University Lowbrow Astronomers

Fall 2000 Saturday Morning Physics: X-ray Astronomy.

by Dave Snyder
Printed in Reflections: January, 2001.

Saturday Morning Physics has covered a variety of topics.  This fall there were nine lectures, six of which covered X-rays (three on X-ray astronomy and three covered other aspects of X-rays).  Dr. Martin Sulknen gave the three X-ray astronomy lectures.  He covered the history of X-ray Astronomy, current X-ray observatories including Chandra and plans for future X-ray observations.

Visible light has been used to explore the universe since the 1600’s.  Astronomers have gained insights from visible light, however as astronomers have viewed the skies in other parts of the EM spectrum, they have gained knowledge that was not possible with visible light.  In 1932, Jansky observed radio waves that came from space.  Visible light and radio waves can be detected from earth’s surface, however UV and X-rays are absorbed by the atmosphere.  The only practical way to observe UV or X-rays is to have an observatory outside the earth’s atmosphere.  The earliest attempts used captured V2 rockets:  In 1946 a rocket observed UV.  The first attempt to observe X-rays failed but a 1949 attempt successfully observed X-rays.  (Both the UV and X-rays came from the sun).  After detecting X-rays from the sun, in 1962 a different rocket found the next brightest X-ray source in the sky.  It is located in Scorpio and is now known as Scorpius X-1.

A variety of devices can detect X-rays.  The 1949 attempt used a device known as a proportional counter.  It is also possible to detect X-rays with photographic plates or with Geiger-Muller Counters that are equipped with special filters.  However proportional counters are able to determine the energy of X-ray photons, which is not possible with either photographic plates or Geiger Counters.  More recent instruments use electronic detectors such as the Charge Coupled Detector known as ACIS.

However simple detection of X-rays is not enough.  It is important to collimate the X-rays (in other words only detect X-rays from specific directions), without collimation astronomers would have no way to determine the location of X-ray sources.  In addition astronomical X-rays are generally dim.  It is necessary to focus X-rays to obtain enough signal to detect any sources beyond the sun.  You cannot focus X-rays with glass lenses or glass mirrors.  X-rays reflect off Nickel, Gold and Iridium if hit at a grazing angle.  A cone made of any of these metals can both collimate and focus X-rays.  A set of cones placed one inside the next (sort of like a set of Russian Dolls) works even better than a single cone.

Most X-ray observations since the 1970’s have been made from satellites (rather than rockets) equipped with nested metal cones as the focusing mechanism.  In the 70’s three satellites, SAS 1 (better known as Uhuru), HEAO 1 and HEAO 2 (the later known as Einstein) conducted systematic searches for X-ray sources.  In the early 1990’s, two new satellites were launched:  Rosat and Asuka.

These satellites lead NASA to embark on a grand project:  a set of four observatories in earth orbit which collectively can observe most of the EM spectrum.  This project included

Chandra was named for Subrahmanyan Chandrasekhar (1910-1995, awarded the Nobel Prize in 1983).  Chandrasekhar is best known for his theory that predicts (among other things) that stars with mass greater than 1.44 solar masses would rapidly collapse into a black hole.

Chandra has a collection area of 300 square centimeters (for comparison, an eight inch telescope has a collection area of 324 square centimeters), a focal length of 10 meters and can detect X-rays that have energies anywhere from 100 to 10,000 electron volts.  It consists of a single telescope along with a detector and a spectroscope.  The spectroscope is useful only for small and relatively bright sources.  The electronic detector can detect X-ray energies (like the spectroscope).  While it doesn’t have the resolution of the spectroscope, it can be used for dim or spread out sources.  In addition, it has an optical telescope which is used to guide the X-ray telescope.

Chandra took eight years to construct.  Great care was needed to build the metal cones which must be very smooth.  The shape takes into account gravitional forces (when Chandra was placed into orbit, the reduced gravity caused the cones to assume the correct shape).  Chandra was put into a highly elliptical orbit.  Such an orbit was designed so that Chandra can avoid the Van Allen radiation belts.  (It has to shut down when it is inside the radiation belts, but is able to make observations the rest of the time).

