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TeV gamma-ray astronomy
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 87 (2000) 335-344 www.elsevier.nl/locate/npe
TeV Gamma-Ray Astronomy Eckart Lorenza aMax Planck Inst. for Physics Foehringer Ring 6, 80805 MUNICH, FRG TeV gamma-ray astronomy is becoming an important tool to gain insight into the relativistic (nonthermal) universe. The status of observations and prospects for the next years will be presented.
1. INTRODUCTION Information about our universe is retrieved through the detection of electromagnetic waves, respectively electromagnetic particles at higher energies. The main tool for the observation of the relativistic (astronomers prefer 'nonthermar) universe is gamma ray (shortcut y) astronomy. In the keVGeV energy range observations are carried out by means of balloon borne or satellite borne instruments because the earth atmosphere shields efficiently ys in this energy range. Above = 1 TeV observations can only be carried out by ground based instruments with many orders of magnitude larger collection areas compared to that of satellite borne detectors. The primary motivation for y astronomy was the search for the sources of cosmic rays (CR), mostly charged particles, which bombard constantly the earth from outer space. Charged CRs were discovered in 1912 by Victor Hess, ref. 1. The origin of CRs is still unresolved. The CR spectrum extends to at least 1020 eV; it follows a rather smooth power law with a coefficient of -2.6 to -3.4. The presence of the steady flux of these energetic particles is a clear sign that ultrarelativistic particle processes play an important role in our universe. The challenge to find the locations of production of these energetic charged particles cannot be solved by extrapolating their direction of incidence back to cosmic accelerators, very likely stellar objects. Charged particles are deflected by the weak galactic magnetic field and are thus completely scrambled in direction. Only neutral particles, ys, v s and neutrons, can be back extrapolated to their origin. Neutrons are only useful for short distance observations due to their limited
lifetime and vs require huge detection volumes because of their extreme small interaction cross section. Therefore only ys are viable tracer particles. Unfortunately ys are only a minuscule fraction (< I05) of the total cosmic ray flux and their identification against the charged CRs poses a major experimental challenge. This difficulty was also the main reason why the search for y sources was unsuccessful over many decades. In the past three decades more than 250 y sources were discovered in the MeV-GeV energy range by satellites, mostly by EGRET on board of the Compton Gamma Ray Observatory. TeV y astronomy is trailing these efforts by about 20 years but has succeeded in the last decade to discover about 12 sources above 300 OeV (see chapter 5). Here we present a short overview of the different aspects of TeVy-astronomy. In the following chapter we elaborate on some of the current physics questions related to y-astronomy, followed by a short overview of y-production, transport through our universe and the principal detection methods. In chapters 5 some highlights of recent observations will be presented while chapter 6 deals with the prospects of yastronomy in the next years.
2. PHYSICS OBJECTIVES The full extends of the current physics objectives of TeV y-astronomy cannot be discussed here. Therefore only a short list of some candidates for y emission and some of the physics objectives are given.
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1) Search for recently (up to a few 104 years ago) exploded Super Novae. The remnants are considered to be efficient particle accelerators and thus the origin of the charged CRs. 2) Plerions (Pulsars) which are potentially also electron accelerators and are expected to show in certain energy bands pulsed "/emission 3) Radioloud, compact binaries. 4) Diffuse y emission from the galactic disc (from charged CRs interacting with the interstellar gas, e.g., information about the gas density in the galactic plane) 5) Active Galactic Nuclei (AGN) with central supermassive black holes accreting mass and driving relativistic jets releasing well above 1040 ergs/sec, yastronomy would shed light onto this energetic process, black holes and indirectly about the cosmological infrared (LR) background from yabsorption processes. 6) Gamma-Ray bursts (GRB) which occur 1-2 times per day and are still in many aspects enigmatic. GRBs must be extremely powerful short-term processes 7) Search for possible Topological Defects, left over from the early universe 8) Search for the lightest supersymmetric particles, ys would be generated in annihilation processes. 9) Tests of quantum gravity effects.
TeV ys I need parent particles of higher energy (masses). Possible generation processes are:
i) High energy hadronic interactions with > TeV r~° production and their subsequent decay into yy. ii) Inverse Compton (IC) scattering of > TeV electrons on low energy photons. iii) Decays of supermassive particles, not necessarily of high energy. Current models of "/ production assume that process i) or ii) are responsible for energetic "/s from stellar sources. Process iii) is at present only speculative. Supermassive particle candidates are topological defects or relic particles assumed to be left over from the early universe. Process i) would be a natural link to the energetic charged CRs as these must be part of the general acceleration process of also the hadronic parents of ys. The necessary target material, say, a beam dump is normally found close to stars or galaxies in form of gas clouds. Hadronic interactions produce in first order the same amount of ys and vs. Ones v detectors of sufficient sensitivity are build one should be able to identify from simultaneous ¥ and v observations identify the hadronic accelerators in our cosmos. The underlying mechanism of charged particle acceleration is assumed to be shockwave acceleration in rapidly expanding gas clouds interacting with gas of lower velocity. Strong shocks are expected for example in supernovae remnants (SNR). It is predicted that in the aftermath of a SN explosion particles can be accelerated up to at least 1015 eV. These particles might interact with nuclei of gas clouds ejected before the SN explosion. Electrons might equally be accelerated in shock waves or alternatively in timevariable magnetic fields, e.g., in the strong magnetic fields of pulsars. High energy electrons might upscatter low energy photons, which are normally plenty in the vicinity of any hot stellar object. Alternatively these photons might be generated by synchrotron radiation of electrons moving through magnetic fields. The concept of shockwave acceleration has been originally proposed by Fermi, ref 2, and has been since then steadily been refined. SN explosions are sufficiently frequent and energetic to explain most of the flux of the CRs observed on earth up to = 1015 eV. Therefore one of
1Here we use still the notation of TeV "/-astronomy for groundbased observations although in the last
years the threshold of the best instruments has been lowered to 300 GeV
Observations would reveal in first order ¥ fluxes (-> information about the energetics of cosmic reactors), energy spectra (-> energy of the involved parent particles)-and time variability (-> information about the time structure of relativistic processes). Discussion about GRBs is omitted because the subject is covered by Eli Waxman in a separate contribution to this conference.
