Pergamon Chin. Asfro~ Asrrophps. Vol. 22, No. 4, 409-418, pp. 1998

A translation of Acta Astrophys. Sin. Vol. 18, No. 3, 243-252, pp. 1998 0 1998 Elsevier Science B.V. All rights reserved

Printed in Great Britain 0275-1062/98 $19.00 + 0.00

PII: SO275-1062(98)00052-6

Evolutionary relationship between the y-ray flares and millimeter wave outbursts in

PKS 0528+134t

QIAN Shari--jie ZHANG Xi-zhen

Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080

S. Britzen A. Witzel T. Krichbaum A. Kraus

Maz-Planck Instit& fiir Radioa8~ronomie, Bonn, Germany

E. Valtaoja

Metsihova Radio Research Station, Helsinki Univ. of Technology, Finland

H. D. Aller

Astronomy Department, University of Michigan, USA

Abstract During the period of 1991-1993 two strong high energy T-ray flares

were observed by the Compton Gamma Ray Observatory in the flat spectrum ra-

dio source PKS 0528+134. They were associated with strong mm-radio outbursts

with a few months time-delays. In this paper the spectral energy distributions

(SED) of the radiations in the r-hard X-ray and the IR-optical bands are anal-

ysed. It is shown that the high energy r-ray radiation may be due to the inverse

Compton scattering of the ambient UV and soft X-ray photons by the relativis-

tic electrons in the jet. Basing on the comparison between the properties of the

synchrotron radiation of the T-ray source and the spectral evolution of the mm-

radio outbursts, the evolutional relationship between the r-ray emitting blobs

and the mm-radio emitting blobs is discussed.

Key words: r-ray flares-radio outburstspectral evolution-PKS 0528+134

1. INTRODUCTION

Up to the present, Compton Gamma-Ray Observatory (CGRO) has detected r-ray emission

in some 50 active galactic nuclei *[1-5l; these are mostly high (optical) polarization quasars

t Supported by National Natural Science Foundation and CAS Astronomy Commission

Received 1997-02-04; revised version 1997-12-08

409

410 QIAN Shari--jie et al.

(HPQs) and BL L ac objects. In the radio range, they are all strong, compact, flat-spectrum,

violently variable sources, and in many of them, superluminal motion has been observed on

VLBI scales. Hence, there may be a close connection between the emission of the superlumi-

nal knots accompanying radio outbursts and the y-ray flares. In fact, in the region of r-ray

emission, interaction between the y- and X-rays generates electron-positron pairs and causes

serious absorption of the y-emission. Studies on the time scale of variation of the r-rays

and their photon-photon absorption showed that the region of y-emission must be moving

at relativistic speeds at small angles towards the observer, with Lorentz factors near the

VLBI observed values. Hence it is of especial importance to study the correlation between

the y-ray and radio emissions (particularly for the outbursts)16-‘1. Of course, correlation

between activities in y-ray on one hand and those in the infrared, optical, ultraviolet and

X-ray is also important. Observational studies of the whole electromagnetic spectrum from

r-ray to radio are necessary for the study of r-ray active galactic nuclei. In this field there

has been rapid progress in joint international projects, and many interesting phenomena

have been discovered. 1) in the quasar 3C 379, high (low) activities in r-ray appeared at

the same time as high (low) activities in optical and X-ray, while no simultaneous variations

were found from infrared through radio llOl. 2) In the quasar 0420-014, optical bursts were

observed at the same time as y-ray bursts llll. 3) In the quasar 1406-016, the GeV y-ray

burst was seen some 22 hours after the optical burstI121. 4) in the BL Lac object Mrk421,

a TeV burst and a hard X-ray burst were observed to occur at the same time, while there

was no trace of any activity in GeV, ultraviolet, optical and mm rangesl13J41.

