulge on the directly behind the companion star.) (b) tensity is observed. The azimuthally asym- outer edge of the accretion disk.

42 Spring 1986 LOS ALAMOS SCIENCE Cygnus X-3

(a) SURFACES OF CONSTANT GRAVITATIONAL POTENTIAL

Star

proposed an accretion model as an expla- i nation for the x rays. That is, matter is being transferred from a more or less normal companion star to a compact star (a white dwarf, a neutron star, or a black hole), and, as it falls toward the surface of that star, its gravitational potential energy is converted into kinetic energy. Interac- (b) FORMATION OF ACCRETION DISK tions of this energetic matter with other matter in the vicinity of the compact star produce the x rays. If the companion star has a low mass (equal to or less than about

Roche lobe, the accreting matter forms a disk around the compact star and, losing angular momentum through some viscous process, gradually spirals toward the surface (Fig. 3). If the companion star is more massive, matter accretes directly onto the compact star from a stellar wind leaving the companion star. Data gathered in the early seventies by the Uhuru and Copernicus satellites revealed a 4.8-hour periodicity (more ex- actly, a 4.79-hour periodicity) in the x-ray Fig. 3. The gravitational potential in the vicinity of a binary system can be visualized by intensity of Cygnus X-3. The period is means of surfaces on which the gravitational potential is constant. Shown in (a) are cross now known to be slowly lengthening. In- sections (through the orbital plane) of three such equipotential surfaces for a binary system terpretation of this periodicity as modula- composed of a neutron star and a low-mass companion star. Note the change in topography tion by orbital motion of a binary system of the equipotential surfaces, as the gravitational attraction decreases, from two discon- was natural, although two of its features nected surfaces, one surrounding each star, to a single connected surface surrounding both caused some doubt. The period was stars. Of particular interest is the surface consisting of two so-called Roche lobes with a shorter, by a factor of 10, than the orbital single point in common, for this is the smallest equipotential surface providing an period of any other x-ray binary then energetically easy path for transfer of matter from one star to the other. In (a) the known, and the modulation was quasi- companion star is smaller than its Roche lobe, but during the course of its stellar evolution, sinsuoidal and asymmetric about a mini- it may expand and fill its Roche lobe, as shown in (b). Then matter from the companion star mum intensity equal to about 40 percent can move through the connecting point to the Roche lobe of the neutron star and be captured of the maximum (Fig. 4). Today, other x- in its gravitational field. The result is an accretion disk, coplanar with the orbital plane, ray binaries with comparably short orbital from which matter gradually spirals toward the surface of the neutron star, losing angular periods and similar orbital modulations momentum and gravitational potential energy and gaining kinetic energy. Interactions of are known, and, despite lack of conclusive this energetic matter with the neutron star or with other matter in its vicinity produce x rays. evidence, most astrophysicists are confi- dent that Cygnus X-3 is a binary system. If so, it is an extremely close binary system. X Rays. The field of x-ray astronomy was galactic x-ray sources first discovered Kepler’s third law, together with the or- launched in the early sixties when a series exhibited periodic, abrupt, and total de- bital period and reasonable estimates for of rockets earned gas-filled proportional creases in x-ray intensity. Such an in- the component masses, tells us that the counters with thin plastic windows a hun- tensity modulation is consistent with oc- two stars can be no farther apart than dred miles above the earth. (X rays and cultation of a point source of x rays by a ultraviolet light are the forms of electro- larger object as they orbit about each other sun. magnetic radiation most highly absorbed (Fig. 2). Astronomers therefore identified Data about the x rays from Cygnus X-3 by the earth’s atmosphere.) Some of the these sources as binary systems and have been collected over an energy range

LOS ALAMOS SCIENCE Spring 1986 43 Cygnus X-3

X-RAY LIGHT CURVE The inherent x-ray luminosity of out of the line of sight with the rotation OF CYGNUS X-3 Cygnus X-3, which can be calculated from period of the neutron star. If Cygnus X-3 its observed x-ray spectrum and its esti- contains a pulsar surrounded by a corona, mated distance from the earth (about the corona could scatter the x rays so much 500 which more below), varies from about 1037 that this lighthouse effect is destroyed. to 1038 ergs per second. (For comparison, the sun’s total luminosity is 4 X 1033 ergs Radio-Frequency Radiation. Following per second.) Such a high x-ray luminosity, the discovery of Cygnus X-3 as an x-ray together with the high luminosities of the source, its radio-frequency counterpart source in other spectral regions, makes it was identified and monitored sporadi- unlikely that the compact star in the sys- cally. Although the brightest of the known tem is a white dwarf. If the compact star is radio counterparts of galactic point x-ray 0.5 0 a neutron star. as seems likely, and if the sources, it was nevertheless a very weak 4.8 -hour Orbital Phase mass of that neutron star is similar to the radio source. Then, on 2 September 1972 a measured masses of other neutron stars Canadian radio astronomer by chance ob- Fig. 4. This sketch of the x-ray light curve of Cygnus X-3 is based on EXOSAT data of Cygnus X-3 is very close to the Ed- taken in September 1984 by the author, K. dington limit. This limit, which equals O. Mason, and N. E. White. Note the broad FREQUENCY DEPENDENCE dip to the minimum intensity, the asym- OF RADIO BURST of mass M, is the radiation rate beyond metry of the dip about the minimum, and which the outward radiation force on ac- the large and rapid variations in intensity creting matter is greater than the inward outside the dip. gravitational force. 22 0.808 x 10’0 A satisfactory model has recently been t , hertz from 1 to 150 keV during numerous developed for an x-ray binary with an balloon- and satellite-mounted experi- orbital modulation similar to that of Cygnus X-3 (see “X1822-371 and the Ac- X 10i7 hertz). The observed x-ray spec- cretion-Disk Corona Model”). According trum shows a sharp decrease in flux at to that model the x rays originate from a energies below a few keV (see Fig. 1). This point source, but the source appears ex- cutoff is attributed to absorption by inter- tended because the x rays are scattered stellar gas and possibly by a medium local into the line of sight by an extensive co- to the source. EXOSAT data show that the rona of ionized matter surrounding the intensity of x rays with energies between 6 point source (see Fig. 2). Such a corona and 30 keV is less modulated by orbital could be formed in an accretion disk by motion than that of x rays with energies the radiation pressure of a compact star between 3 and 6 keV and that the intensity radiating near the Eddington limit. The ratio of the harder to the softer x rays existence of a corona in Cygnus X-3 could increases at the x-ray minimum. These explain why it is difficult to determine observations must be accounted for in any whether the system contains a neutron Fig. 5. The initial stages of the first de- model of the source. star. A sure signature of a neutron star, and tected radio-frequency burst from Cygnus The x-ray intensity of Cygnus X-3 one that has served to identify neutron X-3 were observed at two frequencies. Note shows intrinsic temporal variations as well stars as components of many other x-ray that the maximum energy flux per hertz as the 4.8-hour orbital variation. On short binaries, is a rapid pulsation in x-ray in- attained is greater at the higher frequency, time scales (50 to 1500 seconds) the intensity. The pulsation results when matter and that the maximum at the lower fre- tensity of the softer x rays sometimes accretes onto a rotating, magnetized neu- quency occurred later in time. These fea- undergoes Quasiperiodic oscillations. On tron star (a pulsar) whose rotation and tures have been found to be characteristic of longer time scales (weeks to months) the magnetic axes are not aligned. Then accre- the radio-frequency bursts and are sug- average intensity level varies randomly, tion occurs preferentially on the magnetic gestive of a possible mechanism (see text). by a factor of 10 or more, between low and poles of the neutron star, creating a beam- (Figure adapted from R. M. Hjellming, high states. like pattern of x rays that swings into and Science 182(1973):1089.)