3. GENERATION OF ~ AND ABSORPTION LOSSES IN THE UNIVERSE
E. Lorenz~Nuclear Physics B (Proc. SuppL) 87 (2000) 335-344
the main aims of TeV ?-astronomy is to find these SNRs. Active galactic nuclei (AGN) are also candidates for very powerful particle acceleration and y-emitters. Here shock wave acceleration can occur inside blobs formed in relativistic jets, i.e., the ?s are Doppler boosted along the jet axis and very high energy ?s might be observed at earth, provided the jet points towards us. Also, any time variability in the center of mass system of the blobs in the jets would manifest itself as a much more rapid variation for the earth observer. Gamma rays have to pass significant distances between their generation location and the terrestrial observer. Becauseys are massless their arrival time is a true image of the time structure at the origin 2. The universe is not fully transparent to high energy ys because it is filled with various low energy photon fields, such as from the 2.7 ° microwave background, IR light, starlight or radiowaves. High energy ?s can interact with these low energy photons forming e+epairs with a peak rate at threshold. These electrons might be deflected in the weak extragalactic magnetic fields and eventually interact by inverse Compton scattering with photons of the above noted photon fields. This process generates lower energy "/s, which do not conserve the original direction. Fig 1 shows a calculation of the transmission length of ¥s as a function of energy, ref. 3. The strongest attenuation occurs for 1015 eV ?s that interact with the 2.7 ° MWBR, with an attenuation length of about the distance of the center of our galaxy to the earth. Extragalactic sources are not observable in the 1015 eV band. While the 2.7 ° MWBR is well studied, the knowledge of the IR background is poor. Its origin, amount and distribution over the sky are not known and are difficult to measure from earth due to the much stronger foreground IR radiation of our galaxy. It is speculated that a significant amount of the cosmic IR background is due to the earliest (shortly after the big bang) formed stars and galaxies that are now cooled down and invisible. The determination of the IR background from studies of absorption of ys
2Recently strong evidence for neutrino mixing with a small mass difference has been found. Observation of ?s and vs from short time events of cosmological distance, such as from GRBs, offer a unique opportunity to measure v masses.
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between 1010 and 1013 eV is therefore an important issue for y-astronomy of extragalactic objects as well as for cosmology. 1029
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4. DETECTION METHODS OF TeV Ts The detection of TeV ys with ground based instruments is indirect. Gamma Rays (as well as the charged CRs) interact in the upper atmosphere and initiate electromagnetic (hadronic) cascades which, depending on the energy, stop in the atmosphere or penetrate down to ground level. The vertical atmosphere at sea level corresponds to 27 radiation length (to) and 13 hadronic absorption length (Xo). Measurable quantities from the shower can be detected at ground level, such as air scintillation, Cherenkov light or in case of high primary energies the shower tail particles. From these quantities one can deduce the incident energy and direction and also sometimes information to identify the primary particle. In principle, all air shower detectors are calorimetric devices similarly to the ones used in high energy accelerator experiments. One can classify the air shower detectors into 3 groups: i) Ground based charged particle detector arrays tail catcher calorimeters- for the detection of showers above a few 1013 eV. These detectors operate 24 hours a day. it) Air fluorescence detectors. These detectors, such as for example the Fly's Eye, are true fully active calorimetric devices with imaging. Due to the low scintillation light yield (O 10-5 of the initial energy) and the isotropic light emission these
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E. Lorenz~Nuclear Physics B (Proc. Suppl.) 87 (2000) 335-344
detectors are only useful for observations above =1017 eV. iii) Detectors making use of the Cherenkov light. These are in first order also fully active calorimeters with strong directional effects. All discoveries of TeV Y sources were made by the latter type of detectors because they have the lowest threshold and the best y/hadron (y/h) selectivity of all ground based instruments. In the following emphasis will therefore be put on these instruments. Cherenkov photons are peaked along the direction of the shower axis. The combined effect of multiple scattering of the electrons (Pt kick of secondaries in hadronic showers) and the altitude dependent Cherenkov emission angle (0.3-1.3 °) illuminates nearly uniformly a disc of about 120 m radius on ground3. Any high sensitivity photon detector placed anywhere in this disc is able to detect high energy Ts; therefore these detectors have typical collection areas of a few 104 m2. A typical detector consists of a large diameter curved mirror that focuses the Cherenkov light onto an array of densely packed fast photomultipliers (PM), the camera. These so-called air Cherenkov telescopes (ACT) with a limited field of view have to be pointed towards a source candidate and must be permanently moved to counteract the rotation of the earth. The observation of the Cherenkov light does not require that the shower tail reaches ground, thus a lower threshold compared to tail-catching array detectors can be achieved. Low energy showers are detectable as long as their signals stand out against the night sky light background of typically 2"1012 photons/m 2 sec sr between 300 and 550 nm (star light, ionospheric fluorescence, and sometimes man made light pollution). As the ratio of signal to background increases typically with the square root of the mirror collection area the tendency is to build large diameter dishes. The presently largest ACT is the 10 m Whipple telescope, ref. 4. Jelley, ref. 5, already proposed Cherenkov light detectors for CR observations in the 60's. Some major hurdles have to be mastered with ACTs for Yastronomy 3this radius is altitude dependent. Also, some light is spread towards larger distances due to rare large Pt kicks.
i) In contrast to satellite borne y detectors with a charged particle anticoincidence system no device for the suppression of the enormous hadronic CR background exist. ii) ACTs have only a few msterad angular acceptance, thus some guidance for potential sources is needed. iii) In contrast to calorimeters for accelerator experiments the atmospheric calorimeter has a rather similar response to incident electromagnetic and hadronic particles (consequence of the low of the atmosphere). Also the calorimeter is constantly changing its parameters during the observation due to the rotation of the earth and -more critical- due to atmospheric transmission changes. iv) ACTs (as well as other Cherenkov light detectors or air fluorescence detectors) can only operate during clear and moonless nights. The real breakthrough in T-astronomy came with the introduction of efficient methods for y/h separation by analysis of the Cherenkov light image recorded in the telescope's focal plane, see for example refs. 6, 7. In an ACT with 3-4 ° camera diameter one might see a rather compressed and sometimes complex image of the shower. Fig 2 shows simplified examples of an electromagnetic and a hadronic shower and their 'images'4. While Y and hadron showers are nearly indistinguishable at tail level (except by the number of rare muons) the Cherenkov images of the entire showers are quite different in longitudinal and transversal structure. When searching for ys from a point source to be aligned with the telescope the principal shower axis should point to the center of the camera while the direction of hadronic showers should be randomly distributed Therefore a good angular resolution is mandatory for a point source search. Hadronic shower images are in general slightly wider and less concentrated than that of Ys. Experimenters have developed a series of image classifying parameters (Width, Length, Concentration, Miss, Alpha etc.) that allow one to select y candidates against hadronic showers. Modem single 'imaging' air Cherenkov telescopes (IACT)
4due to the peaked direction of the Cherenkov light one sees only a part of the shower. For large angles between shower particle- and telescope direction no image can be seen by the Cherenkov ltelescope.
