• Abstract and Figures
• Public Full-text

by France Anne-Dominic Cordova lthough the visible radiation of Cygnus A X-3 is absorbed in a dusty spiral arm of our gal- axy, 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 instru- ments, 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 consist- ing 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 inter- ferometer 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 sat- ellite 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 fre- quencies 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.

Load more

  22

Copy Link