44 Spring 1986 LOS ALAMOS SCIENCE Cygnus X-3

Fig. 6. A chronological sequence of color- ativistic electrons that have been injected high total luminosities of the systems in- coded radio-frequency intensity maps of the into opposing jets of plasma emanating dicate that these stellar objects may be binary system SS 433 obtained with the from the source. The speed of the jets is small-scale versions of active galactic nu- VLA during the first four months of 1981. about one-fourth the speed of light, which is clei and quasars. (The images, made by R. The low-intensity (blue) regions moving comparable to that of the jets emanating M. Hjellming and K. J. Johnston, are the outward from the source are interpreted as from Cygnus X-3. The existence of oppos- property of the National Radio Astronomy synchrotrons radiation emitted by rel- ing jets in SS 433 and Cygnus X-3 and the Observatory.) served that the radio intensity of the the relativistic electrons is a burst of the source. The best such estimate, a dis- source was a hundred times greater than gamma rays from the central engine of the tance of at least 11.6 kiloparsecs, was de- usual and increasing. Radio astronomers compact star, but this idea remains to be rived from data taken with the VLA dur- all over the world immediately began to tested with simultaneous gamma-ray and monitor the source at frequencies ranging radio-frequency observations.) Consistent light-years). There is, however, no reliable from about 109 to 10]1 hertz. (Other with this mechanism, which implies that upper limit on the distance. astronomers, too, focused on the source in the radio source should expand when a Very recent radio-frequency observa- the hopes of observing similarly exciting burst occurs, are images acquired with a tions of Cygnus X-3 during its quiet x-ray activity in other spectral regions.) Within very-long-baseline interferometer and the state suggest the occurrence of low- about ten days the radio intensity of the VLA during another spate of radio-fre- amplitude radio-frequency flares every source had decreased to a normal level, quency bursts from Cygnus X-3 in 1982. 4.95 hours. Like the bursts, the flares are but on 18 September another burst began These observations showed high-intensity attributed to injection—but in this case and two more occurred within a week. The radio-frequency emission from extensive, periodic injection—of relativistic elec- publicity accorded these bursts was im- elongated regions that can be interpreted trons. It has been suggested that the perio- mense, occupying, for example, an entire as jets of plasma into which the relativistic dicity of the flares, which is but a few issue of Nature Physical Science. electrons are injected. The speed of the jets percent longer than the x-ray periodicity, The flux-versus-time curves of the 1972 is at least 0.35c, where c is the speed of is due to perturbations of the binary bursts were found to vary with the fre- light. Such extremely fast jets have been separation by the presence in the system of quency at which the observations were observed in only one other galactic radio a more widely separated third body. made: the lower the frequency, the lower source, the x-ray binary SS 433 (Fig. 6). the maximum intensity attained and the Information about the distance to Optical Radiation. Much of our under- later the time at which the maximum in- Cygnus X-3 can be derived from absorp- standing of x-ray sources comes from data tensity occurred (Fig. 5). This frequency tion lines in the radio-frequency spectra about their visible radiation. Unfortu- dependence suggests that the bursts are obtained during its bursts. The absorption nately, Cygnus X-3 is located in or beyond caused by injection of relativistic electrons lines are produced by hydrogen atoms in a dusty spiral arm of our galaxy and can- into an expanding volume of plasma. The intervening clouds of interstellar gas. The not be detected with even the most power- radio-frequency radiation observed is Doppler shifts of the lines from the normal ful of today’s optical telescopes. Models synchrotrons radiation emitted by the elec- wavelength of 21 centimeters, coupled for the origins and observed properties of trons as they interact with the magnetic with a kinematic model of galactic rota- its x-ray emissions are therefore often field of the plasma. (A possible source of tion, yield an estimate of the distance to based on those developed for similar x-ray

LOS ALAMOS SCIENCE Spring 1986 45 Cygnus X-3

Fig. 7. Examples of air showers initiated in particles, and other charged particles, are 1014 eV) can be detected with an array of the earth’s atmosphere by energetic radia- deflected and arrive along random trajec- particle detectors, such as scintillation tion (photons or particles) from astro- tories. Shown above are early stages of counters. Air showers that do not reach the nomical sources. Such an air shower con- typical air showers initiated by (a) gamma surface of the earth but come within a sists of an increasingly numerous assort- rays and (b) protons. Electrons and posi- certain distance (those initiated by pri- ment of secondary photons and particles trons are the most abundant secondary maries with energies greater than about advancing together in a plane perpendicular particles in both gamma-ray- and proton- 1011 eV) can be detected on moonless nights to the direction of motion of the primary initiated air showers, but muons are more with an array of mirrors that gathers photon or particle. The primaries initiating abundant in proton-initiated air showers by the Cerenkov radiation emitted by the the air showers fall into two categories. a factor of about 10. An air shower dis- secondary particles. Proton-initiated air Those in one category, consisting mainly of sipates when the energies of the secondaries showers constitute an unavoidable back- gamma rays but including neutrinos, neu- become so low that processes other than ground among which air showers initiated trons, and other neutral particles, trav- those creating new secondaries are domi- by gamma rays from a source must be el through space undeflected by cosmic nant. At that point the diameter of the disk- detected. The background can be reduced by magnetic fields and arrive at the earth’s shaped shower, which is proportional to the determining the arrival directions of the atmosphere along their original trajec- energy of the primary, can be as large as primaries from a map of the intensity of tories. Primaries in the other, far more several kilometers. Air showers that reach the Cerenkov radiation or from the minute numerous category, consisting mainly of the surface of the earth (those initiated by differences in times of arrival of the secon- protons but including electrons, alpha primaries with energies greater than about dary particles at the detectors. —