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of the currently most advanced camera design allowing for a directional analysis of the shower image (with most telescopes only the around 90 ° folded distribution can be measured), ref 8. The single telescope approach has two deficiencies in the image analysis: a) only one 'projected' Cherenkov light image will be recorded and b) the impact parameter of the shower axis is not well defined. The y/h separation can be improved by observing a shower by a series of 50-200 m spaced telescopes recording images from different directions and angles. This so-called stereo system allows one to improve the angular and energy resolution by = 1.5 and the hadron suppression by = 2-4, albeit with a large increase in costs. Currently the most powerful stereosystem is operated by the HEGRA collaboration, ref. 9, Table 1 lists the leading IACTs.
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Table 1 Parameters of the leading_ACTs. Telescope Whipple CAT Site Mt.Hopkins Themis Longitude -110.5 o -2.00 Latitude 31.4 0 N 42.4 o N Elevation(m asl) 2300 1650 Number of IACTs 1 1 Mirror area (m2) 74 17.5 Number of pixels 151 548+52 Pixel diameter (deg) 0.25 0.10 Threshold(CreV) 250 300 Sensitivityb) (cm-2s-1) 10-11 10-11 at threshold b) Definedas the 5 ~ limitfor 50 h observationtime.
HEGRA Syslcrn LaPalma -17.80 28.8°N 2220 5 5x8.4 5x271 0.25 500 5x10-12
Fig. 3. Distribution of the image parameter ALPHA for the Mkn 501 observation of CAT (ref 6)
HEGRA CT1 LaPalma -17.80 28.80N 2220 1 10 127 0.25 650 10-11
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340 Table 2 Observed TeV '/-sources Source/type Crab/plerion PSR 1706/plerion VELA/plerion SN 1006/SNR CAS A/SNR RXJ 1713/SNR Mkn4211AGN Mkn 50 I/AGN IES 2344/AGN PKS 2155/AGN 3C66/AGN 1ES 1959/AGN
E. Lorenz~Nuclear Physics B (Proc. Suppl.) 87 (2000) 335-344
discovered 1989 1995 1997 1998 1999 1999 1992 1995 1998 1999 1998 1959
reference ref. 10 ref. 11 ref. 12 ref. 13 ref. 14 ref. 15 ref. 16 ref. 17 ref. 18 ref. 19 ref. 20 ref. 21
comments >> 10~, steady, seen by > 9 groups > 10 or, steady, seen by 2 groups > 6 s, steady, seen by 1 group > 6 a, seen by 1group 4.9 a, seen by 1 group 5 a, seen by 1 group >>10 ~, variable, seen by > 3 groups >>10 c~, variable, seen by > 7 groups 6 ~, variable, seen by 1 group 7 a, seen by 1 group 5 a, seen by 1 group = 7 a, seen by 1 group
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5. T H E TeV SKY MAP In the following a brief overview of the status of observations will be given. In the 60-80's sporadic observations of some ,/sources were reported, albeit with rather low significance. The first high significance observation was reported in 1989 by the Whipple group, ref. 10. Their observation of the
Crab nebula opened the new window for TeV yastronomy. Nowadays about a dozen sources have been identified; about half o f galactic and half of extragalactic origin. Some of them have been observed by many groups with in one case a combined significance exceeding 100 a.
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Fig 4 shows a recent sky map of the TeV ,/sources while table 2 lists the sources and references. All these sources are ¥ emitters in lower energy bands. In the following some individual sources will be commented in'more detail.
5.1 The Crab nebula The Crab, a Plerion, was the first source discovered in the TeV region, ref. 10. It is a rather nearby galactic pulsar and the strongest known steady y-source. Broadband emission has been observed spanning from radiowaves up to a few tens of TeV. Below 10 GeV a strong pulsed signal has been observed. In the TeV region no periodic signal has been discovered which would be naturally explained by different emission regions, i.e., keV/MeV ~/ would be produced by electrons accelerated close to the pulsar and the TeV ys by particles accelerated by shocks to higher energies in the surrounding nebula. Even there one assumes that the parent particles are very likely also electrons which produce ys by IC scattering of low energy photons initially radiated by the same electrons by synchrotron radiation close to the pulsar. Calculations based on the so-called synchrotron selfCompton (SSC) model, ref. 22, can explain the spectrum quite well between 1 MeV and = 50 TeV. From the highest part of the spectrum one can estimate the magnetic field of the pulsar to be in the range of a few hundred laG. Two interesting questions for the next generation observations will be: a) to find the pulsed to steady emission transition band (decision between the outer gap and polar cap model of pulsars) and b) any proof of hadronic generation of ys. TeV ys have been observed by at least 9 different instruments. The most recent observation is from the Tibet AS ground based scintillator array, ref. 23. Fig 5 shows a compilation of the spectrum above 300 GeV from a few experiments. Experiments could not find any time variability of the TeV emission; therefore, and because of its large flux, the Crab is nowadays used as the standard candle for ACTs on the Northern Hemisphere. Similarly, PSR 1706-44, a pulsar resembling many features as the CRAB is used as standard candle on the Southern Hemisphere.
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5.2 SNR candidates SNRs are considered to be the sources of the galactic CRs (see section 2, 3). Up to now 3 candidates have been found all of them by one group only and with = 5 o significance reflecting a rather low flux. This low flux together with the nonobservation of TeV y-emission o f some other young SNRs (G 78, IC 443, Tycho,..) casts some doubts that SNRs will be efficient multi-TeV ha&on accelerators. All the up to now observed TeV ,/sources can be explained by being electron accelerators only. In SNRs the location of acceleration and y-emission will be the shell. Therefore the source region of nearby SNRs will be extended and excellent angular resolution of the IACTs will be needed. All current SNR candidates are strong "),-emitters in the MeV/low GeV region. Therefore, it will be one of the main challenges for the next generation IACTs to find the spectral shape in the up to now unexplored energy range between 10 and 300 GeV and clarify the question about the origin of the charged CRs. An overview of the SNR aspects can be found in ref. 24.
