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High Energy Cosmic Ray Astronomy
IUCLEAR PHYSIC ~,
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 48 (1996) 478~479
High Energy Cosmic Ray Astronomy V. Fonseca a
a D e p a r t a m e n t o de Fisica At6mica y Nuclear Facultad de Cieneias Fisicas, Universidad Complutense. 28040 Madrid, Spain A brief introduction to High Energy Cosmic Ray Astronomy is presented. This field covers a 17 decade energy range (2x104 - 102°) eV. Recent discoveries done with gamma-ray detectors on-board satellites and ground-based Cherenkov devices are pushing for a fast development of new and innovative techniques, specially in the low energy region which includes the overlapping of satellite and ground-based measurements in the yet unexplored energy range 20 keV - 250 GeV. Detection of unexpected extremely high energy events has triggered the interest of the internaciona] scientific community.
1. I N T R O D U C T I O N High Energy Cosmic Ray Astronomy is a research field attracting a lot of attention. New detection, analisis and simulation techniques have emerged. Special attention is devoted to the detection of cosmic ?-rays which are not deflected by the interstelar magnetic fields, therefore the measured arrival direction points to their origin. Recently new discoveries have been done and among others 4 high energy 'f-ray sources were detected with ground-based detectors and more than 130 low energy ones with the satellite C o m p t o n G a m m a Ray Observatory (CGRO). Detection of high energy 7-rays from galactic and extragalactic sources add i m p o r t a n t support to the understanding of the 2.7 K and infrared interstelar radiation fields. The cosmic ray spectrum at the high energy side extends up to 102° eV and yet the nature, origin and mechanism by which particles get such an extremely high energy remains a complete mlstery. 2. S a t e l l i t e O b s e r v a t i o n s b e l o w 20 G e V Since the launch of the C G R O in 1992, "fray Astronomy in the energy range 20 keV - 30 Gee is full of i m p o r t a n t discoveries and new unsolved questions. Each of its 4 instruments has released exciting new data. The BATSE and the E G R E T detectors have the highest energy thresholds. BATSE has seen 1122 "f-ray bursts (GRB), 6 of them have been also detected at higher en0920-5632/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PII: S0920-5632(96)00296-4
ergy by E G R E T , in one case a 18 GeV "f-ray was detected 1.5 hour after the lower energy detection. Combined measurements of bursts are being done with ground-based detectors using the B A C O D I N E system triggered from BATSE. The bursts are isotropically distributed over the sky, have durations from fractions of a second to minutes and a big variety of spectral and time structures. Although m a n y theories have been proposed to explain these phenomena, it keeps being an enigma. The E G R E T detector has measured 7 "f-ray pulsars with periods 33 - 237 ms, more t h a n 60 Quasars and BL Lac objects as well as 45 nnidentiffed sources at higher latitudes.
8. G r o u n d b a s e d o b s e r v a t i o n s Current space-based experiments can directly detect "f-rays up to an energy of 30 GeV due to the limited detection area of the instruments and to the low fluxes of the 7-ray sources. Groundbased experiments provide a large collection area and can measure high energy cosmic rays by detecting the extensive air showers (EAS) they produce in the atmosphere. The most a b u n d a n t components of the EASs are relativistic • +, e - ; low energy "f-photons; optical-ultraviolet Cherenkov photons and fluorescence light; all can be detected with ground-based detectors. At present the most successful cosmic "f-ray measurements in the energy range (0.25 - 4) TeV
I( Fonseca/Nuclear Physics B (Proc. Suppl.) 48 (1996) 478-479
were done using the atmospheric Cherenkov telescope (ACT) techniques which fonow two different approaches I) imaging the EAS Cherenkov light on s mnitipixel camera located at the focal plane of a large area mirror and 2) sampling the Cherenkov light pool at different points with light detectors distributed as arrays of single mirrors with one photomultipller at the focal plane. Both methods have found 7-ray sources by effectively reducing the hadronic background. The Whipple Observatory used first the imaging technique and sucessfully detected the Crab, now there are about 20 ACTs world-wide. Two galactic sources -Crab Nebula and PSR 1706-44- and 2 extragalactic ones-Mrk421 and MrkS01-have been detected [I]. These 4 high energy sources added to the 130 lower energy ones seen by CGRO are the driving power for new detectors using innovative approaches. The range 20 - 250 GeV is still unexplored but several detectors are foreseen or under development. The MAGIC detector, a 17m diameter imeging Cherenkov telescope with a high resolution camera and an energy threshold of 20 GeV was presented at this conference [1],[2]. The MILAGRO detector [1], s wide aperture water Cherenkov counter with 250 GeV "f-ray energy threshold is under construction. At energies above 10 TeV wide angular acceptance charged particle detectors distributed in large area arrays at mountain altitudes are used to sample the shower front. These detectors can operate 24 hours and monitor up to 2 sterad of sky in contrast to the present ACTs that can operate only in clear moonless nights with s few degrees angular acceptance. The detected charged particles at the shower front have inherently more fluctuations than the Cherenkov photons measured by the Cherenkov counters, making the identification of "?-rays versus the much more abundant background of charged cosmic rays very difficult. The most notably improved particle arrays are the CASA-MIA (the biggest one), CYGNUS, HEGRA and Tibet (at highest altitude). Recent results from CASA-MIA and HEGRA have been presented at this conference [2]. These experiments cover the energy range 10 TeV - 100 PeV, have large detection areas and