sources that can be studied at optical fre- from Cygnus X-3, astronomers aimed the tion were also observed; these lasted for quencies. 200-inch Hale telescope on Mt. Palomar at times ranging from a few minutes to one- Cygnus X-3 has been assigned an ap- the same location and detected infrared and-a-half hours. The infrared flares may parent visual magnitude of no less than 23. radiation at wavelengths between 1.6 and be radiation from clumps of matter ejected It is therefore at least 6 million times 2.2 microns. (Infrared radiation suffers into the jet emerging from Cygnus X-3. If fainter than a star of magnitude 6, which is less severe extinction by dust than does so, the same clumps of matter should give the faintest star the human eye can detect visible or ultraviolet radiation. ) Observa- rise to a subsequent radio-flare when they on a dark night. tions during the next year revealed a 4.8- are viewed farther out in the jet. This hour periodicity in the infrared source that prediction needs to be tested with simulta- Infrared Radiation. Shortly after the secured its identification with Cygnus X-3. neous infrared and radio-frequency ob- first of the 1972 radio-frequency bursts Large-amplitude flares of infrared radia- servations.

LOS ALAMOS SCIENCE Spring 1986 Cygnus X-3

GAMMA-RAY LIGHT and ultrahigh-energy gamma rays that cause the flux of such high-energy photons CURVE OF CYGNUS X-3 Cygnus X-3 has been accorded a second is quite low. The experiments are made renaissance of interest. (Other astro- even more difficult by the existence of a nomical objects, including the pulsar in high, isotropic background of air showers 1,3 the Crab Nebula, the x-ray binary initiated by similarly energetic but far Hercules X-1, the Andromeda Galaxy, more numerous charged primaries, and the radio source Centaurus A have mainly protons. (The trajectories of also tentatively been identified as sources charged particles, whatever their source, of TeV and/or PeV gamma rays.) are randomized by interaction with the Low- and high-energy gamma rays from inhomogeneous magnetic field of the gal- an astronomical source can be detected axy.) This background can be reduced by with satellite-borne instruments (spark determining the arrival directions of the chambers, scintillation counters, and air showers detected. In addition, if the Compton telescopes, for example). source being investigated exhibits a peri- Whether Cygnus X-3 is a source of such odic intensity variation in other spectral radiation is uncertain. Data gathered in regions, detection of the same periodicity 1973 by the SAS-2 satellite were reported in the intensity of air showers from the to reveal a 4.8-hour periodicity in the in- direction of the source is conclusive 4.8-hour Orbital Phase tensity of 35- to 200-MeV gamma rays evidence that the primary gamma rays from the direction of the source, whereas originate from the source. Fig. 8. This light curve is based on detection data gathered by the COS-B satellite be- More than a dozen groups have re- of Cerenkov radiation emitted by secon- tween 1975 and 1982 were reported to ported detection of gamma rays from 11 dary particles in air showers. The energies include no gamma rays with energies be- Cygnus X-3 with energies of at least 10 of the primary gamma rays are greater than tween 70 and 5000 MeV that could be eV. The first report came from a Soviet about 1 TeV. The ordinate is the ratio of the attributed to the source. This null result is group in 1972, shortly after the radio- Cerenkov radiation intensity with the apparently not related to the x-ray state of frequency bursts. The tell-tale 4.8-hour source inside the field of view to that with Cygnus X-3, since the COS-B observations periodicity in the gamma-ray flux was the source outside the field of view. Note the covered both high and low x-ray states. subsequently detected by the same group prominent peak at a phase of about 0.75. TeV and PeV gamma rays have such and by American observers. Some in- Other investigators have reported slightly low interaction probabilities that their de- vestigators report peaks in the gamma-ray different phase values for this peak and a tection with conventional instruments is light curve at a phase of about 0.2 (relative second prominent peak at phase values near not practical. Instead, these gamma rays to the x-ray minimum), others at a phase 0.2. (Figure adapted from S. Danaher, D. J. are observed by detecting the air showers of about 0.65, and still others at both Fegan, N. A. Porter, and T. C. Weekes, they initiate high in the earth’s atmosphere phases (Fig. 8). (The exact phase values Nature 289(1981):568.) (Fig. 7). A gamma-ray-initiated air shower reported for the peaks differ slightly. It has consists of an assortment of photons and been suggested that the differences may be secondary particles (mostly electrons and due to an inherent bias in the data result- positrons) that advances toward the earth ing from the close coincidence of a 24- Gamma Rays and Cosmic Rays. The in a plane perpendicular to the direction of hour day to an integral multiple of the term ‘gamma ray’ is applied to photons motion of the primary gamma ray. Air 4.79-hour period of Cygnus X-3. This with energies greater than about 106 eV showers are detected by sensing either the coincidence, together with the fact that and covers a wide range of energies. Cerenkov radiation emitted by the secon- gamma-ray sources are usually observed Gamma rays (and high-energy particles) dary particles or the secondary particles near zenith, when the signal-to-noise ratio are therefore often subdivided as follows: themselves. (The latter technique is lim- is greatest, implies that data gathered at a low-energy, or ‘MeV,’ gamma rays (E < ited to air showers that reach the surface of particular site during a few days of ob- 108 eV); high-energy, or ‘GeV,’ gamma the earth, that is, to those initiated by servation cover only a small portion of the rays (108 eV < E < 1011 eV); very-high- primaries with an energy of at least 1014 Cygnus X-3 cycle.) Sporadic flux increases energy, or ‘TeV,’ gamma rays (1011 eV < E eV.) lasting on the order of minutes are also <1014 eV); and ultrahigh-energy, or ‘PeV,’ Experiments aimed at observing TeV observed (Fig. 9). gamma rays (E> 1014 eV). and PeV gamma rays from an astro- PeV gamma rays from the direction of It is as a possible source of very-high- nomical source are time-consuming be- Cygnus X-3 and with its 4.8-hour perio-

