Gamma-rays of TeV energy from plerions and results of CANGAROO project

A& Space Rex Vol. 25, No. 314, pp. 641-646.2000 0 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/00 $20.00 + 0.00

Pergamon www.elsevier.nlllocate/asr

PII: SO273-1177(99)00816-9

GAMMA-RAYS OF CANGAROO

T. Kifune

of TeV ENERGY PROJECT

for CANGAROO

Institute for Cosmic

FROM PLERIONS

AND RESULTS

Group’

Ray Research,

University

of Tokyo, 3-2-l

Midori,

Tanashi, Tokyo 188-8502,

Japan

[email protected]

ABSTRACT Copious production of electrons and positrons results in very high energy gamma-rays from the sources The emission at TeV energies is characterized and such as pulsar nebulae and supernova remnants. controlled by the interaction of high energy photons and electrons. The radiation of electrons links the TeV region tightly to the other bands, giving us the means of ‘multiwavelengths analysis’ to investigate production, acceleration and interaction of energetic particles. The absoption of TeV gamma-rays due to creation of electron and positron pairs puts constraints on the emission size of detectable sources. The point-like source of gamma-rays by proton progenitor still remains to be uncovered. 0 2000 COSPAR. Published by Elsevier Science Ltd.

1

INTRODUCTION

The way of seeing the Universe with High Energy (HE; at 100 MeV - 1OGeV energies) and Very High Energy (VHE; in the region of 100 GeV to TeV energies) gamma-rays is characterized by the interactions of energetic particles. The observation window at the shortest wavelengths of electromagnetic radiation was made widely open thanks to Compton Gamma-Ray Observatory (CGRO) launched in 1991 as well as due to the success of ground-based technique for TeV gamma-rays. The current status of gamma ray astronomy can be said, when taking the number of point sources for comparison, to be similar to that of X-ray band in 1970’s. The EGRET instrument of CGRO has discovered about 200 HE sources which consist of pulsars, AGNs (active galactic nuclei) and unidentified sources. The number of VHE sources is approaching 10; several EGRET pulsars, a few AGN and a supernova remnant. The VHE gamma-ray astronomy in ‘the CGRO Era’ is summarized and discussed in Weekes et al. 1997 and references therein. In Section 2 is presented a summary of CANGAROO (Collaboration between Australia and Nippon for a GAmma Ray Observatory in the Outback) observations in South Australia (e.g. Kifune et al. 1997) and the implication of our results is argued about in Section 3 paying attention to the ‘multiwavelength data’. The unique feature of VHE photons is their conversion into/from electrons (and positrons) via soft ambient photons. The process depends on, for example, the spatial size of VHE emission, possibly affecting the type of the sources so far discovered (Section 4). ‘WWW: http://icrhp9.icrr.u-tokyoac.jp; T.Kifune, S.Le Bohec, S.A.Dazeley, P.G.Edwards, S.Gunji, S.Hara, T.Hara, J.Holder, S.Kamei, R.Kita, A.Kawachi, Y.Matsubara, Y.Mizumoto, M.Mori, M.Moriya, H.Muraishi, Y.Muraki, T.Naito, K.Nishijima, S.Ogio, J.R.Patterson, M.D.Roberts, G.P.Rowell, T.Sako, KSakurazawa, N.Sato, R.Susukita, R.Suzuki, T.Tamura, T.Tanimoti, G.J.Thornton, S.Yanagita, T.Yoshida, T.Yoshikoshi 641

T. Kifune

642

CANGAROO

2

The first object

and VHE

firmly confirmed

group, or the break-through

as a VHE

was almost

simultaneous

pulsars of EGR.ET

detection. following

et al.

1995) and Vela pulsar

sources.

The VHE signals arc unpulsed; star.

non-thermal

X-rays

direct

Three

AGNs classified

as blazars

Efforts

of CANGAROO

to detect

negative

results

(Roberts

at redshift

et al.

