Hanle is an ideal site for capturing the afterglow of gamma rays at a low threshold in view of its altitude. While the threshold for space telescopes can hardly exceed 30 GeV, ground-based telescopes usually detect only above 200 GeV. An array at high altitude on the ground like Hanle will fill the gap in the range of detection. The non-imaging gamma ray telescope at Hanle is located at slightly lower altitude (4,300 m) than the optical telescope; but still it is at a higher location than the MAGIC array in the Canary Islands. Some basics would enable us to appreciate the advantage of the location.
The Himalayan Gamma Ray Array (HAGAR) at Hanle consists of six telescopes placed at equidistant locations on a circle (with a radius of 50 m), while the seventh telescope is at the center with a total effective aperture of 30 square meters. HAGAR uses a non-imaging technique to study gamma rays, called wavefront sampling of the Cerenkov radiation. The wavefront is sampled to detect the direction and energy of the incident gamma rays. The wavefront cone is mapped on the basis of the output of photo multipliers attached to each of the telescopes. The photometers function with better than nanosecond accuracy. The arrival time of the light of different telescopes varies in terms of a few nanoseconds.
Each of the seven telescopes has as many mirrors with an individual diameter of 90 cm. All the 49 mirrors have been kept in an excellent condition with a recent aluminium coating at site. Light is received by more than one telescope as it improves the angle and direction of the incoming light. Synchronized operation of all the seven telescopes from a central control computer has been successfully carried out.
All the telescopes can be commanded to look at one source or some of then can be turned to a second source simultaneously. Initially expert observers would be at the control room in Hanle itself. Eventually, remote control through a satellite link may be introduced.
The optical telescope at Hanle monitors the after-glow. Even during the science verification phase at Hanle, astronomers monitored gamma ray bursts (GRBs). Whenever GRBs appear, the schedule and observation programmes are paused to observe unexpected phenomenon, as targets of opportunities.
IIA has designed the structure of the telescope, which was fabricated in the private sector in India TIFR has designed and developed the sensors. Its experience in detector software would help in converting the raw data into meaningful information. Interestingly, the detectors should be sensitive enough to distinguish gamma rays from the more predominant (almost 90 per cent) cosmic rays.
HAGAR is followed by gamma ray imaging telescope arrays that image the Cerenkov cone. Known as the Major Atmospheric Cerenkov Experiment (MACE), it is developed by IIA in collaboration with TIFR and the Bhabha Atomic Research Centre (BARC). MACE envisages four telescopes (20 m diameter), to be designed by BARC. The fist one will be installed at Hanle. In this type of telescopes, a large primary mirror is required for imaging with a large number of photomultipliers in the focal plane. It can push the threshold of observation of gamma rays to 5 to 10 GeV, which would be one of the best in the world.
A Close Watch
As GRB coordination network is emerging, comprising observatories at Pachmarhi, Madhya Pradesh (run by TIFR) and at Mt Abu (operated by BARC), ARIES in Nainital and the Giant Metrewave Radio Telescope (GMRT) in Pune. A multiple imaging Cerenkov telescope is being constructed at Mount Abu.
As soon as the coordinator in-charge of information the Network receives emails on the sighting of a GRB, a message is broadcast to all observatories in the country in 15 to 30 minutes, disclosing the coordinates for pinpointing the source. A large database of the whole sky facilitates those in charge of individual observatories. They can identify a GRB, as it would be a bright source, which had not shown up in earlier images and as the intensity of radiation changes. The GRB detectors are turned a catch certain frequencies provision is made to put aside normal work to make use of any target of opportunity, as the source many linger on only for a while. The observatories are designed to detect the optical signature created by the charged particles of gamma rays in the Earth’s atmosphere.
Radio astronomers at the Giant Metrewave Radio Telescope near Pune and optical astronomers at the one-metre Sampurnanand Telescope, Nainital and HCT have been observing GRB afterglows in a coordinated manner. The optical and infrared telescopes observe the radiation caused by the explosive shocks that occur near the GRB source. The radio radiation from the shock lasts longer after the burst.
Pachmarhi has an array of 25 telescopes for catching gamma rays. Set up by TIFR, Mumbai, the array has seven parabolic reflectors of 90-cm diameter, as against 50-cm dishes at Hanle, where the base of the Cerenkov radiation cone is lower, almost half compared to Pachmarhi. Each of the telescopes at Pachmarhi can be individually steered and the entire operation can be done by remote control. The sources of gamma rays can be monitored in real time.
Located at an altitude of 1075 m above sea level, the Pachmarhi array is planned for what is known as Very High Energy Gamma Ray Astronomy observations in the unexplored energy range of 10 to 250 GeV. On dark, moonless nights, the optical reflectors at the telescopes catch the faint Cerenkov radiation from gamma rays and photo multipliers convert the optical signals into electronic signals to record the event.
Mt Abu in Rajasthan, famous for the Dilwara Jain temples, is cloud-free most of the year. At 1722 m above sea level, it is higher than Pachmarhi, though at about the same longitude. A Gamma Ray Astrophysics Coordinated Experiment (GRACE) was inaugurated at this summer resort in 1997. It is designed to probe the entire gamma ray spectral band from a single location and detect high-energy processes in and around black holes; study X-ray binaries and neutron stars (pulsars), remnants of supernova and central power sources in active galactic nuclei.