A number of mechanisms of r-emission have been proposed, and the one that is most

discussed is inverse Compton scattering. But there are two views on the origin of the

soft photons: 1) they come from the synchrotron radiation in the self-same region of y-

emission (the SSC mechanism 13~15~161); 2) they come from outside that region (the EICS

mechanism116-1gl). Besides the inverse Compton scattering mechanism, cascading type syn-

chrotron emission driven by extremely energetic protons 1201 has also received much atten-

tion. Recently, Blandford et a1.121-231 put forward an “electron pair jet model” which gave

a consistent interpretation of the jet dynamics and emissions in y-ray and other wavelength

ranges. In this paper we mainly analyze the evolutionary connection between the bursts in

y-ray and mm-radio in the flat-spectrum radio source PKS 0528+134.

2. THE BURST ACTIVITIES OF PKS0528+134

PKS0528+134 is a large redshift (Z = 2.06), low (optical) polarization quasar (LPQ). We

discuss its strong activities in y-, X-, optical through radio ranges in the years 1991-1993.

2.1 y-ray

Between 1991-1993, two y-ray events were observed by EGRET (30 MeV-30GeV),

Comptel (0.73-30 MeV) and OSSE (50 keV-1OMeV) on board CGRO, showing that, at its

high activity state, PKS 0528+134 is one of the brightest r-ray sources, with a time scale of

variation of a few days1415~24-261. See Fig. 1. For the 1991.35 event, the mean photon flux is

F(E > 100MeV) - 10v6 PCS; for the 1993.23 event, it is - 3 x 10e6 photon cmp2 s-l. These

PKS 0528t134 411

.O 92.0 93.0 94.0 9

F(> 100MeV) @I

30 - lo” (photons ati* 5.‘) +

2O- l

lo- + + +

I * I + I

91.0 92.0 93.0 94.0 95.0

Fig. 1 mm-radio outbursts and r-ray flares.

(a) light curves at 150, 86 and 32 GHz; (b)

r-flares monitored by EGRET (1991.35 and

1993.23 events)

values are -3-10 times higher than in

the low activity state. The correspond-

ing apparent luminosities are as high as -

3 x 1041 and 1O42 J/s, the highest so far ob- served (Bo = lOO(km/s)/Mpc, ~0 = 0.5). The spectral index is cr N -1.6 in the GeV range and - -0.4 in the soft X-100MeV range, thus there is a spectral discontinu- ity of Acu - 1.2, which again is the largest among r-ray sources[271.

14 - .’ , I

t. \

813 - _

z-a .

312 , ,’ ----,

Yf, K1

z 211 - :: ‘?

,,!-- ***

I’ ,* *,

: \

\ ,I* :

.A

4 . 10

/i

p

. #‘, ---.

‘\ , I I . .

I \ 9

10 15 20 25

log VW)

Fig. 2 The overall spectral energy

distribution log I/ -log v of PKS 0528+134

2.2 IR-Optical and X-Ray

There is little IR, optical or X-ray observations made at the same time as the r- ray events11~5~281. For the 1993.23 event, a simultaneous optical burst in the R band was observed, the magnitude brightened from 20”’ to -17.5”’ (Wagner, private correspondence). The observational data in these ranges are collected in Fig. 2. Of these the far infrared data12’l are particularly important, for these rather clearly expressed the size of the peak in the synchrotron spectrum. We note from Fig. 2 that the peak in the y-ray is - 30 times the peak in IR-one of the highest ratios among y-ray sources. We also note that the synchrotron radiation begins to be cut off near the optical range.

2.3 Millimetre and Radio Waves

In these ranges we have available multi-frequency monitoring observations of PKS 0528+134 over quite a long time Is?7~2s~sol. The radio activity can be summarized as fol-

lows: 1) Before - 1994.1, PKS 0528+134 had been in the low flux state for nearly 2 years, the spectral peak was at - 7GHz, so it was a GPS type source. But at - 1991.5, an outburst occurred, which started in the mm waves (see Fig. l), the peak having shifted to - 60GHz. This outburst lasted till - 1992.8. At the same time, another outburst began,

which lasted till - 1994.0. We note that the 1993.35 y-burst observed by EGRET occurred in a quiet period in radio (mm waves), at a time when a radio outburst was just starting, while the 1993.23 r-event occurred during the rising phase of the second radio outburst.