1998).

remnants,

with one arcminute

3

VHE

result

B1706-44

Koyama

motivated

X-ray

in HE band.

which corotates

with

one from the pulsar nebula.

plerion,

et al.

our VHE

emission

(1995)

et al.

to search

199813).

by the SN shock,

As

detected

observation

(Tanimori

accelerated

0.03 are found as VHE emitters

The

and suggests

by the Whipple

from the AGNs in the southern

CANGAROO

has also attempted

(Sako et al. 1997), X-ray binaries,

and ‘after glow’ of GRB

observation

PSR

at the shell of the SN.

TeV gamma-rays

binary pulsar such as PSR B1259-63 supernova

-

pulsars,

magnetosphere

by unpulscd

without

electrons

are also likely to be shock-acceletated

for VHE signal in the commenced

by the pulsar spin period as detected

This

of -1OOTeV

searches

have been found to be VHE gamma-ray

in the pulsar

of the non-thermal

evidence

by the Whipple

after a long ‘dark age’,

Two gamma-ray

1997),

presumably

from the shell of SN 1006.

from the direction

signal provides

that protons

et al.

at higher energies

remnant

The detection

Telescope)

of CANGAROO

telescope.

originates

The signal is replaced

nebula.

and has stimulated

not modulated

of GeV gamma-rays

SN (supernova)

TeV gamma-rays VHE

(Yoshikoshi

SOURCES

Air Cerenkov

The 3.8m telescope

the Whipple

(Kifune

the neutron

(Imaging

with the launch of CGRO,

in 1992, as the second IACT

for the shell-type

RAY

source is the Crab

achieved by IACT

gamma-ray

The pulsed emission

GAMMA

(g amma

ray bursts)

observations

EGRET

group.

sky have so far been of Centaurus

unidentified

which BeppoSAX

A,

sources and

satellite

detected

accuracy.

GAMMA

RAY

EMISSION

and MULTI-WAVELENGTHS

SPECTRUM 3.1

Crab and other nebulae: Any prototype ?

The ‘entire’

spectrum

over ‘multiwavelength’

profile is consistently mechanism,

explained

i.e. synchrotron

is reasonably

by a magnetic

emission

the Crab is the only pulsar nebula in the EGRET with weaker magnetic

has shown that the Crab spectrum suggest deviation luminosities electrons

process

the Compton background) Electrons

or a proton

Lsync and Li, of synchrotron scattering

The strong magnetic 100MeV.

GeV band, while no unpulsed GeV gamma-rays The CANGAROO

up to several tens of TeV (Tanimori possibly

component

due to gamma-rays (e.g.

Aharonian

and inverse Compton

1997).

radiation

from the

observation

et al. 1998a),

frorn either another

Thus,

and may

population

The ratio between

of

the two

from the common

progenitor

field (WB) to the seed photons

undergoing

(Wphotun). By using the luminosity of the VHE gamma rays and X-rays as Lsync Wr,hoton to be the energy density W2.7~ of 2.7K MWB (micro wave

radiation,

we obtain

=

B -

a few ILG for the pulsar nebulae monochromatic

radiation

of PSR

at, the photon

B1706-44

100 TeV electrons

field is not much stronger

and Vela.

energy of

(1)

04

where E is the energy of target same -

scattering.

The (SSC)

and by putting

of energy E emit approximat,ely

k‘sync

nebula.

self Compton’

into the energy region as high as -

is equal t,o the ratio of the energy density of magnetic

and Lit, respectively,

and ‘synchrotron

field in those pulsar nebulae.

extends

from the SSC spectrum;

the inverse Compton

well known only for the Crab 200pG

photons served as the target of the Compton

field allows the nebula to have synchrotron others are consistent

field B -

photons

radiate

X-rays

involved in Compt,on

scattering.

and VHE gamma-rays

than 1pG and 2.7K MWB

As the relation

under the condition

is the rnain contributor

(1) shows, the

that the magnetic

to the Compton

scattering.

CANGAROO

Results on TeV Gamma-Ray?