479
improved angular resolution which is tested by the measurement of the shadow cast of the Moon and the Sun on the cosmic ray. These detectors have not seen 7-ray sources above 10 TeV. Neverthelcss this technique remains a powerful and complementary method to the ACT techniques. Improvements to the EAS experiments oriented to reduce the energy threshold, angular resolution and to improve the cosmic ray composition measurements have been realised by addition of Cherenkov light counters and muon counters operating in coincidence with the part/cle arrays. For example, the HEGRA multi-array detector has the AIROBICC detector since 1992, a wide angular acceptance array sampling at several points the Cherenkov light front with bear photomultip]iers operating in coincidence with the other detector arrays. 4. T h e H i g h e s t E n e r g i e s The highest energy cosmic ray ever detected was observed on October 15, 1991 by the Fly's Eye cosmic ray detector in Utah. By measuring the fluorescent Light produced by the charged particles of s cosmic ray air shower an energy of 2 • 1020 eV was estimated. On December 3, 1993 the AGASA array which detects the particles of the shower front recorded a cosmic ray with an energy of about 3.2 • 1020 eV. No known acceleration mechanism can explain such extremely high energy. These exciting events have triggered the interest of the international scientific community to build a giant array capable to measure with enough statistics these events. As a result the Pierre Auger Observatory project [3], the largest cosmic ray detector ever built and presented at this meeting [2] has already passed the design phase which was done by scientists of 20 courttries. REFERENCES I.
Proceedings 24th Internacional Cosmic Ray Conference, Roma 1995 2. See these proceedings. 3. The Pierre Auger project design report. Fermilab (October 1995).
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 48 (1996) 478~479
High Energy Cosmic Ray Astronomy V. Fonseca a
a D e p a r t a m e n t o de Fisica At6mica y Nuclear Facultad de Cieneias Fisicas, Universidad Complutense. 28040 Madrid, Spain A brief introduction to High Energy Cosmic Ray Astronomy is presented. This field covers a 17 decade energy range (2x104 - 102°) eV. Recent discoveries done with gamma-ray detectors on-board satellites and ground-based Cherenkov devices are pushing for a fast development of new and innovative techniques, specially in the low energy region which includes the overlapping of satellite and ground-based measurements in the yet unexplored energy range 20 keV - 250 GeV. Detection of unexpected extremely high energy events has triggered the interest of the internaciona] scientific community.
1. I N T R O D U C T I O N High Energy Cosmic Ray Astronomy is a research field attracting a lot of attention. New detection, analisis and simulation techniques have emerged. Special attention is devoted to the detection of cosmic ?-rays which are not deflected by the interstelar magnetic fields, therefore the measured arrival direction points to their origin. Recently new discoveries have been done and among others 4 high energy 'f-ray sources were detected with ground-based detectors and more than 130 low energy ones with the satellite C o m p t o n G a m m a Ray Observatory (CGRO). Detection of high energy 7-rays from galactic and extragalactic sources add i m p o r t a n t support to the understanding of the 2.7 K and infrared interstelar radiation fields. The cosmic ray spectrum at the high energy side extends up to 102° eV and yet the nature, origin and mechanism by which particles get such an extremely high energy remains a complete mlstery. 2. S a t e l l i t e O b s e r v a t i o n s b e l o w 20 G e V Since the launch of the C G R O in 1992, "fray Astronomy in the energy range 20 keV - 30 Gee is full of i m p o r t a n t discoveries and new unsolved questions. Each of its 4 instruments has released exciting new data. The BATSE and the E G R E T detectors have the highest energy thresholds. BATSE has seen 1122 "f-ray bursts (GRB), 6 of them have been also detected at higher en0920-5632/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PII: S0920-5632(96)00296-4
ergy by E G R E T , in one case a 18 GeV "f-ray was detected 1.5 hour after the lower energy detection. Combined measurements of bursts are being done with ground-based detectors using the B A C O D I N E system triggered from BATSE. The bursts are isotropically distributed over the sky, have durations from fractions of a second to minutes and a big variety of spectral and time structures. Although m a n y theories have been proposed to explain these phenomena, it keeps being an enigma. The E G R E T detector has measured 7 "f-ray pulsars with periods 33 - 237 ms, more t h a n 60 Quasars and BL Lac objects as well as 45 nnidentiffed sources at higher latitudes.