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5.3 The AGNs Mkn 421 and Mkn 5 0 1 Another class of fascinating TeV y-emitters are some radioloud, BL-Lac type AGNs. EGRET has found up to now over 60 sources in the MeV region. In the TeV range 6 sources have been identified, two of them, Mkn 421 and Mkn 501 with very high significance. Within the statistical errors all sources show rapi d time variability. In some cases a strong correlation with X-Ray variability was observed, favoring again electron acceleration as the primary production mechanism. The first discovered source, Mkn 421, ref. 16, showed sometimes intense flaring up to 8 times the Crab flux with doubling time of less than 1/2 hour ref. 25. This fast flaring is an indication that the acceleration volume must be very small. Even taking some typical gamma factor of 10 into account, the volume can only have an extension of a few tens of lighthours. In a multiwavelength observation, fig. 6, rapid repeated flaring in the XRay domain has been observed. If one superimposes the X-Ray lightcurve -after the subtraction of a smoothly rising background- one finds excellent agreement in the by daylight interrupted TeV light curve. The mechanism of this variability has to be explained. A general observation of all TeV sources is that y-production can be explained by electron acceleration and IC upscattering of low energy photons (mainly manifesting itself by a pronounced X-Ray and a TeV peak in the vFv plot). No clear evidence for hadron acceleration has been found. Only recently, at the 27th ICRC, a group of COMFI'EL, ref. 26, reported sizeable y emission at around 1 MeV from Mkn 421. This is inconsistent with the low flux expected in the minimum between the X-Ray and TeV peak. Very likely the future satellite INTEGRAL, optimized for observations in the 1-10 MeV energy band, will help to clarify the situation. The source Mkn 501 showed a remarkable variation in intensity. In its year of discovery (1995) the mean flux was only a few % of that of Crab. Nearly throughout 1997 Mkn 501 showed persistent but seemingly chaotic flaring of up to 10 times the Crab flux. At the same time a significant increase in the X-Ray spectrum was observed, but nearly no change in the radioemission or visible spectrum. Since than the TeV flux is dropping. Fig. 7 shows the
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X-Ray and TeV light curve. From the analysis of the time variability in both the TeV and X-Ray data of 1997 two groups found indications of quasiperiodic variability with a period of 23.9 days, ref. 27 and 23 days, ref. 28, using slightly different data sets. This observation still needs to be explained. Anyhow, such a QPO must be observed again before being accepted as real. Due to their redshift of z= 0.03 (Mkn 421) and z = 0.034 (Mkn 501) the y spectra of both AGNs (as well as that of the other more distant extragalactic sources) could be affected by interaction with the still unquantified IR BG. Fig. 7 shows both spectra in the TeV region, ref. 29. The one of Mkn 501 shows a strong deviation from the generally expected power
343
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law shape, thus this could be interpreted as a cut-off due to IR interaction. On the other hand the spectrum of Mkn 421 shows within the statistics no such attenuation, therefore the turnover of the Mkn 501 is more likely due to an energy cut-off of the acceleration process. Nevertheless, from the current observations one can already conclude, that the IR
density must be very low, i.e., the star density shortly after the Big Bang must have been rather low. It will be one of the most important goals of future lower energy observations to measure the ¥ spectra of high redshift AGNs to determine the cut-off as a function of z.
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Current experiments deliver a clear message: future detectors must have a) a lower threshold and b) the sensitivity (collection area and ~//h separation) of the instruments must be increased. In the current observation gap between satellites and ground based ACTs between, say, 10 and 300 GeV, many new results as well as fundamental changes in the acceleration, respectively absorption processes are expected. The recent successes in y astronomy have triggered a world-wide activity to construct more powerful telescopes. Efforts concentrate on a) larger collection mirrors, either single or multimirrors/telescopes, b) higher quantum efficiency
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photosensors, and c) improved cameras with finer pixels and a more refined, higher rate data acquisition. Two distinct directions are followed. A number of groups tries to build ACTs on the basis of large, readily available solar heliostat fields. All current projects, CELESTE, GRAAL, SOLAR II and STACEE, refs. 30-33, are already in the test phase. Some of these experiments hope to achieve a threshold as low as = 20 OeV. Their main advantage is the relatively modest investment and ready availability of up to a few 1000 m 2 mirror collector while their main deficiency is a very modest y/h separation and a rather restricted collection area. Other groups aim for new, optimized ACTs by either bundling telescopes of the proven 10 m class to stereo systems such as CANGAROO III, HESS and VERITAS (refs 34-36) or for a single very large mirror MAGIC, MASE and PETAL (refs. 37-39). These experiments will have thresholds between 15 and 50 GeV, very high ~/h separation, collection areas up to 1 km 2 above 1 TeV and see 'first light' in about 2-4 years. Fig 9 shows the sensitivity for some current and proposed new generation ACTs in comparison with EGRET and the future leading ¥ satellite project, GLAST (to be launched in 2006). With a threshold as low as 15 GeV one might explore intense y sources up to about 8 billion light years distance. A 'T
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Fig. 9 ACT sensitivities as a function of energy for some current and future instruments, ref. 36. The CANI3AROO II and HESS sensitivity are similar to that of VER1TAS.