412 QIAN Shan-jie et al.

But this may be mere phenomenological description: the second outburst may have been a

superposition of several separate bursts, and the y-event was linked to the starting phase

of one of these. For this event was accompanied by an optical burst, and optical bursts are

generally thought to be precursors of emission by superluminal knots and radio outbursts.

2) VLBI observations 16*28y311 showed that PKS 0528+134 had a core-jet structure, and there

is a difference of - 60’ - 90” in the directions of the sub-ma8 and mas structures. The knots

near the core showed superluminal motion (VT,,, - 5c), while those further out did not do

so clearly. The 1991.35 r-event was clearly accompanied by the emission of a new VLBI

knot Nr and the start of the radio outburst 1311. Hence, flux monitoring in the mm waves

and VLBI observation are of particular significance in establishing connection between mm

wave and r-ray burst81321.

Fig. 2 displays the entire spectrum of PKS 0528+134 from radio to r-ray, including the

two y-ray events of 1991.35 and 1993.23, and the observational data in X-ray, uv, optical,

ir, mm and radio (mostly taken at different times) 15~26~28~29~301. In the radio range, the lower

curve gives the quiet level and the upper curve, the burst peak value (taken at different

times, as the burst moved from high to low frequencies). The following features may be

noted: 1) There are two peak8 in the overall spectrum, one in the infrared range, one in the

r-ray range. 2) The ratio between the two peaks is - 20 - 30, that is, the emitted energy

is mainly concentrated in the y-rays. 3) The spectrum begins to be cut off at the optical

range. We now proceed to give a physical explanation of the overall energy spectrum.

3. MODEL OF 7 BURST EMISSION

We first discuss the emission mechanism of r-bursts.

3.1 EICS Mechanism

As Fig. 2 shows, for the r-bursts observed in PKS0528+134, the emitted energy is

concentrated in the high-energy T-ray range. In this, it is similar to the r-ray sources 3C 279,

1633+134, etc.. As stated by Ghisellini and Maraschi[181, 7- ray active galactic nuclei can be

roughly divided into two types: 1) high polarization quasar (HPQ) r-ray sources, where the

energy emitted in y-rays far exceed8 that in the IR-optical range, and 2) BL Lac type objects

where the emissions in the two ranges are comparable. The emission mechanism may be

different for these two different types of source, for the former, it may be the mechanism of

exterior inverse Compton scattering (EICS), for the latter, it may be that of self Compton

scattering (SSC). For the first type, since the r-emission is particularly strong, the electron

energy spectrum is usually cut off at re < 104, and their synchrotron spectrum is then

cut off in the optical-uv range. For the second type, because the y-emission is relatively

weaker, the cutoff in the electron spectrum can be as high as re > 105, and the synchrotron

spectrum can extend to the uv and X-ray ranges and the self Compton radiation can reach

-TeV waves (as is observed in Mrk 421). For PKS 0528+134, its high r-ray luminosity, large

discontinuity between the y- and X spectra (Aa - 1.2), and the cutoff of the synchrotron

spectrum in the optical-uv range all favour the choice of the EICS mechanism116~‘8~211. A

most basic feature of the EICS mechanism is this: viewed from the coordinate frame of

the plasmon moving with Lorentz factor I?, the surrounding soft photons (UV-soft X) are

PKS 0528+134 413

blueshifted, and the energy density of the seed photons they generate will be enhanced by

a factor of - 12, and so the inverse Compton scattering will be greatly strengthened.

3.2 Specific Model and Results of Calculation

In the frame of EICS, the calculation of inverse Compton scattering (Thomson approxi-

mation) is as follows: we assume the T-ray source to be a sphere of radius R, inside which the

relativistic electrons have a power-law energy distribution: x(7,) = Ke7rb (s = 2a, + 1).

