X-ray universe

Astrosat is the latest instance of the revolution in X-ray astronomy, which, by mapping high-energy phenomena, gives us vital clues about the dynamics of the universe’s evolution.

Figure 1: Rings of X-ray light centered on V404 Cygni, a binary system containing an erupting black hole (dot at center), were imaged by the X-ray telescope aboard NASA's Swift satellite from June 30 to July 4, following the detection of the X-ray outburst on June 15. Color indicates the energy of the X-rays, with red representing the lowest (0.800-1.5 kilo electron volts, keV), green for medium (1.5 keV-2.500 keV), and the most energetic (2.5- 5.0 keV) shown in blue. For comparison, visible light has energies ranging from about 2 to 3 eV. The dark lines running diagonally are artifacts of the imaging system.

As Astrosat, the multi-wavelength X-ray astronomy satellite of the Indian Space Research Organisation (ISRO), sits on the launch pad at the Satish Dhawan Space Centre in Sriharikota waiting to be lifted into the skies by the Polar Satellite Launch Vehicle (PSLV) on September 28 (see “India’s eye in the sky”, Frontline , October 2), the question one might ask is, why is X-ray astronomy important?

Consider what happened on June 15. The X-Ray Telescope (XRT) aboard the National Aeronautics and Space Administration’s (NASA) Swift satellite detected an extraordinary outburst of high-energy light from a black hole system called V404 Cygni in the constellation Cygnus, which is located about 8,000 light years away in our Milky Way. Figure 1 shows the image of that event, resembling a shooting target, captured by the satellite. It represents nested rings of X-ray light centred on an erupting black hole. The nested rings are basically X-rays reflected by galactic dust layers around the black hole and coming towards us.

X-rays point to some of the most energetic phenomena in our universe, such as stellar explosions, powerful outbursts and black holes devouring matter from their neighbourhoods, and what was detected on June 15 is only one of the thousands that keep occurring in different parts of the cosmos.

V404 is a binary system of a black hole, with 10 times the solar mass, and a sun-like star with half the solar mass orbiting around each other. Such an outburst from V404 was last seen in 1989, and this black hole system had been quiet since then. In the V404 type of binary systems, matter flows from the star towards the black hole and gathers in a disc, where it gets heated to extremely high temperatures and shines brightly in the visible, ultraviolet (UV) and X-ray wavelengths before spiralling into the black hole (Figure 2). When the piled-up matter reaches a critical state, the black hole’s devouring rate changes dramatically, leading to events like this for a short period.

During this event, V404 became the brightest object in the X-ray sky, up to 50 times brighter than the Crab Nebula, one of the brightest X-ray sources in the high-energy sky and whose emission intensity, in fact, is used as a unit in X-ray astronomy for comparing X-ray emissions from different objects.

As the event was really intense, even ground-based telescopes picked up light from V404’s high-energy outburst across different wavelengths. Astronomers will now combine high-energy observations made in X-ray and gamma ray with optical and radio measurements to get a complete picture of what is going on in this black hole system.

X-rays are produced by high-energy particles. Cosmic phenomena, such as very high gravitational fields found around collapsed stars like black holes, neutron stars and white dwarfs; strong magnetic fields around neutron stars; very high temperature regions and plasmas such as coronae of stars; matter ejected from supernova explosions; and matter trapped and heated in deep gravitational potential of galaxies and clusters of galaxies, impart very high energies to particles, resulting in the emission of X-rays. Essentially, therefore, the X-ray images of the universe reveal the hot spots and turbulent regions in the universe.

Riccardo Giacconi, the 2002 Nobel Physics Prize-winner for the discovery of cosmic X-ray source, said in his Nobel lecture: “Gone is the classical conception of the universe as a serene and majestic ensemble whose slow evolution is regulated by the consumption of nuclear fuel. The universe…is pervaded by the echoes of enormous explosions and rent by abrupt changes of luminosity on large energy scales. From the initial explosion to formation of galaxies and stars, from the birth to the death of stars, high-energy phenomena are the norm and not the exception in the evolution of the universe.”

Indeed, the X-ray sky looks completely different from the visible sky (Figure 3), and the centre of our galaxy, for example, looks much brighter in X-rays than in the optical band because of heavy absorption of visible light by interstellar gas and dust. X-rays of energy greater than several hundreds of electronvolts (eV) can penetrate interstellar gas over distances comparable to the size of the Milky Way. Higher energy X-rays of a few keV can penetrate the entire intervening galactic medium and reach us even from distances comparable to the radius of the universe. In the electromagnetic spectrum, from X-ray to gamma ray energies, the flux of X-ray photons is the most abundant. So X-ray astronomy can reveal the high-energy events in the universe.