X-rays are expected within extreme environments such as gas at 10 million degrees Kelvin, strong gravitational fields (for example near a neutron star or a black hole), strong magnetic fields, or the shock waves from supernovae.

Since its launch in the July 1999, Chandra has been able to make numerous contributions to science by looking at such regions.  Chandra has taken photographs of Cassiopeia A, the Crab Nebula, the Vela Pulsar, the Orion Nebula, the Antenna Galaxies the Andromeda Galaxy, the black hole region of our galaxy, the black hole region of M82, brown drawfs and Eta Carninae (among others).  These photographs have as much detail as photographs taken with visible light.  Chandra has also produced spectra, a variety of elements have been detected from Chandra’s spectra, including Iron, Calcium, Argon, Sulfur, Silicon, Magnesium, Neon and Oxygen.  The observations of Cassiopeia A suggest it is an unusual object known as a magnetar (these objects have enormous magnetic fields that could explain the X-ray emissions).  Astronomers have discovered small black holes (about 1 solar mass) and large black holes (about 1 million solar masses), but black holes with intermediate sizes had never been observed.  The black hole at the center of M82 appears to be such a mid-sized black hole.  In addition Chandra has collected observations that may explain faint emissions of X-rays that can be seen in all directions within the celestial sphere.  This required precise imaging to detect the numerous objects that make up this background radiation.

In addition, Chandra has observed galaxy clusters (there is little matter and no stars at the gravity well of a galaxy cluster, but X-rays form in the gravity wells); the central black hole of both Centaurus A and Pictor A (the central black hole of these galaxies have jets of material that produce X-rays, such jets are common but poorly understood); and Comet S4 (Linear) (a few recent comets have been observed to emit X-rays).

In December 1999, the X-ray Multi-Mirror Mission-Newton Observatory (XMM for short) was launched.  It has one tenth the resolution of Chandra, but has three times the collection area.  XMM and Chandra complement each other.  XMM does better at spectroscopy, but Chandra produces more detailed photographs.  XMM imaged the Coma Cluster, a source of hot gas and hence X-rays and has provided data on the central black holes for a number of galaxies.

However XMM and Chandra do not tell astronomers everything they would like to know.  If we had better resolution we could image the accretion disks around supermassive black holes, the corona of stars (currently we can do that only for our own sun), use X-rays to provide an independent measurement of the Hubble Constant and presumably discover new phenomena.

A new mission called Constellation is planned for 2015.  Unlike previous X-ray observatories, it will use flat metal mirrors (the X-rays reflect when they hit at a grazing angle) instead of cones.  It will use inferometry to achieve its increased resolution.  A set of spacecraft will collect X-rays and send them to a detector located in another spacecraft located 500 kilometers away (approximately 300 miles).  Positioning the telescope requires moving the collectors hundreds of kilometers and rotating the detector so that everything lines up within a few millimeters, in the process the distance of 500 kilometers must be maintained within 10 meters (slewing is a very very slow operation).  This will allow much greater resolution than is possible with either XMM or Chandra.  Unlike previous detectors, Constellation will use calorimeters to detect X-rays, allowing high resolution imaged spectroscopy.

The first step toward Constellation will be the Micro Arcsecond X-ray Inferometry Mission (known as MAXIM Pathfinder), planned for 2010.  This is the detector component of Constellation; it have 0.0001 arcsecond resolution and can detect the polarization of X-rays (which has previously been impossible).  We expect that ability will allow astronomers to study sources of synchrotron radiation (synchrotron radiation emits polarized X-rays).  Some five years later the rest of the spacecraft are expected to be launched.

For more information

See the web site for the Chandra Observatory, http://chandra.harvard.edu/

The following two articles cover much (but not all) of the information in this article.

Ron Cowen.  October 21, 2000.  Science News.  “Invisible Universe.”  pp. 266-268.

Ron Cowen.  October 28, 2000.  Science News.  “X-ray Visionaries.”  pp. 282-283.

In addition, the following article discusses one of the many observations of Chandra:

_____.  October 2000.  Sky & Telescope.  “Brown-Drawf Flare.”  p. 29.

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This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
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