Assume that in the coordinate system comoving with the source, the field of seed photons

also have a power-law form, t+,h+(z+,h+) oc vLhtph (the asterisk indicates comoving frame). We think that, in the comoving frame, the intensity of the seed photons is anisotropic, but

for simplifying the calculation we may use a suitable mean isotropic field as an approxima-

tion and this would be sufficient for the purpose of this paper. Hence, the emissivity of the

inverse Compton scattering is

&e(b)= CUph*(&*)&Tr /

ymsr 2apb-2a,-l 7e d7e , (1)

rmin

where v,, is the frequency of emission, bT is the Thomson scattering cross section, (7d,,, rmax)

is the energy range of the electrons producing this v e+ emission, and is related to the fre- quency range of the seed photon field (Y$‘, $$y) (for standard method of calculating

inverse Compton scattering, see Refs. [33,34]). From (1) we can get the emission luminosity

L - 47rR3c&,(v,,), hence the observed luminosity (at the observed frequency vc), Y,. -

(2)

where ye = [b/(1+ z)13yc* and 6 is the Doppler factor of the source.

Consider now the scattering of UV-soft X rays of the accretion disk by the seed pho-

tons coming from cloudlets in the broad line region 116+l. Denoting the spectrophometric

luminosity of the disk emission in the galaxy frame by Lext(vext), the corresponding spectral

density of the seed photons is

(3)

where OL is the re-emission coefficient of the cloudlets of the broad line region (Q - O.l), and

r is the distance of the region boundary from the core. Thus, the photon spectral density

in the

where

comoving frame is

+h+(Vph+) = ~%t(~ext),

Vphr = %,t. Referring to Ref. [35]. we take Lext(vext) to have the following form:

vext 5 1014.5 Hz,

10i4.‘Hz < Y ext 5 10’5.5 Hz ,

vest > 10’5.5 Hz .

The form is displayed in Fig. 3.

414 QIAN Shan-jie et al.

I 8 I I I

13 14 15 16

1% vex

Fig. 3 The UV-soft X-ray spectral luminosity of the accretion disk used in the EICS model

In our calculation we took the electron energy distribution (isotropic) to be composed

of 3 power-law segments (Fig. 4):

Fig. 4 The spectral distribution of the

electron energy in the r-ray emitting source

1 -Ye2.0 -Ye I Ybl > iv, lx ye3.O rbl < Ye < ‘yb2 ,

co Yb2 < 7e 2 rcut *

In Fig.2, the solid curve in the

y-range is a fit to the observations of the 1993.23 event and was calculated for the following parameter values: R = 4x1017cm, a = 0.1, T = lpc, Lext,i = 4x 1O23 J/Hz.s, Lext,2 = 2x 1O23 J/Hz.s, Kr = 3.8x103,r = 20,8 = 2.?,7,, =

50, YF,2 = 750~ Tcut = 2.8x 103. The dashed curve is a fit to the 1991.35 event, calcu;

lated with the same parameter values, ex- cept Ki = 1.3x103.

4. SYNCHROTRON SELF-COMPTON RADIATION

After using the EICS mechanism to explain the r-ray emission, we still have to discuss its

synchrotron self-Compton (SSC) radiation. As most of the source parameters have been

determined, we can use the flux of the optical burst observed at the same time as the

1993.23 event to estimate the internal magnetic field of the source region. The calculation

of SSC followed the standard procedure133~34~361, fo r a field of B = 3.8 G. The energy spectra

PKS 0528+134 415

obtained are shown in Fig. 2. For the 1993.23 event, the calculated spectrum (solid curve)

fits quite well the observed data of the high activity state in the IR-optical range.

5. SPECTRAL EVOLUTION OF THE MILLIMETRE-RADIO BURST

For the mm-radio outburst that accompanied the 1991.35 y-event, its evolutionary

features have been analyzed in Ref. [30], w h ere the flux curves at 12 frequencies were fitted

using the author’s “model of explosive injection 771331. For the 1993.23 event, if we regard it to mark the beginning of a new mm-radio burst, then we have to resolve the observed flux

curve into 2 or more bursts and link one of them with the event. Fig. 5 shows the results

at six frequencies, when the outburst is resolved into 3 similar bursts using the explosive

injection model. An important handle in this determination is the descending part of the

burst curve at the higher frequencies.