8. G r o u n d b a s e d o b s e r v a t i o n s Current space-based experiments can directly detect "f-rays up to an energy of 30 GeV due to the limited detection area of the instruments and to the low fluxes of the 7-ray sources. Groundbased experiments provide a large collection area and can measure high energy cosmic rays by detecting the extensive air showers (EAS) they produce in the atmosphere. The most a b u n d a n t components of the EASs are relativistic • +, e - ; low energy "f-photons; optical-ultraviolet Cherenkov photons and fluorescence light; all can be detected with ground-based detectors. At present the most successful cosmic "f-ray measurements in the energy range (0.25 - 4) TeV
I( Fonseca/Nuclear Physics B (Proc. Suppl.) 48 (1996) 478-479
were done using the atmospheric Cherenkov telescope (ACT) techniques which fonow two different approaches I) imaging the EAS Cherenkov light on s mnitipixel camera located at the focal plane of a large area mirror and 2) sampling the Cherenkov light pool at different points with light detectors distributed as arrays of single mirrors with one photomultipller at the focal plane. Both methods have found 7-ray sources by effectively reducing the hadronic background. The Whipple Observatory used first the imaging technique and sucessfully detected the Crab, now there are about 20 ACTs world-wide. Two galactic sources -Crab Nebula and PSR 1706-44- and 2 extragalactic ones-Mrk421 and MrkS01-have been detected [I]. These 4 high energy sources added to the 130 lower energy ones seen by CGRO are the driving power for new detectors using innovative approaches. The range 20 - 250 GeV is still unexplored but several detectors are foreseen or under development. The MAGIC detector, a 17m diameter imeging Cherenkov telescope with a high resolution camera and an energy threshold of 20 GeV was presented at this conference [1],[2]. The MILAGRO detector [1], s wide aperture water Cherenkov counter with 250 GeV "f-ray energy threshold is under construction. At energies above 10 TeV wide angular acceptance charged particle detectors distributed in large area arrays at mountain altitudes are used to sample the shower front. These detectors can operate 24 hours and monitor up to 2 sterad of sky in contrast to the present ACTs that can operate only in clear moonless nights with s few degrees angular acceptance. The detected charged particles at the shower front have inherently more fluctuations than the Cherenkov photons measured by the Cherenkov counters, making the identification of "?-rays versus the much more abundant background of charged cosmic rays very difficult. The most notably improved particle arrays are the CASA-MIA (the biggest one), CYGNUS, HEGRA and Tibet (at highest altitude). Recent results from CASA-MIA and HEGRA have been presented at this conference [2]. These experiments cover the energy range 10 TeV - 100 PeV, have large detection areas and
479
improved angular resolution which is tested by the measurement of the shadow cast of the Moon and the Sun on the cosmic ray. These detectors have not seen 7-ray sources above 10 TeV. Neverthelcss this technique remains a powerful and complementary method to the ACT techniques. Improvements to the EAS experiments oriented to reduce the energy threshold, angular resolution and to improve the cosmic ray composition measurements have been realised by addition of Cherenkov light counters and muon counters operating in coincidence with the part/cle arrays. For example, the HEGRA multi-array detector has the AIROBICC detector since 1992, a wide angular acceptance array sampling at several points the Cherenkov light front with bear photomultip]iers operating in coincidence with the other detector arrays. 4. T h e H i g h e s t E n e r g i e s The highest energy cosmic ray ever detected was observed on October 15, 1991 by the Fly's Eye cosmic ray detector in Utah. By measuring the fluorescent Light produced by the charged particles of s cosmic ray air shower an energy of 2 • 1020 eV was estimated. On December 3, 1993 the AGASA array which detects the particles of the shower front recorded a cosmic ray with an energy of about 3.2 • 1020 eV. No known acceleration mechanism can explain such extremely high energy. These exciting events have triggered the interest of the international scientific community to build a giant array capable to measure with enough statistics these events. As a result the Pierre Auger Observatory project [3], the largest cosmic ray detector ever built and presented at this meeting [2] has already passed the design phase which was done by scientists of 20 courttries. REFERENCES I.
Proceedings 24th Internacional Cosmic Ray Conference, Roma 1995 2. See these proceedings. 3. The Pierre Auger project design report. Fermilab (October 1995).