LOS ALAMOS SCIENCE Spring 1986 47 Cygnus X-3

dicity were first reported in 1983 by a West German group. Their finding has since been verified by groups working in Eng- land, the United States, India, and Italy. The PeV gamma rays peak at approx- imately the same phases as do the TeV gamma rays. The extremely energetic gamma rays emanating from Cygnus X-3 are un- doubtedly the products of interactions between even more energetic particles within the source, mainly protons. Cygnus X-3 is thus the first astronomical object to be identified with reasonable certainty as a source of cosmic rays. (The term ‘cosmic ray’ is applied to any cosmic radiation with an energy greater than about 108 eV. Cosmic rays include protons (92 percent), helium nuclei (6 percent), electrons (1 percent), gamma rays (<0.1 percent), and small percentages of heavier nuclei and other elementary particles.) Calculations based on the observed flux of TeV and PeV gamma rays from Cygnus X-3 in- 23:00 dicate that only a very small number of 00:00 01:00 Sidereal Time sources of like nature would be required to produce most of the observed high-energy Fig. 9. Cygnus X-3 is an intrinsically variable gamma-ray source, as demonstrated by this cosmic rays. plot of an 8-minute burst. The energies of the primary gamma rays are greater than about 1 The question that has aroused so much TeV. (Figure adapted from T. C. Weekes, Astronomy and Astrophysics 121(1983):232.) interest is how such energetic protons can be produced in Cygnus X-3. Two of several responses to the question invoke ac- remains moot, but the pulsar model seems Cygnus X-3. These reports come from cretion as the ultimate energy source: in to have several points in its favor. It may groups searching for evidence of proton one of these models, the unipolar inductor explain the two peaks in the TeV and PeV decay in detectors far underground, one in model, protons are accelerated by the elec- gamma-ray light curves (as interaction of the Soudan iron mine in Minnesota and tric field induced in an accretion disk by the proton beam with the atmosphere of the other in the Mont Blanc Tunnel in the magnetic field of a slowly rotating the companion star or with some structure Europe. Both groups report that the flux of neutron star (that is, a neutron star with a on an accretion disk), and, if the accelera- muons recorded by their detectors exhibits rotation period of about 1 second); in the tion mechanism produces 1017-eV an enhancement in the direction of other protons are accelerated by shocks in protons, it can reproduce fairly well the Cygnus X-3 and with its 4.8-hour perio- the flow of matter accreting onto a neutron observed spectrum of TeV and PeV dicity. (Muons are among the secondary star or a black hole. A third model iden- gamma rays. Furthermore, evidence for particles formed by interaction of tifies the energy source as the rotational the existence of a pulsar in Cygnus X-3 (in primaries with the earth’s atmosphere. energy lost by a rapidly rotating, magne- the form of a 12.59-millisecond peri- The relative abundance of muons among tized neutron star (a pulsar) in the process odicity in the TeV gamma-ray flux) was the products depends on the identity of the of gradually winding down. Electric fields very recently reported but has not yet been primary; in particular, protons produce a sufficiently high to accelerate protons to confirmed. greater number of muons than do gamma 1016 eV are possible near a pulsar with a rays (see Fig. 7).) magnetic field of about 1012 gauss and a New Particles? Two recent, and con- What are the primaries responsible for rotation period of about 10 milliseconds. troversial, reports have given an addi- these muons? The directionality and Which, if any, of these models is correct tional boost to the resurgence of interest in periodicity of the muons, and thus of the

48 Spring 1986 LOS ALAMOS SCIENCE Cygnus X-3

primaries, eliminate as candidates all charged particles, for the reason men- tioned above. Neutrons would decay during the 11.6-kiloparsec journey from Cygnus X-3, unless they had the un- reasonably high energy of about 10]8 eV. Neutrinos, oblivious to the intervention of the earth between their source and a detector, would produce a flux of muons that is independent of the position of the source when the observations are made. But the Soudan group reports that the enhancement in muon flux reaches a maximum when Cygnus X-3 is overhead. More detailed arguments provide limits on the masses and lifetimes of the primaries that eliminate all other known neutral particles. Gamma rays should be the most likely candidates for the primaries, but the enhancement in muon flux reported by the Soudan group is much too large to have been produced by the flux of TeV and PeV gamma rays observed above ground. Moreover, the West German group had previously reported a muon content in air showers initiated by PeV radiation from Cygnus X-3 (presumably but possibly not gamma rays) that also was too high for gamma-ray initiation. Thus, if the experimental evidence is confirmed, no known particle can be the primary responsible for the enhancements in muon flux. One response to this puzzle has been the conjecture that some previously unobserved neutral particle is Fig. 10. Brenda Dingus, a University of Maryland graduate student, examining one of emanating from the source. The compo- seventy scintillation counters being installed on the grounds of the Los Alamos Meson sition of some of the proposed candidates Physics Facility. The counters compose one element of an experiment aimed at resolving a has led in turn to the suggestion that the current controversy about Cygnus X-3. (The experiment is a collaborative effort by the compact star in Cygnus X-3 consists pri- University of Maryland, the University of New Mexico, the University of California, marily of matter containing a substantial Irvine, and Los Alamos National Laboratory.) The counters are distributed uniformly fraction of strange quarks (see “Does within a 60-meter-radius circle centered on the other essential element of the experiment—a Cygnus X-3 Contain a Strange Neutron spark-chamber detector already in use in a study of the scattering of accelerator-produced Star?"). neutrinos by electrons. The array of scintillation counters will provide highly accurate data However, the statistical analyses of the about the direction of the air showers detected and thus permit selection of air showers Soudan and Mont Blanc groups have been initiated by electrically neutral primaries from Cygnus X-3; the spark-chamber detector challenged, and confidence in their find- will provide data about the muon content of those same air showers. If the muon content of ings has been lessened by more recent the selected air showers is inconsistent with that theoretically predicted for gamma-ray- reports. Analyses of data from another initiated air showers, then either the theories of nuclear interactions need modification or proton-decay detector (in the Silver King some as yet undiscovered neutral particle is originating from Cygnus X-3. continued on page 53

