X-ray variability in BL Lac objects

Adv. Space Res. Vol. 25, No. 314, pp. 723-728,200O 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.nl/locate/asr

X-RAY

Fumiyoshi

VARIABILITY

PII: SO273-1177(99)00829-7

IN BL LAC OBJECTS

Makino’

1 Institute of Space and Astronautical

Science,

3-l-l

Yoshinodai,

Sagamiham,

Kanagawa

229-8510,

Japan

[email protected]

ABSTRACT X-ray properties of BL Lac objects are classified into two categories. One is low energy peaked BL Lacs (LBLs). The X-ray spectrum of LBLs is a power law type with an index of about 1.7 and is flatter than those in optical and UV bands. X-ray flux varies in correlated way with optical flux for a major outburst. The other catgory is high-energy peaked BL Lacs (HBLs), whose X-ray spectra are either single power law type or power law type with gradual steepening toward higher energy. The spectra are steeper than those of LBLs and can be connected smoothly from optical and UV bands. X-rays from HBLs are characterized by correlated flux variation with spectral index and soft lag. The spectra becomes steeper with decreasing flux, and the peak or bottom of the light curve in the soft band is delayed from those in hard band. These properties are interpreted by a synchrotron-cooling model, and cooling time, escape time, and injection time of synchrotron emitting electrons are determine! d by fitting the light curves with the model. The flux increase and hardening of X-rays during the rise of the outburst was expressed by Fermi acceleration of low energy electrons and hard lag of the start time of the burst was strong support for the acceleration. On the other hand, Rat X-ray spectrum of LBLs is attributed to inverse Compton scattering of internal and/or external seed photons. 0 2000 COSPAR. Published by Elsevier Science Ltd.

1

INTRODUCTION

BL Lac objects are a class of active galactic nuclei characterized by featureless spectrum, high variability of optical flux, and high polarization. The energy spectrum f(v) (v:frequency) is flat or of positive index in radio bands and gradually steepens toward UV and soft X-ray bands through IR and optical. The vf (v) vs. v plot of continuum spectrum from radio to UV shows a maximum near IR/optical frequencies or near UV/soft X-ray bands. The former and the latter are called low energy peaked BL Lac (LBL) and high energy peaked BL Lac (HBL) respectively (Urry et al. 1996, Sambruna 1997 and references there in). The hard X-ray spectrum of LBL is flatter than the extrapolation of UV/soft X-ray spectrum whose index is approximately 1.7, and whose spectral properties are similar to optically violent variable (OVV) quasars. HBLs are connected smoothly from the UV region and can be expressed by a power law with an index of about 2.5. There is another classificat! ion, radio-selected BL Lacs (RBLs) and X-ray-selected BL Lacs (XBLs), which mostly corresponds to HBLs and LBLs respectively (Urry et al. 1996). Examples of LBLs are BL Lacertae (Tanihata et al. 1998), OJ 287 (Idesawa et al. 1996), and examples of HBLs are Mkn 421, Mkn 501, and PKS 2155-304. Combined Ginga, ROSAT, ASCA, SAX;‘and OSSE spectra of Mkn 501 might have suggested a transition from LBL to HBL (Kataoka et al. 1998). GeV r-rays were detected from both LBLs and HBLs by EGRET aboard CGRO. However, air Cherenkov detectors detected TeV y-rays only from HBLs. The y-ray spectra were of power law type and their indices were as flat as those of X-rays from LBLs.These 723

124

F. Makino

spectral properties suggest that soft component from radio to soft X-rays in LBLs and to hard X-rays in HBLs are synchrotron radiation and that the high energy component of flat spectra is produced by inverse Compton scattering of internal or external soft photons. Strong beaming has been invoked to avoid Compton catastrophe and its observational evidence is thought to be superluminal motion of the radio sources in blazars.