UY) 10 -

(JY) 230 GHz

(a) 10 - 150 GHz (b)

5-

‘:__a&1

Y Y

I I I I 1 I I I 92.5 93.0 93.5 94.0 94.5 92.5 93.0 93.5 94.0 94.5

(JY) 10

(JY) 10 Cd)

I 94.0 9,

(JY) 10 -

32 GHz

5

‘J’;‘o - 22GHi

I 1

92.5 93.0 93.5 94.0 s

Fig. 5 The separation of the mm-radio outburst: associated with the 1993.23 r-ray event

416 QIAN Shan-jie et al.

The evolutionary tracks in the (Sm, vm) plane for the two accompanying mm-radio outbursts are shown in Figs. 6a and 6b. Both have the form of the three-stage evolution1331.

In Fig. 6b, the ascending stage v,,, is increased somewhat. This reminds us of the thickening

behaviour seen in a certain knot in 3C 3451371.

+1 1 (a) (b)

: :. : ._.~ ^x

_ ., I” .

3 ,’ ,‘,’

ro - ,*- :‘-.‘._* m

z ,’ 0..

,‘Y ,’ ,, .’ ,,’ ,o.. ..a . . .

; ,’ .’ :. ‘, ,,’ *,.

. .

Fig. 6 The synchrotron spectrum of the r-ray emitting source and the spectral evolution track

(%I, vrn) of the associated mm-radio outbursts: (a) 1991.35 r-ray event; (b) 1993.23 r-ray event

6. EVOLUTIONARY RELATION BETWEEN GAMMA AND

RADIO BURSTS

To discuss evolutionary relation between mm-radio and 7 bursts, we added the synchrotron

spectrum of the 7 source to Figs. 6(a) and 6(b). The spectrum refers to the burst maximum.

Because of limitation of observed data we are unable to have the evolutionary track in the

(S, -urn) diagram for the y-ray spectrum, and this limits our discussion on the evolutionary

relation. Moreover, the results shown in Fig. 6 are model dependent. Nevertheless, our

discussion may shed some light. For the evolutionary track for the radio burst shown in

Fig. 6 is typical and the 7 spectrum also contains typical featuresl’611sl.

We see clearly from Fig. 6 that the spectral maximum as specified by the inversion

frequency and flux does not lie on the extension of the ascending part. According to some

jet-shock burst model, the r-ray emitting source (or plasmon) and the radio emitting source

(or plasmon) are produced by the same shock propagating outward, hence, the evolution of

their synchrotron spectra should be continuous. This is to say, if we suppose that the track

of the synchrotron spectrum of the 7 source falls rapidly after the y-burst maximum, then

in order that this track extend to the radio track, there must be some physical mechanism to change the falling track into a rising one (as shown by the arrow). We can imagine two

possibilities. One, in some jet region of transition from 7 to radio emission enhancement of particle acceleration and magnetic field takes place. Two, acceleration of the shock takes place in such a region. These proposals are tentative, for under the EICS mechanism, as the

shock passes the boundary of the broad line region, the r-emission suddenly falls, and so does the Compton dissipation. Hence acceleration of electrons will be relatively greatly enhanced. At the same time acceleration of the whole jet may possibly occur. To solve the evolutionary

relation between the 7 and radio burst plasmons, monitoring of the synchrotron emission

from the 7 source is particularly important (mainly in the optical-IR-submillimetre-mm

range).

PKS 0528+134 417

7. CONCLUDING REMARK

In this paper we take PKS 0528+134 as example and discuss the evolutionary relation between the sources of r-rays and radio bursts. The discussion is made from the point of view of the evolutionary track in the (S,,, - vm) diagram. To ascertain the relation and to extract therefrom importance physical information, the key is the determination of track of the r-ray source from observations, and this will principally involve monitoring the optical, IR, submillimetre and mm ranges. The results of this paper are preliminary, but some of them may be checked as more multi-frequency observation becomes available.

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