LOS ALAMOS SCIENCE Spring 1986 49 eep underground proton-decay charge must be zero; otherwise the direc- a sufficiently long lifetime, of order a year detectors in the Soudan iron mine tionality and timing of the signal would be in its rest frame, that it not decay en route D in Minnesota and under the Mont destroyed by galactic magnetic fields. from the source. (Lorentz dilation in- Blanc have recorded very energetic muons Secondly, the mass of the primary must be creases the observed lifetime of a rapidly coming from the direction of Cygnus X-3 less than its energy by a factor of about 104; moving particle by the ratio of its energy with its 4.79-hour periodicity. These ob- otherwise differences in travel times of to its mass.) The known neutral particles servations, if confirmed, present a very primaries with different energies would with such properties are photons, neu- challenging puzzle. What is the primary wash out the periodicity of the primaries trinos, and neutrons, but arguments cosmic-ray particle that produces the and hence that of the muons. (A 1OO-GeV- presented in the main text appear to rule muons at the earth, and how is such a mass particle, for example, would arrive these out. Briefly, the reported flux of particle produced in Cygnus X-3? One of about 1 hour sooner if it had an energy of muons is too high to be attributed to the more coherent explanations is that the 12 TeV than if it had an energy of 10 TeV gamma rays (high-energy photons), the primaries originate as exotic hadrons (1 GeV = 109 eV and 1 TeV = 1012 eV).) observed dependence of the muon flux on (strongly interacting particles, not yet To produce muons with sufficient energy zenith angle rules out neutrinos, and neu- made in laboratories) chipped off the neu- to penetrate the overlying rock and reach trons would decay in flight unless their tron star in Cygnus X-3, a star itself made the great depths of the detectors (equiva- energy was unacceptably large. entirely of matter containing a substantial lent to 2 to 5 kilometers of water), the The only remaining possibility is a fraction of strange quarks (Fig. 1). energies of the primaries are likely to be in previously unobserved particle, a ‘cygnet.’ The detection of a periodic muon signal the range 10 to 100 TeV; the mass of the The large flux of muons (comparable to deep underground constrains the prop- primaries is therefore likely to be at most 1 the observed flux of gamma rays), and erties of the primary. Firstly, its electric to 10 GeV. Lastly, the primary must have hence of cygnets, suggests that cygnets are

50 Spring 1986 LOS ALAMOS SCIENCE Star?

Does Cygnus X-3 Contain a Strange

Consultant to the Theoretical Division from the University of Illinois

made at a rapid rate through strong inter- ness of 1, as when a lambda decays into a A more likely possibility is that the cygnet actions rather than through the slower nucleon. is accelerated from a neutron star bound to electromagnetic or weak interactions. One How might cygnets be made in Cygnus charged particles in the form of an exotic promising candidate for this strongly in- X-3? To generate the high-energy gamma nucleus. Free cygnets could then be re- teracting particle is the H particle, earlier rays believed responsible for the extensive leased by fragmentation of such a nucleus proposed by Robert L. Jaffe of MIT, com- air showers observed, Cygnus X-3 must when it strikes a particle in the atmosphere posed of two up, two down, and two have an accelerator capable of producing of the companion star, in a process similar strange quarks in a closely bound state; the charged particles with energies up to 1016 to proton-nucleus fragmentation observed His thus a particle with a strangeness of 2 eV. Cygnets might be produced as the in the laboratory. and a baryon number of 2, the same quan- energetic charged particles accelerated The next question is how exotic nuclei tum numbers as two lambda particles. If from a neutron star interact with the at- might be produced and emitted from a the mass of the H is less than that of the mosphere of the companion star. How- neutron star. A first possibility is that they lambda (1.1 16 GeV) plus that of the neu- ever, since the cross section for this proc- are made by bombardment of the surface tron (0.938 GeV), then the lifetime of the ess would have to be large to produce them of the neutron star by particles accelerated H could be sufficiently long for it to be a in quantities comparable to those of the onto it. (In the electromagnetic accelera- candidate for the primary, since in this gamma rays, we would expect to have seen tion process electron-positron pairs will be case it could not undergo the rapid decay cygnets produced in laboratory accelerator produced, and if, for example, positrons into a lambda and a neutron. Decay of the experiments. (The cygnet mass should be are accelerated away, then the electrons H into two neutrons would be very slow relatively low, so the energy threshold for will be accelerated back to the surface, at since it involves a change in strangeness of producing them should be well below the energies of a TeV or greater, and cause 2, a rarer process than a change in strange- energies available at current accelerators.) substantial spallation of the surface.) This

LOS ALAMOS SCIENCE Spring 1986 51 Cygnus X-3—A Strange Neutron Star?

Exotic Charged Underground Particles Cygnets Detector

Neutron

Muons

Comparion Star FROM STRANGE QUARK MATTER TO MUONS

Fig. 1. A scenario for the observation, in of a strange neutron star cause ejection of star. The cygnets released travel un- underground detectors, of muons with the exotic charged nuclei, which are accel- deflected to the earth’s atmosphere, where directionality and periodicity of Cygnus erated outward and fragment as they pass they produce muons that penetrate to the X-3. Particles accelerated onto the surface through the atmosphere of the companion underground detectors.