REFERENCES 1. V. Hess, Phs, Z 13 (1912)1084 2. E. Fermi, Phys. Rev. 75(1949)1169 3. J. Wdowczyk, A. Wolfendale, J. Phys. A5 (1972) 1419F 4. T. Weekes et al., ApJ. 342 (1989) 379 5. J. Jelley: Cherenkov Radiation and its Appplication, Pergamon Press (1958) 6. M. Hillas, Procs 19th ICRC (1985) Vol 3, 445 7. M Punch et al., Prec.: Towards a Major Atmospheric Cherenkv Detector II, Calgary, (1993) 163 8. A. Djannanti-Atai et al, astro-ph 9906060 (1999) 9. A. Daum, Prec.: Towards a Major Atmospheric Cherenkov Detector V(1997) 178 10. T.C. Weekes et al., ApJ. 342(1989)379 11. T. Kifune et al., ApJ. 438(1995)L91 12. T. Yoshikoshi et al.,ApJ. 487(1997)L65 13. T. Tanimori et al., ApJ. 497 (1998)L25 14. G. Puelhofer et al, Prec. 26th ICRC, OG. 2.2.17 15, H. Muraishi et al., Prec. 26th ICRC, OG. 2.2.20 16. M. Punch et al., Nature 358 (1992) 477 17. J. Quinn et al., ApJ 456(1996)L83 18. M. Catanese et al., ApJ., 501 (1998) 616 19. P. Chatwick et al., ApJ. 513 (1998) 161 20. Yu. I. Neshpor et al., Astro. Lett 24(1998) 139 21. N. Nithiyama et al., Prec. 26th ICRC, OG. 2.1.21 22. O.C. DeJager et al., ApJ., 457(1996) 253 23. M. Amenomori et al., Proc 2@ ICRC, OG 2.2.04 24. J. Gaidos et al., Nature 383 (1996)319 25. L. O. Drury et al., A&A. 287 (1994)959 26. W.Collmar et al., Prec. 26th ICRC, OG 2.1.22 27 D. Nishikawa et al, Prec. 26th ICRC, OG 2.1.17 28. D. Kranich et al., Prec. 26th ICRC, OG 2.1.18 29. E.A. Aharonian et al., Prec. 26th ICRC, OG 2.1.16 30. E. Pare Prec.: Towards a Major Atmospheric Cherenkov Detector V(1997)228 31. F. Arqueros et al., Prec. 26th ICRC, OG 4.3.07 32. J.A. Zweerwink et al., Proc. 26th ICRC, OG 4.3.09 33.R.A. Ong et al., Prec.: Towards a Major Atmospheric Cherenkov Detector V(1997)246 34. M. Mori: The Cangaroo HI project. Proc. TeV 3(ray workshop. Snowbird, Utah, 13-16. 8.99 35. A. Kohnle el al., Prec. 26th ICRC, OG 4.3.245 36. T. Weekes: Veritas Proposal to SAGENAP 37. M. Martinez, Prec. 26th ICRC, OG 4.3.08 38. C. L. Bhat, private communication 39. D. Eichler, D. Faiman private communication
Nuclear Physics B (Proc. Suppl.) 87 (2000) 335-344 www.elsevier.nl/locate/npe
TeV Gamma-Ray Astronomy Eckart Lorenza aMax Planck Inst. for Physics Foehringer Ring 6, 80805 MUNICH, FRG TeV gamma-ray astronomy is becoming an important tool to gain insight into the relativistic (nonthermal) universe. The status of observations and prospects for the next years will be presented.
1. INTRODUCTION Information about our universe is retrieved through the detection of electromagnetic waves, respectively electromagnetic particles at higher energies. The main tool for the observation of the relativistic (astronomers prefer 'nonthermar) universe is gamma ray (shortcut y) astronomy. In the keVGeV energy range observations are carried out by means of balloon borne or satellite borne instruments because the earth atmosphere shields efficiently ys in this energy range. Above = 1 TeV observations can only be carried out by ground based instruments with many orders of magnitude larger collection areas compared to that of satellite borne detectors. The primary motivation for y astronomy was the search for the sources of cosmic rays (CR), mostly charged particles, which bombard constantly the earth from outer space. Charged CRs were discovered in 1912 by Victor Hess, ref. 1. The origin of CRs is still unresolved. The CR spectrum extends to at least 1020 eV; it follows a rather smooth power law with a coefficient of -2.6 to -3.4. The presence of the steady flux of these energetic particles is a clear sign that ultrarelativistic particle processes play an important role in our universe. The challenge to find the locations of production of these energetic charged particles cannot be solved by extrapolating their direction of incidence back to cosmic accelerators, very likely stellar objects. Charged particles are deflected by the weak galactic magnetic field and are thus completely scrambled in direction. Only neutral particles, ys, v s and neutrons, can be back extrapolated to their origin. Neutrons are only useful for short distance observations due to their limited
lifetime and vs require huge detection volumes because of their extreme small interaction cross section. Therefore only ys are viable tracer particles. Unfortunately ys are only a minuscule fraction (< I05) of the total cosmic ray flux and their identification against the charged CRs poses a major experimental challenge. This difficulty was also the main reason why the search for y sources was unsuccessful over many decades. In the past three decades more than 250 y sources were discovered in the MeV-GeV energy range by satellites, mostly by EGRET on board of the Compton Gamma Ray Observatory. TeV y astronomy is trailing these efforts by about 20 years but has succeeded in the last decade to discover about 12 sources above 300 OeV (see chapter 5). Here we present a short overview of the different aspects of TeVy-astronomy. In the following chapter we elaborate on some of the current physics questions related to y-astronomy, followed by a short overview of y-production, transport through our universe and the principal detection methods. In chapters 5 some highlights of recent observations will be presented while chapter 6 deals with the prospects of yastronomy in the next years.
2. PHYSICS OBJECTIVES The full extends of the current physics objectives of TeV y-astronomy cannot be discussed here. Therefore only a short list of some candidates for y emission and some of the physics objectives are given.
0920-5632/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PI1 S0920-5632(00)00696-4
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1) Search for recently (up to a few 104 years ago) exploded Super Novae. The remnants are considered to be efficient particle accelerators and thus the origin of the charged CRs. 2) Plerions (Pulsars) which are potentially also electron accelerators and are expected to show in certain energy bands pulsed "/emission 3) Radioloud, compact binaries. 4) Diffuse y emission from the galactic disc (from charged CRs interacting with the interstellar gas, e.g., information about the gas density in the galactic plane) 5) Active Galactic Nuclei (AGN) with central supermassive black holes accreting mass and driving relativistic jets releasing well above 1040 ergs/sec, yastronomy would shed light onto this energetic process, black holes and indirectly about the cosmological infrared (LR) background from yabsorption processes. 6) Gamma-Ray bursts (GRB) which occur 1-2 times per day and are still in many aspects enigmatic. GRBs must be extremely powerful short-term processes 7) Search for possible Topological Defects, left over from the early universe 8) Search for the lightest supersymmetric particles, ys would be generated in annihilation processes. 9) Tests of quantum gravity effects.