X-ray astronomy actually began with the serendipitous discovery of the first X-ray source outside the solar system, Sco X-1, in the constellation Scorpius, in 1962 by Giacconi and others in a joint rocket experiment conducted by the Massachusetts Institute of Technology and American Science and Engineering Inc. Until then, the only X-ray source known was the sun, whose X-ray emissions from the corona were detected by Herbert Friedman and others at the Naval Research Laboratory in 1949. But given the low X-ray output from the sun, most astronomers thought that detection of extrasolar X-ray sources would not be possible. However, Giacconi’s experiment to look for reflected solar X-rays from the moon accidentally found this strong X-ray source in Scorpius. It is the brightest X-ray source known until now.

It was only with the progress in space technology that X-ray astronomy emerged as an important part of astronomical studies. This is because, since X-rays are absorbed by the atoms in the earth’s atmosphere, X-ray astronomy cannot be done using ground-based instruments (Figure 4). This may seem strange when you consider that X-rays pass through our bodies, which are much denser than the atmosphere. Even though the density of the atmosphere is low, the total thickness of the atmosphere is so large that an X-ray photon has a negligible probability of reaching the ground. Just 10 cm of air is sufficient to absorb 90 per cent of the photons in a beam of 3 keV X-rays.

Besides X-rays, the atmosphere is also opaque to far-IR (infrared), UV and gamma rays from cosmic sources. In comparison, the visible wavelength photons interact weakly with atmospheric atoms and reach optical telescopes with little absorption. So, X-ray telescopes have to be placed above the atmosphere. This could only be achieved with developments in satellite technologies as well as X-ray telescope technologies and detector technologies, which enabled flying experimental payloads with space-qualified X-ray detectors and components. While X-ray optics was well understood in 1959 (X-rays can be reflected only at grazing angles; at larger angles, X-rays will simply shoot through material), fabrication of X-ray mirrors and telescopes took two decades, and the first X-ray telescope was made only in 1979.

Before the advent of satellite technology, X-ray studies were done using rockets and balloons. The use of sounding rockets was one of the earliest platforms for X-ray astronomy. The detector, which is placed in the nose cone of the rocket, remains above the atmosphere (at altitudes of about 200 km) for only 5 to 20 minutes and then falls back to the earth. This short duration was enough only to make meaningful observation of a single bright source. This limited exposure time and limited field of view (FOV) were the major drawbacks of rocket-based studies. Also, a sounding rocket launched from a place like Thumba in Kerala will not be able to see sources in the northern hemisphere and vice-versa.

Balloon flights can carry detectors to altitudes of 35 to 40 km, which is above 99.7 per cent of the atmosphere. X-ray and gamma ray photons of energies greater than about 30 keV can reach the instrument. Unlike a rocket, balloons can stay aloft much longer and take data for longer durations. But soft X-rays, which come from many interesting astrophysical phenomena and sources, cannot reach a balloon’s altitude and cannot, therefore, be studied. Also, balloons can carry only limited payloads.

Satellites provide an excellent platform and can observe the full range of the X-ray spectrum and for as long as the instruments operate. X-ray satellites began to be launched in 1970, the first of which was Uhuru (meaning freedom in Swahili), launched by the United States from Kenya. It performed the first comprehensive survey of the entire sky for X-ray sources, with a sensitivity of about 0.001 times the intensity of the Crab Nebula. In 1977, the U.S. launched the High Energy Astronomical Observatory (HEAO-1). During its two-year lifespan, it scanned the X-ray sky almost three times over in the energy range 0.2-10 MeV, provided nearly constant monitoring of X-ray sources near the ecliptic poles and made detailed studies of a number of objects through pointed observations.

Its successor, HEAO-2, later called the Einstein X-ray Observatory, was the first large X-ray telescope to be placed in orbit around the earth. It was launched in 1978 and carried a single large “grazing-incidence focussing X-ray telescope”, which provided high sensitivity levels (hundreds of times better than previously achieved), arc-second angular resolution of point sources and extended objects in the 0.2-3.5 keV energy range. It produced high-resolution images and accurate locations for thousands of cosmic X-ray sources. The other X-ray satellite missions that followed include EXOSAT (by Europe in 1983), ROSAT (a U.S.-U.K.-Germany collaborative mission launched in 1990), ASCA (by Japan-U.S. in 1993) and Beppo-SAX (a Dutch-Italian collaborative launch in 1997).

The high point of this satellite revolution in X-ray astronomy was reached with the launch of NASA’s Chandra X-ray Observatory (CXO), which marked a major milestone in the evolution of X-ray astronomy, particularly in the area of X-ray mirror fabrication and instrumentation. It carries a 13.8-metre-long X-ray telescope capable of producing the sharpest X-ray images of stars and galaxies and has been sending spectacular images and data since August 1999. Its capabilities are unlikely to be surpassed in the near future, and the CXO images and spectral data contained therein have already led to a host of discoveries about the X-ray universe in the energy range 0.1-10 keV.