exceptionally long, at least the time re- a large density fluctuation. If the strange quired for diffusion, of order 105 years. state is lower in energy per baryon than the The final possibility is that the entire neu- normal state of nuclear matter, then once tron star is made of strange matter, and formed the seed will begin to convert the surface spallation throws exotic nuclei up matter around it into strange matter, as a into the beam of particles accelerated away fire spreads through flammable material. from the neutron star. The ‘burning’ front would first convert the Neutron stars may very well be made of liquid core of the neutron star to exotic matter containing a substantial fraction of matter; the heat ahead of the front would strange quarks if, as Edward Witten of melt the crust of the neutron star, as well Princeton conjectured, the absolute as melt the nuclei in the crust into normal Matter ground state of matter might not be the fluid nuclear matter, and within an hour Density familiar material nuclei are made of, but or so the entire star would be converted rather is ‘strange quark matter’ in which into a strange neutron star. Fig. 2. If, as illustrated here, the minimum the quarks, a substantial fraction of which One important consequence of this in the energy per baryon versus density are strange, are not confined within indi- scenario is that if the compact star in curve for normal nuclear matter is higher vidual nucleons but are free to roam Cygnus X-3 is a strange neutron star, then than that for strange quark matter, then throughout. By having less zero-point, or many, if not all, neutron stars should also, normal matter, which in its ground state Fermi, energy, such matter could be stable as a result of the same burning process, be sits at the minimum of the normal matter compared to ordinary nuclear matter (Fig. strange. Strange neutron stars are expected curve, would be unstable against transition 2). (We need not worry about ordinary to cool more rapidly than normal stars to strange quark matter. This transition nuclei turning into strange nuclei if strange since they can emit neutrinos more could result in a neutron star composed matter is the lowest energy state only when rapidly. This enhanced cooling should be entirely of strange quark matter. a finite percentage of the baryons are observable in measurements with future x- strange.) ray telescopes of the surface temperatures mechanism might produce exotic nuclei Imagine then a neutron star being of neutron stars. from normal nuclei, but one is faced with formed (of normal nucleons) in the core of The Cygnus X-3 muon data sugest the the question of why, if correct, it has never a supernova explosion. At the very high existence of a new and unusual particle been observed in the laboratory. A second densities in the center (an order of magni- produced in a new and unusual way. If possibility is that exotic nuclei are tude above the density of laboratory nu- future measurements confirm these data, produced in the core of the neutron star clei, some 3 X 104 grams per cubic cen- the underground experiments will have and then diffuse to the surface. But the timeter), a seed of strange quark matter led to a remarkable discovery of new lifetimes of the exotic nuclei must then be can form either spontaneously or through physical phenomena. ■

52 Spring 1986 LOS ALAMOS SCIENCE Cygnus X-3

Orbital Phase gators will soon be carrying out an experiment directed toward that goal (Fig. 10). 0.1 0.3 0.5 0.7 0.9 1.1 Of course, the apparent conflict in the 30 experimental evidence may be due to other factors, such as instrumental error or some intrinsic variability of the source. 2.0 More data are needed before the new- particle hypothesis can be either accepted or rejected.

Attempts at a 25 More Complete View 1.6 The first attempt at simultaneous multifrequency observation of Cygnus X-3 was made in 1973 by an international group using the Copernicus x-ray satellite and various radio and infrared telescopes. The 1.2 group collected data between June and 20 October of that year but achieved simultaneity for times totaling only two hours at all three frequency ranges and only several hours at infrared and x-ray frequencies, Unfortunately, these simultaneous data, 0.8 and somewhat more extensive simultaneous data obtained by the same group in September 1974, were too discontinuous 15 to permit inference of temporal relations among the emissions of the source at different frequency ranges, The data as a 0.4 whole did, however, reveal some puzzling aspects of the orbital modulation: its constant presence at x-ray frequencies, its lesser magnitude and sometimes absence at infrared frequencies, and its complete 10 0 absence at radio frequencies. (The coming 08:00 09:00 10:00 11:00 12:00 13:00 and going of the infrared orbital modula- Universal Time tion in these early data is now attributed to the combined effects of intrinsic varia- Fig. 11. Simultaneous infrared (red) and x-ray (blue) data taken by the author and K. O. bility and the low sensitivity and temporal Mason on 2 September 1984 are plotted here with the infrared and x-ray minima resolution of the instruments used. The superposed to emphasize the almost identical shapes of the orbital modulations. The flares absence of the radio-frequency orbital so evident in the infrared data have no apparent x-ray counterparts. modulation is attributed to the large size of the radio-emitting region. Ten years later the author and two co]- continued from page 49 muon content in air showers initiated by leagues, taking advantage of the greater mine in Utah) and from a neutrino de- radiation from the direction of the source sensitivity and temporal resolution of the tector (in the Homestake mine in South that is consistent with gamma-ray initia- x-ray monitor aboard EXOSAT and the Dakota) reveal no evidence of a muon flux tion. Resolution of this muon-content dis- infrared telescope on Mauna Kea, made attributable to primaries from Cygnus agreement is a matter of high priority, and the second attempt at simultaneous x-ray X-3, and Japanese investigators report a a group including Laboratory investi- and infrared observations of Cygnus X-3.