TeV ys I need parent particles of higher energy (masses). Possible generation processes are:
i) High energy hadronic interactions with > TeV r~° production and their subsequent decay into yy. ii) Inverse Compton (IC) scattering of > TeV electrons on low energy photons. iii) Decays of supermassive particles, not necessarily of high energy. Current models of "/ production assume that process i) or ii) are responsible for energetic "/s from stellar sources. Process iii) is at present only speculative. Supermassive particle candidates are topological defects or relic particles assumed to be left over from the early universe. Process i) would be a natural link to the energetic charged CRs as these must be part of the general acceleration process of also the hadronic parents of ys. The necessary target material, say, a beam dump is normally found close to stars or galaxies in form of gas clouds. Hadronic interactions produce in first order the same amount of ys and vs. Ones v detectors of sufficient sensitivity are build one should be able to identify from simultaneous ¥ and v observations identify the hadronic accelerators in our cosmos. The underlying mechanism of charged particle acceleration is assumed to be shockwave acceleration in rapidly expanding gas clouds interacting with gas of lower velocity. Strong shocks are expected for example in supernovae remnants (SNR). It is predicted that in the aftermath of a SN explosion particles can be accelerated up to at least 1015 eV. These particles might interact with nuclei of gas clouds ejected before the SN explosion. Electrons might equally be accelerated in shock waves or alternatively in timevariable magnetic fields, e.g., in the strong magnetic fields of pulsars. High energy electrons might upscatter low energy photons, which are normally plenty in the vicinity of any hot stellar object. Alternatively these photons might be generated by synchrotron radiation of electrons moving through magnetic fields. The concept of shockwave acceleration has been originally proposed by Fermi, ref 2, and has been since then steadily been refined. SN explosions are sufficiently frequent and energetic to explain most of the flux of the CRs observed on earth up to = 1015 eV. Therefore one of
1Here we use still the notation of TeV "/-astronomy for groundbased observations although in the last
years the threshold of the best instruments has been lowered to 300 GeV
Observations would reveal in first order ¥ fluxes (-> information about the energetics of cosmic reactors), energy spectra (-> energy of the involved parent particles)-and time variability (-> information about the time structure of relativistic processes). Discussion about GRBs is omitted because the subject is covered by Eli Waxman in a separate contribution to this conference.
3. GENERATION OF ~ AND ABSORPTION LOSSES IN THE UNIVERSE
E. Lorenz~Nuclear Physics B (Proc. SuppL) 87 (2000) 335-344
the main aims of TeV ?-astronomy is to find these SNRs. Active galactic nuclei (AGN) are also candidates for very powerful particle acceleration and y-emitters. Here shock wave acceleration can occur inside blobs formed in relativistic jets, i.e., the ?s are Doppler boosted along the jet axis and very high energy ?s might be observed at earth, provided the jet points towards us. Also, any time variability in the center of mass system of the blobs in the jets would manifest itself as a much more rapid variation for the earth observer. Gamma rays have to pass significant distances between their generation location and the terrestrial observer. Becauseys are massless their arrival time is a true image of the time structure at the origin 2. The universe is not fully transparent to high energy ys because it is filled with various low energy photon fields, such as from the 2.7 ° microwave background, IR light, starlight or radiowaves. High energy ?s can interact with these low energy photons forming e+epairs with a peak rate at threshold. These electrons might be deflected in the weak extragalactic magnetic fields and eventually interact by inverse Compton scattering with photons of the above noted photon fields. This process generates lower energy "/s, which do not conserve the original direction. Fig 1 shows a calculation of the transmission length of ¥s as a function of energy, ref. 3. The strongest attenuation occurs for 1015 eV ?s that interact with the 2.7 ° MWBR, with an attenuation length of about the distance of the center of our galaxy to the earth. Extragalactic sources are not observable in the 1015 eV band. While the 2.7 ° MWBR is well studied, the knowledge of the IR background is poor. Its origin, amount and distribution over the sky are not known and are difficult to measure from earth due to the much stronger foreground IR radiation of our galaxy. It is speculated that a significant amount of the cosmic IR background is due to the earliest (shortly after the big bang) formed stars and galaxies that are now cooled down and invisible. The determination of the IR background from studies of absorption of ys
2Recently strong evidence for neutrino mixing with a small mass difference has been found. Observation of ?s and vs from short time events of cosmological distance, such as from GRBs, offer a unique opportunity to measure v masses.
337
between 1010 and 1013 eV is therefore an important issue for y-astronomy of extragalactic objects as well as for cosmology. 1029
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4. DETECTION METHODS OF TeV Ts The detection of TeV ys with ground based instruments is indirect. Gamma Rays (as well as the charged CRs) interact in the upper atmosphere and initiate electromagnetic (hadronic) cascades which, depending on the energy, stop in the atmosphere or penetrate down to ground level. The vertical atmosphere at sea level corresponds to 27 radiation length (to) and 13 hadronic absorption length (Xo). Measurable quantities from the shower can be detected at ground level, such as air scintillation, Cherenkov light or in case of high primary energies the shower tail particles. From these quantities one can deduce the incident energy and direction and also sometimes information to identify the primary particle. In principle, all air shower detectors are calorimetric devices similarly to the ones used in high energy accelerator experiments. One can classify the air shower detectors into 3 groups: i) Ground based charged particle detector arrays tail catcher calorimeters- for the detection of showers above a few 1013 eV. These detectors operate 24 hours a day. it) Air fluorescence detectors. These detectors, such as for example the Fly's Eye, are true fully active calorimetric devices with imaging. Due to the low scintillation light yield (O 10-5 of the initial energy) and the isotropic light emission these
338
E. Lorenz~Nuclear Physics B (Proc. Suppl.) 87 (2000) 335-344
detectors are only useful for observations above =1017 eV. iii) Detectors making use of the Cherenkov light. These are in first order also fully active calorimeters with strong directional effects. All discoveries of TeV Y sources were made by the latter type of detectors because they have the lowest threshold and the best y/hadron (y/h) selectivity of all ground based instruments. In the following emphasis will therefore be put on these instruments. Cherenkov photons are peaked along the direction of the shower axis. The combined effect of multiple scattering of the electrons (Pt kick of secondaries in hadronic showers) and the altitude dependent Cherenkov emission angle (0.3-1.3 °) illuminates nearly uniformly a disc of about 120 m radius on ground3. Any high sensitivity photon detector placed anywhere in this disc is able to detect high energy Ts; therefore these detectors have typical collection areas of a few 104 m2. A typical detector consists of a large diameter curved mirror that focuses the Cherenkov light onto an array of densely packed fast photomultipliers (PM), the camera. These so-called air Cherenkov telescopes (ACT) with a limited field of view have to be pointed towards a source candidate and must be permanently moved to counteract the rotation of the earth. The observation of the Cherenkov light does not require that the shower tail reaches ground, thus a lower threshold compared to tail-catching array detectors can be achieved. Low energy showers are detectable as long as their signals stand out against the night sky light background of typically 2"1012 photons/m 2 sec sr between 300 and 550 nm (star light, ionospheric fluorescence, and sometimes man made light pollution). As the ratio of signal to background increases typically with the square root of the mirror collection area the tendency is to build large diameter dishes. The presently largest ACT is the 10 m Whipple telescope, ref. 4. Jelley, ref. 5, already proposed Cherenkov light detectors for CR observations in the 60's. Some major hurdles have to be mastered with ACTs for Yastronomy 3this radius is altitude dependent. Also, some light is spread towards larger distances due to rare large Pt kicks.