643

by The approximation of putting LsynclLic = Lx/L, can be justified a posterior-i and self-consistently a few PG in the Vela and PSR B1706-44 nebulae. However, the consequent result, i.e. magnetic field stronger justification must await more detailed energy spectra in the X-ray and TeV gamma-ray bands as well as for the ‘entire’ spectral profile through X-rays to gamma-rays.

3.2

Relic and escaping electrons: Multiple populations ?

The emission region of the VHE gamma rays is displaced from the position of the Vela pulsar by about O.l”, apparently in accordance with the birth place of the pulsar. The radiation can be from the electrons which have survived since the pulsar was born (Harding et al. 1997). A comparison with the upper limit, on VHE gamma-rays from the compact nebula at the pulsar position would put constraints on the evolution of the Vela pulsar nebula during its life time. de Jager and Baring 1997 studied multiwavelength spectrum of the Vela compact, nebula by including the radio and CGRO OSSE data, which suggest B = 6 - 20/1G. Although de Jager and Baring (1997) presumed that the OSSE flux at MeV energies is from the compact nebula, we may argue that it contains contribution from the birth place of the Vela pulsar or the direction of the VHE emission. If so, the synchrotron counterpart of the TeV gamma-rays is in the MeV gammaray band instead of the X-ray band, and the TeV luminosity should be compared with the MeV one to give a higher value of WB /Wphotoll,which then implies; lower energy of the progenitor electrons, stronger B and/or a contribution of infrared photons to Wphoton. In addition, the site of the synchrotron and the inverse Compton radiations can differ from each other, because WB and Wphotonas well as distribution of progenitor electrons generally have spatially varying structure. As discussed in Aharonian et al. 1997 for the case of PSR B1706-44, a magnetic field as strong as 20pG can be compatible with the X- and VHE gamma-ray data when we assume electrons at N 20TeV have an escape time of N 10 yrs from the central N 1 pc to the outer region of B N 1pG. We have uncertainty about the size of the emission region which are not yet spatially resolved from the X-ray and VHE observations.

3.3

Supernova remnants: Proton progenitor ?

The Galactic disk is the most intense HE gamma-ray source and widely believed to be due to cosmic ray protons. There are several unidentified EGRET sources associated with SN remnant. If the GeV gamma-rays are from the protons accelerated in the SN shock, the GeV gamma-rays will have an energy spectrum of power index = -2, which suggests detectable flux of VHE gamma-rays. However, efforts have so far failed to detect the VHE gamma-rays from these objects. Interestingly, SN 1006 is a ‘reversed case’ in which TeV gamma-rays are enhanced compared to the GeV gamma-rays. If the VHE gamma-rays from the SN 1006 are due to protons, GeV gamma-rays are expected with intensity above the EGRET upper limits. Thus, the VHE gamma-rays are likely to be from the non-thermal electrons. By using the radio to X-ray data to infer the spectrum of progenitor electrons and by assuming that the seed photons for Compton scattering is dominated by the 2.7K MW13, t)he magnetic field is estimated to be about 10pG. Similarly, the emission of the unidentified EGRET sources with SN remnants could be explained by electrons radiating dominantly into the GeV gamma-ray region. We need to carefully investigate the following possibilities; the association with t,he SN remnant may be accidental; the GeV gamma rays are from the electrons concentrated in the supernova shell; effects of environmental conditions in the individual objects such as possible injection of electrons from associated plerion and various matter density sorrounding the SN remnant; the shock acceleration mechanism is to be developed to reconcile with the observations (e.g. Volk 1997; de Jager and Baring 1997). More examples of VHE emission are necessary, and CANGAROO recently made observation of R.XJ 1713.7-3946 (G347.5-0.5), a SN remnant quite similar to SN 1006, showing non-thermal X-ray emission.

644

4

T. Kifune

SIZE AND

In the EGRET outer nebula

pulsars,

OF EMISSION

region apparently

gamma-ray

energy

suffer from the cascade Let

changes

source, t,he radiation

becomes

process of electromagnetic

stronger,

interactions.

a simplified

more intense,

tend more likely to can be annihilated

and are prevented

density

to the

as the emission

field becomes

The TeV gamma-rays

case in which the number

-..