LOS ALAMOS SCIENCE Spring 1986 53 Cygnus X-3

This attempt yielded continuous simulta- the visibility of this corona and causes the gamma-ray activity, and between spectral neous coverage of an entire orbital period. asymmetric shape of the x-ray modulation changes and flux changes. A striking feature of the simultaneous light (see Fig. 2). To produce an infrared mod- curves is the clear presence in both of an ulation of the same shape, the infrared The Future orbital modulation of nearly identical radiation must originate from the corona shape, although of different magnitude as bremsstrahlung. This model, like the Many other astronomical objects de- (Fig. 11). These features must be ac- stellar wind model, invokes unmodulated serve simultaneous multifrequency ob- counted for in any model of the source. bremsstrahlung and photoelectric absorp- servation, and, indeed, some have re- Two models that may be applicable to tion to explain the other features of the ceived it. The results, especially in the case Cygnus X-3 are the stellar wind model and infrared and x-ray modulations. of flare stars and BL Lacertae objects, have the accretion-disk corona model; both can The data obtained during the Septem- been most informative. But the experi- qualitatively explain the identical shapes ber 1984 observations also revealed a con- ments, as now conducted, are difficult to of the x-ray and infrared modulations, the tinual succession of infrared flares lasting organize and expensive of limited re- asymmetry of the modulations about the between 2 and 10 minutes (see Fig. 11). sources, such as human time, telescope minima, the lesser magnitude of the in- Except for the largest, these flares would time, money, and fuel for maneuvering frared modulation, and the greater magni- not have been detected with the instru- satellites into position. Furthermore, the tude of the x-ray modulation at low (less ments used in 1974. The maximum in- goal of achieving simultaneity is often not than 6 keV) x-ray energies. frared luminosity of the source during the met: scheduling problems, failure of one In the stellar wind model x rays from largest flare, which occurred during the x- or more of the detectors or data-acquisi- the compact star are scattered into the line ray minimum, was very high, 1037 ergs per tion systems, and lack of cooperation by of sight by an optically thick cloud of second. No corresponding x-ray flares are the weather can lead to gaps in what was plasma evaporated from the companion obvious, although they may be obscured intended as simultaneous coverage. star (a stellar wind). In this model the by other variable x-ray activity. These difficulties could be alleviated by asymmetric modulation of the x rays is The October 1985 multifrequency cam- outfitting satellites with as many detectors attributed to a wake formed in the cloud paign described at the beginning of this as possible, each covering a different fre- by radiation pressure of the compact star article was launched in the hopes of ob- quency range. In line with this goal, Labo- as it moves through the cloud. To produce taining longer periods of continuous si- ratory astronomers and collaborators x-ray and infrared modulations of the multaneous coverage of Cygnus X-3 over from abroad have prepared a proposal for same shape, the cloud must present an a greater frequency range. Serendipitously, addition of optical and ultraviolet effective photosphere of the same radius the radio, infrared, and x-ray observations monitors to future American and Euro- to both types of radiation. Electron scatter- were made during an epoch of large radio- pean x-ray satellites. Another possibility ing, the dominant mechanism for scatter- frequency flares. Analysis of data from the being considered is the mounting of a ing the x rays, must therefore also be the Soudan proton-decay detector reportedly multifrequency observing platform on the dominant mechanism for scattering the reveals a correlation between muon events American space station now being infrared radiation. The lesser magnitude and these flares. planned. of the infrared modulation is attributed to Too little time has elapsed for detailed Such arrays of instruments will make dilution by unmodulated bremsstrahlung analyses of all the other data, but we do simultaneous coverage much easier and from the far reaches of the cloud, and the know that Cygnus X-3 was behaving un- more certain and, when directed toward a greater magnitude of the x-ray modulation usually not only at radio frequencies but particular astronomical object, will ac- at low x-ray energies is attributed to also at infrared and x-ray frequencies. Its celerate progress toward its understand- photoelectric absorption of low-energy x x-ray flux was greater than ever before, ing. An equally, if not more, compelling rays within the cloud. and, compared to the previous year, its argument for deployment of detectors in In the accretion-disk corona model infrared flux was greater by a factor of 2 to this manner lies in the history of astron- (details of which are presented in 4 and its infrared flares were more intense omy, and all of science. X-ray sources, “X1822–371 and the Accretion-Disk Co- and lasted longer. Apparently infrared pulsars, sources of gamma-ray bursts, the rona Model”) x rays from the compact star flares, like radio flares, are characterized microwave background-all were dis- are scattered by an optically thick corona by a large spectrum of durations and covered by instruments aimed at the evaporated from an accretion disk rather amplitudes. Analyses of the data will focus heavens for other purposes. We cannot than from the companion star. Asym- on searching for correlations between the know in advance what there is to know, metric azimuthal variation in the height of times at which infrared and radio flares but we dare not let pass the opportunity to the accretion disk at its outer edge limits begin and peak, between x-ray and discover it. ■

54 Spring 1986 LOS ALAMOS SCIENCE X1822 – 371 and the Accretion-Disk Corona Model

by France Anne-Dominic Cordova

he galactic x-ray source known by source as a binary system. (In contrast, mum) of the narrow dip to the angle its coordinates as X 1822—371 is of Cygnus X-3 can at present only be pre- subtended by the companion star at the Tparticular interest because its x-ray sumed to be a binary system. ) neutron star suggested that this feature light curve, like that of Cygnus X-3, is The picture of X 1822–371 that is most was due to occultation by the companion unusual for an eclipsing binary. However, consistent with spectroscopic and star of a luminous accretion disk in a X1822–371, unlike Cygnus X-3, can be photometric studies of its optical radiation system with a binary inclination near 90”. observed at optical frequencies, and much is that of a binary system composed of a (The inclination of a binary system is the information basic to the development of low-mass, late-spectral-type companion angle between the axis of rotation of the models for x-ray binaries is obtained from star filling its Roche lobe, an accretion system and the line of sight.) The shape of optical data. The models fashioned for disk, and a neutron star emitting x rays as the broad dip suggested that the luminous X1822–371 illustrate well how astrophys- matter accretes onto its surface from the region was being obscured by some ex- icists infer the existence and properties companion star. Estimates for the masses tended structure, perhaps a bulge on the of structures they cannot image directly. and radii of the component stars and for outer edge of the accretion disk or the X1822–371 first came to notice in the the binary separation are listed in the ac- stream of accreting matter between the early seventies through detection of its x companying table. companion star and the disk. rays by the Uhuru satellite. Not until 1978, Qualitative attempts by K. O. Mason These suggestions, together with the ob- however, was its faint optical counterpart and colleagues to explain the optical light servational data of many astronomers, led identified. A 5.57-hour periodicity in the curve of X1822—371 led to suggestions to the development by N. E. White and S. intensity of its continuum optical radia- about the source of the optical radiation S. Holt of a model (the accretion-disk co- tion was discovered soon thereafter, and and the existence in the system of some rona model) for the x-ray light curve of the same periodicity was subsequently de- occulting structure in addition to the com- X1822–371, which, like the optical light tected in its x-ray, ultraviolet, and infrared panion star. As shown in Fig. 1, the optical curve, exhibits a narrow and a broad dip in emissions and in the intensities and Dop- light curve exhibits two dips in intensity: a intensity (see Fig. 1). In this model the pler shifts of its optical emission lines. The narrow dip to the minimum intensity and narrow dip in the x-ray light curve is at- periodic variation in the Doppler shifts a broader asymmetric dip. The near equal- tributed to occultation by the companion permitted positive identification of the ity of the fractional width (at half mini- star of an extended x-ray source centered