i) In contrast to satellite borne y detectors with a charged particle anticoincidence system no device for the suppression of the enormous hadronic CR background exist. ii) ACTs have only a few msterad angular acceptance, thus some guidance for potential sources is needed. iii) In contrast to calorimeters for accelerator experiments the atmospheric calorimeter has a rather similar response to incident electromagnetic and hadronic particles (consequence of the low
4due to the peaked direction of the Cherenkov light one sees only a part of the shower. For large angles between shower particle- and telescope direction no image can be seen by the Cherenkov ltelescope.
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of the currently most advanced camera design allowing for a directional analysis of the shower image (with most telescopes only the around 90 ° folded distribution can be measured), ref 8. The single telescope approach has two deficiencies in the image analysis: a) only one 'projected' Cherenkov light image will be recorded and b) the impact parameter of the shower axis is not well defined. The y/h separation can be improved by observing a shower by a series of 50-200 m spaced telescopes recording images from different directions and angles. This so-called stereo system allows one to improve the angular and energy resolution by = 1.5 and the hadron suppression by = 2-4, albeit with a large increase in costs. Currently the most powerful stereosystem is operated by the HEGRA collaboration, ref. 9, Table 1 lists the leading IACTs.
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Table 1 Parameters of the leading_ACTs. Telescope Whipple CAT Site Mt.Hopkins Themis Longitude -110.5 o -2.00 Latitude 31.4 0 N 42.4 o N Elevation(m asl) 2300 1650 Number of IACTs 1 1 Mirror area (m2) 74 17.5 Number of pixels 151 548+52 Pixel diameter (deg) 0.25 0.10 Threshold(CreV) 250 300 Sensitivityb) (cm-2s-1) 10-11 10-11 at threshold b) Definedas the 5 ~ limitfor 50 h observationtime.
HEGRA Syslcrn LaPalma -17.80 28.8°N 2220 5 5x8.4 5x271 0.25 500 5x10-12
Fig. 3. Distribution of the image parameter ALPHA for the Mkn 501 observation of CAT (ref 6)
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340 Table 2 Observed TeV '/-sources Source/type Crab/plerion PSR 1706/plerion VELA/plerion SN 1006/SNR CAS A/SNR RXJ 1713/SNR Mkn4211AGN Mkn 50 I/AGN IES 2344/AGN PKS 2155/AGN 3C66/AGN 1ES 1959/AGN
E. Lorenz~Nuclear Physics B (Proc. Suppl.) 87 (2000) 335-344
discovered 1989 1995 1997 1998 1999 1999 1992 1995 1998 1999 1998 1959
reference ref. 10 ref. 11 ref. 12 ref. 13 ref. 14 ref. 15 ref. 16 ref. 17 ref. 18 ref. 19 ref. 20 ref. 21
comments >> 10~, steady, seen by > 9 groups > 10 or, steady, seen by 2 groups > 6 s, steady, seen by 1 group > 6 a, seen by 1group 4.9 a, seen by 1 group 5 a, seen by 1 group >>10 ~, variable, seen by > 3 groups >>10 c~, variable, seen by > 7 groups 6 ~, variable, seen by 1 group 7 a, seen by 1 group 5 a, seen by 1 group = 7 a, seen by 1 group
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5. T H E TeV SKY MAP In the following a brief overview of the status of observations will be given. In the 60-80's sporadic observations of some ,/sources were reported, albeit with rather low significance. The first high significance observation was reported in 1989 by the Whipple group, ref. 10. Their observation of the
Crab nebula opened the new window for TeV yastronomy. Nowadays about a dozen sources have been identified; about half o f galactic and half of extragalactic origin. Some of them have been observed by many groups with in one case a combined significance exceeding 100 a.
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E. Lorenz~Nuclear Physics B (Proc. Suppl.) 87 (2000) 335-344
Fig 4 shows a recent sky map of the TeV ,/sources while table 2 lists the sources and references. All these sources are ¥ emitters in lower energy bands. In the following some individual sources will be commented in'more detail.
5.1 The Crab nebula The Crab, a Plerion, was the first source discovered in the TeV region, ref. 10. It is a rather nearby galactic pulsar and the strongest known steady y-source. Broadband emission has been observed spanning from radiowaves up to a few tens of TeV. Below 10 GeV a strong pulsed signal has been observed. In the TeV region no periodic signal has been discovered which would be naturally explained by different emission regions, i.e., keV/MeV ~/ would be produced by electrons accelerated close to the pulsar and the TeV ys by particles accelerated by shocks to higher energies in the surrounding nebula. Even there one assumes that the parent particles are very likely also electrons which produce ys by IC scattering of low energy photons initially radiated by the same electrons by synchrotron radiation close to the pulsar. Calculations based on the so-called synchrotron selfCompton (SSC) model, ref. 22, can explain the spectrum quite well between 1 MeV and = 50 TeV. From the highest part of the spectrum one can estimate the magnetic field of the pulsar to be in the range of a few hundred laG. Two interesting questions for the next generation observations will be: a) to find the pulsed to steady emission transition band (decision between the outer gap and polar cap model of pulsars) and b) any proof of hadronic generation of ys. TeV ys have been observed by at least 9 different instruments. The most recent observation is from the Tibet AS ground based scintillator array, ref. 23. Fig 5 shows a compilation of the spectrum above 300 GeV from a few experiments. Experiments could not find any time variability of the TeV emission; therefore, and because of its large flux, the Crab is nowadays used as the standard candle for ACTs on the Northern Hemisphere. Similarly, PSR 1706-44, a pulsar resembling many features as the CRAB is used as standard candle on the Southern Hemisphere.