Generally,

and gamma-rays

pairs when they collide with the infrared photons,

us consider

REGION

from the pulsar magnetosphere

from GeV to TeV band.

smaller or nearer to the central compact field in the pulsar magnetosphere

into electron-positron outwards.

DENSITY

the emission

when we increase

region becomes the magnetic

ENERGY

from escaping

nir of infrared

photons

is

Gamma Ray Burst

A tive Galactic

Nuclei

Log (Size r (cm)) Figure 1: The energy density of radiation from interesting

objects

is calculated

T from the central

compact

kpc and 100 Mpc,

respectively,

object. that

point source of a given luminosity ones (a) and (b).

is plotted against spatial size. The energy density W of photons from L/(4 n?c) and plotted in the vertical axis against the distance The line (a) and (b) corresponds to the least luminosity located at 3 is detectable

with the VHE sensitivity

is to follow the relation

The line (c) indicates

the condition

r2W

that

=constant,

of N lo-l2

erg cmP2 s-l.

a line in pararell

A

with the

the size r is equal to the mean free path

of electron-positron itself.

pair creation by VHE photons in collision with the ambient radiation of the source The dashed line (d) corresponds to the compactness parameter (see text) 1 = 1, and is plotted for

comparison.

The density

of 2.7K MWB

proportional

to the luminosity

L averaged over wavelength

field WSoUTCc,

The

mean

the distance greater

line.

and thus also to the energy density of radiation

L W .sOuTCe n zv = -----~-. 47rr2c E &

where c is the light velocity, erg).

is also shown by the dotted

free path

(2)

and E the energy of the target photons (in the infrared band, i.e. z lo-‘” l/(nirgPpair) of p air creation with the cross section gppair varies with

X =

r from the emission center. In order that the VHE gamma-rays can escape, A must be The condition is as an order of estimate, z 1014cm. (L/(1033ergs-‘)(10-2eV/E).

than r = r,in,

schematically gamma-rays the detection

illustrated

in Fig. 1 as a function

of the energy density

W and the spatial size r. The VHE

are free from absorption threshold

indicated

if the source is below the line (c). The luminosity must be above by the line (a) and (b) w h’lc h are for the cases of 3kpc and 1OOMpc

distance,

respectively, for the source to be observable. Thus, the VHE objects are to be in the shaded region (A) or (B) in Fig. 1. The known Galactic sources, pulsar nebulae and SN remnants, are located in the region (A), demonstrating

that such ‘fairly extended’

sources

(angular

size is smaller

than the field

CANGAROO

of view of the VHE

telescope)

are appropriately

1 = La~/(47rrm,c2)

= r/AM,”

is commonly

and m, are the Thomson of Thomson

scattering

1 = r/XMev

detectable

technique.

the compactness

The parameter

of the AGNs, where ffT

and electron

mass, respectively, and XM~~ is the mean free path density’, i.e. W,,,,,, /(m,c”). The compactness parameter line (d), and most of the AGNs are outside the allowed region of

line (c) of r/X = 1. The detection

10 enhancing

with the current

used for estimating

‘MeV photon

= 1 along the dashed

the boundary factor b -

cross section

against

645

Results on TeV Gamma-Rays

the luminosity

of the VHE signals from blazars

when observed from the jet direction.

is due to the beaming

GRB

and the sizes less than

N 10” cm are also outside the region, implying that we can not, expect easy, ‘straightforward’ VHE gamma-rays and simplified

from compact

assumptions

as asymmetrical time variable blazars,

can make the constraint

made on stable,

sources,

‘DC’ sources

as well as objects

will be of increasing

of

The result in Fig. 1 is, however, based on the static conditions the radiation field, and for example, t,ime-variable phenomena as well

about

geometries

have been mainly

detection

sources.

less tight.

with the phenomena

importance,

Observations

(at least, for the Galactic

possibly

providing

of the VHE

objects).