LOS ALAMOS SCIENCE Spring 1986 55 Xl 822–371

on the neutron star. This source—a coro- Table na of x-ray-scattering plasma extending Basic properties of the x-ray binary X1822–371. The mass listed for the neutron star is above and below the plane of the accretion typical of those measured for pulsating neutron stars; the radius is that derived from disk—may be formed as matter is theoretical calculations. The binary inclination and the distance to the source were evaporated from the inner portion of the inferred from a fit of the optical light curve to the accretion-disk corona model. The other accretion disk by the radiation pressure of properties were derived from spectroscopic and photometric studies of optical radiation the neutron star. Compton scattering was from the source. Complete references are provided in the bibliography at the end of the assumed to be the dominant scattering article. mechanism in the corona, since the observed x-ray spectrum of Xl 822–371 Property Value Reference could be interpreted as resulting from Comptonization of a hard x-ray spectrum Neutron star mass by an optically thick corona. Neutron star radius -10 kilometers White and Holt showed how the broad dip in the x-ray light curve could arise Companion star mass Mason et al. 1982 from occultation of the corona by a promi- Companion star radius Mason et al. 1980 nent bulge on the outer edge of the accretion disk at the confluence of the disk and Binary separation Mason et al. 1980 the stream of accreting matter. This bulge Binary inclination 76° to 84° Mason and Cordova 1982 may be caused by turbulence. A smaller Distance 2 to 3 kiloparsecs Mason and Cordova 1982

OPTICAL LIGHT CURVE OF X1822--371 X-RAY LIGHT CURVE OF X1822–371

0.5 0 0 0.5 0 0.5 5.57-hour Orbital Phase 5.57-hour Orbital Phase

Fig. 1. Both the optical and the x-ray light reproduced with the accretion-disk corona Tuohy, and L. K. Hunt, The Astrophysical curves of X1822—371 show a narrow dip to model (see text). (Optical light curve Journal 242(198O):L1O9. X-ray light curve the minimum intensity convoluted with an adapted from K. O. Mason, J. Middleditch, adapted from N. White and K. Mason, earlier, broader dip. Both curves can be J. E. Nelson, N. E. White, P. Seitzer, I. R. Space Science Reviews 40(1985):167.)

56 Spring 1986 LOS ALAMOS SCIENCE X1822–371

bulge upstream of the prominent bulge is (a) ACCRETION-DISK CORONA MODEL also included in their model. FOR X1822--371 The parameters that can be varied in fitting an accretion-disk corona model to a Cross Section in light curve include the inclination of the Neutron Occulting Orbital Plane binary, the radii of the corona and of the Star Bulge disk, and the height(s) of the bulge(s). From a fit of the model to the x-ray light curve, White and Holt inferred that the inclination of the binary is about 75°, the

half that of the large bulge, the radius of

Figure 2 illustrates their model and its fit to the x-ray light curve. K. O. Mason and the author have found that the accretion-disk corona model also provides good fits to the light curves of Cross Section in Plane X1822–371 in spectral regions other than the x-ray, namely, the infrared, optical, and ultraviolet regions. (Figure 3 shows the fit to the optical light curve.) In their calculations they included contributions to the total radiation from four regions: the accretion disk, the inner surface of the (b) MODEL FIT TO X-RAY DATA thickened outer rim of the disk, the surface of the companion star facing the neutron star (all being heated by x rays from the neutron star), and the outer surface of the Calculated rim. The contribution from each region is X-Ray Light Curve I I modulated differently by orbital motion. Reprocessed x rays are assumed to domi- nate the radiation from the accretion disk and the inner surface of its rim. They found that the best tits to the three light curves were obtained with a binary inclination of about 80°. Their tit to the optical light curve yielded values for the areas of emitting regions; these areas were used to infer a distance to the source of 0.6 0.8 0.0 0.2 0.4 between 2 and 3 kiloparsecs. 5.57-hour Orbital Phase Mason and the author also fitted the observed near-infrared to far-ultraviolet spectrum of X 1822–371 (at maximum Fig. 2. An accretion-disk corona model for X1822–371 containing the structural features light) to a blackbody spectral model. They shown in (a), namely, a scattering corona within the disk and occulting bulges on the outer found that the source could be approx- rim of the disk, can befitted, as shown in (b), to the x-ray light curve with appropriate imated well by a 27,000-kelvin blackbody choices for the geometric parameters that appear in the model (see text). (Adapted from N. slightly reddened by interstellar absorp- E. White and S. S. Holt, The Astrophysical Journal 257(1982):318.) tion. The x-ray luminosity required to heat

LOS ALAMOS SCIENCE Spring 1986 57 X1822–371

MODEL FIT TO OPTICAL DATA

the disk to this temperature is about 1036 ergs per second, a value that is consistent with the observed x-ray flux and the esti- mated distance to the source. Although the x-ray light curve of Cygnus X-3 does not exhibit a narrow dip in intensity, it does exhibit a broad dip that cannot be attributed to photoelectric absorption. The gross morphology of this broad dip can be reproduced with the accretion-disk corona model. The fit to the x-ray light curve yields the following pic- 0 0.5 u 0.5 ture of the system: an inclination of about 5.57-hour Orbital Phase 70°; a corona with a radius equal to three- quarters of the radius of the accretion disk; Fig. 3. If the optical radiation from X1822–371 is assumed to consist of a weighted sum of and, on the outer edge of the disk, a large contributions from four luminous regions, its optical light curve can be reproduced well by bulge with a height equal to at least half the the accretion-disk corona model. The luminous regions are (1) the inner surface of the radius of the disk and subtending an angle thickened rim of the accretion disk, (2) the accretion disk, (3) the outer surface of the of about 40° at the compact star. The thickened rim of the accretion disk, and (4) the face of the companion star illuminated by author and colleagues are currently the neutron star. The contribution from each region is modulated as shown by the analyzing recent infrared data for Cygnus companion star and structures on the accretion disk. (Figure adapted from Keith O. Mason X-3 to see if its infrared light curve also and France A. Cordova, The Astrophysical Journal 262(1982):253.) can be reproduced with this model. ■

France Anne-Dominic Cordova received a B.A. in English literature from Stanford University and, in 1979, a Ph.D. in physics from the Cal- ifornia Institute of Technology. She immediately joined the Laboratory as a staff member in what is now the Space Astronomy and Astrophysics Group. In 1983 she took a year’s professional leave of absence to enjoy a NATO postdoctoral fellowship at the United King- dom’s Mullard Space Science Laboratory, where she analyzed data from the European Space Agency’s EXOSAT satellite. She has made astronomical observations in nearly every region of the electromagnetic spectrum and has written over fifty professional papers and popular astronomy articles. In 1984 she was named by Science Digest as one of America’s hundred brightest scientists under forty. She has served on the National Science Foundation’s Advisory Council and is at present a member of the International Users Com- mittee for West Germany’s Roentgen satellite and president of the Los Alamos Mountaineers.

58 Spring 1986 LOS ALAMOS SCIENCE Cygnus X-3