According to NASA’s High-Energy Astrophysics Science Archive Research Centre database, in the last five decades of X-ray astronomy, around one million X-ray sources have been discovered, most of them by satellite X-ray telescopes and observatories. As of 2000, there were about 340,000 known X-ray sources, with the vast majority discovered by the ROSAT X-ray satellite (about 120,000 sources were found by ROSAT All-Sky Survey and 220,000 from pointed ROSAT observations). Compare this with the one known source Sco X-1 (excluding the sun) in 1962, the 59 known sources in 1970 (all from rocket and balloon observations), about 700 known sources in 1980 (based on Uhuru, Ariel-V and HEAO-1 satellite observations) and about 8,000 by 1990 (derived mostly from observations by the Einstein and EXOSAT satellites).

As of 2013, in its 12.5 years of observations since the turn of the century, the European Space Agency’s (ESA) XMM-Newton X-ray telescope had catalogued about 373,000 X-ray sources. A similar number of about 380,000 had been detected during 12 years of operation by the CXO. Combining both, we get a figure of about 1,100,000 X-ray sources in the sky, which is an increase by a factor of 3.5 since 2000. This number, however, is less than 1 per cent of the known (about 1.5 billion) optical and infrared objects, and an order of magnitude less than the number of radio sources. It is estimated that there are about 300 billion stars in the Milky Way and more than 100 billion galaxies in the observable universe.

One of the important missions launched before CXO was the Rossi X-ray Timing Explorer (RXTE) which, as the name suggests, observed only the time structure of astronomical sources in the energy range 2-250 keV and was thus not an imaging satellite. It was launched in December 1995 and was decommissioned in January 2012. It observed with unprecedented time resolution the fast-moving, high-energy worlds of black holes, neutron stars, X-ray pulsars and bursts of X-rays that light up the sky briefly and disappear forever. Its highlight discoveries include the discovery of Quasi-Periodic Oscillations in binaries at high-kilohertz frequencies, the discovery of the spin periods in Low Mass X-Ray Binary, and observations of the Bursting Pulsar over a broad range of luminosities, providing stringent tests of theories. Like Chandra, it is one of the most successful missions, producing over 1,000 papers. Designed for a lifespan of two years, with a goal of five years, RXTE greatly outperformed expectations and completed 16 years of successful observations.

The important X-ray missions that are currently active, therefore, include Chandra and XMM-Newton (low to mid-energy X-rays in the 0.1-15 keV range); the INTEGRAL satellite of the ESA (launched in 2002); the Swift gamma ray burst mission of NASA (launched in 2004), which caries the Swift X-ray Telescope (XRT) with imaging capabilities in the 0.2-10 keV range; Suzaku (previously known as Astro-E2), the NASA-Japan Aerospace Exploration Agency (JAXA) joint mission (launched in 2005), which covers a very broad energy range of 0.2-700 keV and carries an X-ray spectrometer (XRS); four CCD-based X-ray Imaging Spectrometers (XISs); a Hard X-ray detector (HXD); and four focussing foil X-ray telescopes (XRTs)—the type which Astrosat also uses in its Soft X-ray Telescope —focussing on each of the four XISs, and NASA’s NuSTAR, which too carries an X-ray telescope but, unlike Swift or Suzaku or Astrosat, has imaging capability only in high-energy or Hard X-rays, and not soft X-rays.

Suzaku is soon to be followed up with Astro-H, to be launched next year, one of the new generation X-ray satellites, which will carry a Soft X-ray spectrometer (SXS), a CCD-based Soft X-ray Imager (SXI) and a Hard X-ray Imager (HXI). One had thought that Astro-H, given its capabilities and originally scheduled for 2014, would beat Astrosat in being able to provide new science. But with its delay, Astrosat can indeed deliver unique science. With its suite of five on-board instruments, Astrosat will be able to look at the universe over an unprecedented, very broad X-ray energy range (from 0.3 keV to 100 keV) and in the UV range as well (Figure 5). Though Astrosat’s SXT has roughly the same energy coverage as Chandra and XMM-Newton, it does not have the same imaging capability as those two extraordinary missions. But, coupled with the multi-wavelength data from the other instruments that will all look at a given source simultaneously and its better energy resolution, it will be in a position to deliver interesting physics about the cosmos.

To understand why there have been so many X-ray imaging satellite missions and several ongoing and upcoming missions, including Astrosat of India, we can turn once again to Giacconi, the originator of celestial X-ray astronomy: “The reason is that this radiation reveals the existence of astrophysical processes in which matter has been heated to temperatures of millions of degrees or in which particles have been accelerated to relativistic energies. The X-ray photons are particularly suited to study these processes because they are numerous, because they penetrate cosmological distances, and because they can be focussed by special telescopes. This last property significantly distinguishes X-ray from gamma ray astronomy. However, in a more fundamental way, high-energy astronomy has great importance in the study of the universe, because high-energy phenomena play a crucial role in the dynamics of the universe.”