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5.2 SNR candidates SNRs are considered to be the sources of the galactic CRs (see section 2, 3). Up to now 3 candidates have been found all of them by one group only and with = 5 o significance reflecting a rather low flux. This low flux together with the nonobservation of TeV y-emission o f some other young SNRs (G 78, IC 443, Tycho,..) casts some doubts that SNRs will be efficient multi-TeV ha&on accelerators. All the up to now observed TeV ,/sources can be explained by being electron accelerators only. In SNRs the location of acceleration and y-emission will be the shell. Therefore the source region of nearby SNRs will be extended and excellent angular resolution of the IACTs will be needed. All current SNR candidates are strong "),-emitters in the MeV/low GeV region. Therefore, it will be one of the main challenges for the next generation IACTs to find the spectral shape in the up to now unexplored energy range between 10 and 300 GeV and clarify the question about the origin of the charged CRs. An overview of the SNR aspects can be found in ref. 24.
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5.3 The AGNs Mkn 421 and Mkn 5 0 1 Another class of fascinating TeV y-emitters are some radioloud, BL-Lac type AGNs. EGRET has found up to now over 60 sources in the MeV region. In the TeV range 6 sources have been identified, two of them, Mkn 421 and Mkn 501 with very high significance. Within the statistical errors all sources show rapi d time variability. In some cases a strong correlation with X-Ray variability was observed, favoring again electron acceleration as the primary production mechanism. The first discovered source, Mkn 421, ref. 16, showed sometimes intense flaring up to 8 times the Crab flux with doubling time of less than 1/2 hour ref. 25. This fast flaring is an indication that the acceleration volume must be very small. Even taking some typical gamma factor of 10 into account, the volume can only have an extension of a few tens of lighthours. In a multiwavelength observation, fig. 6, rapid repeated flaring in the XRay domain has been observed. If one superimposes the X-Ray lightcurve -after the subtraction of a smoothly rising background- one finds excellent agreement in the by daylight interrupted TeV light curve. The mechanism of this variability has to be explained. A general observation of all TeV sources is that y-production can be explained by electron acceleration and IC upscattering of low energy photons (mainly manifesting itself by a pronounced X-Ray and a TeV peak in the vFv plot). No clear evidence for hadron acceleration has been found. Only recently, at the 27th ICRC, a group of COMFI'EL, ref. 26, reported sizeable y emission at around 1 MeV from Mkn 421. This is inconsistent with the low flux expected in the minimum between the X-Ray and TeV peak. Very likely the future satellite INTEGRAL, optimized for observations in the 1-10 MeV energy band, will help to clarify the situation. The source Mkn 501 showed a remarkable variation in intensity. In its year of discovery (1995) the mean flux was only a few % of that of Crab. Nearly throughout 1997 Mkn 501 showed persistent but seemingly chaotic flaring of up to 10 times the Crab flux. At the same time a significant increase in the X-Ray spectrum was observed, but nearly no change in the radioemission or visible spectrum. Since than the TeV flux is dropping. Fig. 7 shows the
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X-Ray and TeV light curve. From the analysis of the time variability in both the TeV and X-Ray data of 1997 two groups found indications of quasiperiodic variability with a period of 23.9 days, ref. 27 and 23 days, ref. 28, using slightly different data sets. This observation still needs to be explained. Anyhow, such a QPO must be observed again before being accepted as real. Due to their redshift of z= 0.03 (Mkn 421) and z = 0.034 (Mkn 501) the y spectra of both AGNs (as well as that of the other more distant extragalactic sources) could be affected by interaction with the still unquantified IR BG. Fig. 7 shows both spectra in the TeV region, ref. 29. The one of Mkn 501 shows a strong deviation from the generally expected power
343
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law shape, thus this could be interpreted as a cut-off due to IR interaction. On the other hand the spectrum of Mkn 421 shows within the statistics no such attenuation, therefore the turnover of the Mkn 501 is more likely due to an energy cut-off of the acceleration process. Nevertheless, from the current observations one can already conclude, that the IR
density must be very low, i.e., the star density shortly after the Big Bang must have been rather low. It will be one of the most important goals of future lower energy observations to measure the ¥ spectra of high redshift AGNs to determine the cut-off as a function of z.
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Current experiments deliver a clear message: future detectors must have a) a lower threshold and b) the sensitivity (collection area and ~//h separation) of the instruments must be increased. In the current observation gap between satellites and ground based ACTs between, say, 10 and 300 GeV, many new results as well as fundamental changes in the acceleration, respectively absorption processes are expected. The recent successes in y astronomy have triggered a world-wide activity to construct more powerful telescopes. Efforts concentrate on a) larger collection mirrors, either single or multimirrors/telescopes, b) higher quantum efficiency
344
E. Lorenz~Nuclear Physics B (Proc. Suppl.) 87 (2000) 335-344
photosensors, and c) improved cameras with finer pixels and a more refined, higher rate data acquisition. Two distinct directions are followed. A number of groups tries to build ACTs on the basis of large, readily available solar heliostat fields. All current projects, CELESTE, GRAAL, SOLAR II and STACEE, refs. 30-33, are already in the test phase. Some of these experiments hope to achieve a threshold as low as = 20 OeV. Their main advantage is the relatively modest investment and ready availability of up to a few 1000 m 2 mirror collector while their main deficiency is a very modest y/h separation and a rather restricted collection area. Other groups aim for new, optimized ACTs by either bundling telescopes of the proven 10 m class to stereo systems such as CANGAROO III, HESS and VERITAS (refs 34-36) or for a single very large mirror MAGIC, MASE and PETAL (refs. 37-39). These experiments will have thresholds between 15 and 50 GeV, very high ~/h separation, collection areas up to 1 km 2 above 1 TeV and see 'first light' in about 2-4 years. Fig 9 shows the sensitivity for some current and proposed new generation ACTs in comparison with EGRET and the future leading ¥ satellite project, GLAST (to be launched in 2006). With a threshold as low as 15 GeV one might explore intense y sources up to about 8 billion light years distance. A 'T
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