Studies

similar t,o the relativistic us with information

gamma-rays of violently

jets as we see in

on t,hc central

compact

objects. Simultaneously

with the absorption

the opposite

effect

of the radiation

of the higher

of the VHE

energy

field of the object

density

in the Crab gamma

5

at small r).

nebula

and blazars.

pulsars.

observation

by ROSAT

with synchrotron

In the EGRET

of synchrot,ron

radiated

processes

when assumed

strength

that VHE electrons

phot,ons, known to take place

will contribute

as ~(~11 t,o VHE

in blazars.

the TeV measurements.

of the dependence B1706-44

bands.

on the rotationa.

spectrum

or ‘micro quasars’

by spatial

is remarkably

different

between

in the VHE gamma-rays.

in the form of VHE gamma-rays The results

from Crab,

thus encouraging

from X-ray to VHE gamma-ray and the inverse Compton

from

Vela and

VHE observation

region is necessary

emission.

for

No less important

and optical light, which serve as ‘seed’ photons and also as ‘partner’

of photons

such as GRS

by HEGRA

photons

for annihilation

is coupled to thr VHE gamma-rays. 1915+105

(from which a possible

group) as well as close binary

systems

detection

like PSR B1259-63

size much smaller

if the signal from those are detected, of VHE

than ‘V lpc of pulsar nebula and SN remnant. Thus it will provide a new example and insight into the production and

interaction

mechamism

manifested

itself by the unidentified

EGRET

The time variablit,y

may suggest

1998).

luminosity

energy loss.

VHE gamma-rays

pair. The ‘entire’

of a VHE burst has been reported can be characterized

into the pulsed signals in the

as well as for unpulsed signals from

to argue about such feature

by the radio, infrared

of Compton-scattered

and bursters

pulsars

with young

energy loss is spent into the HE gamma-

which differ from each other,

between the synchrotlron

are the bands of longer wavelengths

pulsars

nebula’

study of a large number of VHE sources will provide an urlderstanding

The broad band energy spectrum

comparison

into electron-positron

‘inverse Compton

of energy output

energy loss from the pulsars manifests

A systematic

of the VHE emission

for the production

of rotational

on the spin-down

It is premature

have shown phenomena

of more pulsars. having better

which also suggests

a larger fraction

The dependence

of the spin-down

the pulsar nebulae.

nebula,

band relative to the spin-down luminosity,

and GeV gamma-ray

A fraction

and ASCA satellit,es have revealed that, most, of the luminous

In Fig. 2 is plotted the fraction

X-ray and GeV gamma-ray

et al.

The relative

into VHE gamma rays (by assuming

is ‘SSC’

seed photons’

X-ray

pulsars,

rays than radio and X-rays.

X-ray

emission.

CONCLUSION

The X-ray

PSR

the VHE

we need to investigate

the abundant 2.7K MWB increases with decreasing 7 M ore number of the ambient soft photons, the ‘seed

The soft phot,ons by other

rays, called as the ‘external

are associated

X-ray

Such example

by pair creation,

enhancing

itself against

M (L/10”3ergs-‘)(r/0.1pc)2. as WsourcelW2.7K photons’, at smaller T can be Compton-scattered

are accelerated

gamma-rays

gamma-rays.

New population

of gamma-ray

sources that seem to exhibit rmission

sources

may have already

violent time variation

from denser radiation

(Tavani

field of size smaller

than

646

T. Kifunr

L

I

I

I

I

I

I

33

34

35

36

37

38

Log(rotational

energy loss [erg/s])

Figure 2: Brightness in X-ray, HE and VHE gamma-ray bands of young pulsars. The luminosity relative to spin-down luminosity (vertical axis) is plotted for individual pulsars versus spin-down luminosity. The data for X-ray (circle) and HE gamma-rays (triangle) is from the pulsed signal, and VHE ones (square) are unpulsed luminosities. - 1 pc of the pulsar

nebulae

or SN remnants,

as in the case of the AGN out,bursts

and GRBs.

The CANGAROO project is supported by a Grant-ill-Aid in Scientific* Research from t,he Japan Ministry of Education, Science, Sports and Culture, and also by the Australian Research Council and t,he Internat,ional Science and Technology Program.

6

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