2

X-RAY

VARIABILITY

IN LBLs

We observed BL Lacertae which is is a prototype of this class of objects during 18.6-19.6 July 1997 with an X-ray astronomy satellite ASCA. The source was in outburst and its average flux was 2.7 x IO-“erg/cm2/s (2 - lOLeV), which was four times higher than that observed in 1988 by Ginga (Kawai et al. 1991). The flux changed by factor of 2 in approximately 5 hours. Two short outbursts were observed in 0.7-1.5 keV band without corresponding variation in 3-7 keV band. The spectra were expressed by a single power law model with Galactic absorption. The photon indices changed from 1.35 f 0.02 to 1.28 f 0.02. The spectra were flatter than those observed by Ginga in 1988 and it was confirmed that BL Lacertae was one of the typical LBLs as suggested by Urry (1996) from ROSAT observations. X-rays were a different component from optical and UV radiation. The spectra of two short flares were also expressed by a single power law model, whose photon indices were 1.54!m0.03 and 1.59 f 0.03, respectively. A two component power law model was also acceptable for soft flares. In this model, the photon index of the soft component was 2.25. However, fast variability of the soft flare suggested that a two component model would be physically plausible. The photon index are plotted in Fig.l(left) as a function of X-ray flux. No correlation can be seen. If two component model was adopted, we can see a weak correlation between the index and the flux of hard X-rays. The hard component spectra become flatter with increasing flux. This trend was also observed from OVV quasar 3C 279 as shown in Fig.1 (right). Flat X-ray spectra of blazars suggest inverse Compton scattering of internal or external photons for X-ray production. We calculated X-ray spectra for high and low states of X-ray flux by synchrotron self Compton (SSC) model. But an acceptable fit was not obtained for any value of magnetic field strength and kinematical Doppler factor. Calculated y-ray flux was much lower than observed flux, which was similar to the 3C 279 spectrum (Makino et al. 1991). An external Compton model or a complicated multi-component model have to be employed to interpret multi-frequency spectra. A weak correlation between the index and the f! lux may suggest a are too uneven samplings flattening of the electron spectrum with increasing flux. Present observations Long term continuous monitoring in various wave in energy and time to derive quantitative conclusion. bands is desired.

3 3.1

X-RAY

VARIABILITY

Observational

IN HBLs

results

X-ray variability in HBLs is characterized by a correlated variation of the spectral index with the X-ray flux and the soft lag. The index-flux correlation was first observed from Mkn 421 (George, et al. 1988) by EXOSAT. Similar results were obtained from almost all the HBLs (Giommi et al. 1990, and Sambruna et al. 1994). The most pronounced spectral variations were observed in the high state of PKS 2155-304 (Morini et al. 1986, and Sembay et al. 1993). Soft lag was first observed from H 0323+022 by Ginga (Kohmura et al. 1994). Two kinds of soft lag which have been reported are peak of the light curve and bottom of the light curve (Takahashi et al. 1996). Looping of data points in flux-index plane would be another manifestation of the flux-index correlation and soft lag. More beautiful light curves were

X-Ray Variability in BL Lx 1.6

1.9

t

1.6

t

t

t

125

Objects

r

1.9

‘t

3 1.4 v C 5 1.3 ‘i f 1.2

t

g 0 C g 1.7 B f

t

1.6 1.1 1

-

0.2

0.1

0.3

--.-.A

X-ray

0.4

0.5

‘.,;

Flux in 3-7 keV ~(cps)

1 X-ray

2

3

4

5

6

Flux in 2-10 keV r(lE-11 erg /cm2/4

Figure 1: The correlation between photon index and X-ray flux of BL Lacertae 1998) and of 3C 279 (right) (Makino and Kii, 1996).

(left) (Tanihata

et al.

observed on 18 May 1994 by ASCA (M a k ino et al. 1996 and Urry et al. 1997), and these are shown in Fig.2. Not only there is a clear correlation in the decay phase of the light c! urve but also there is a slight spectral hardening in the rising phase. The soft lag can be seen clearly between the two light curves. We will discuss only the first major outburst in following sections.

3.2

Synchrotron

cooling

model

A smooth connection of the X-ray spectrum from lower frequency HBLs suggests that X-rays are produced by synchrotron radiation. electrons N,(E,t) is expressed by

region and characteristic variability of The spectrum of synchrotron emitting

and Q(E,t) are electron energy, time, a coefficient of energy loss, escape time, and where E,t,b,T(E) source function, respectively. A general solution of this equation is given by following formulae, for bEt 5 1 and bEt > 1 respectively.

Ne= (1

f

(E, t) _ bEt)2

1

r

N, = where f(E,t) The electron spectrum assumes each electron approximated by

f (E, t’) - t’+l _ bEt,)Zdt'

---) Neo( _;Et+ /,i Q&d

f C&t’) Q(E 1 - bEt’ ” - t’) (1 _ bEt,)2dt’

JbE 0

= ezp{-

lfT-‘(

1 _fEt,,)dt”}

cd

Neo(E) = NJE,O)

is converted to X-ray spectrum employing a monochromatic approximation, emit monochromatic X-ray by synchrotron radiation. The X-ray energy Ex = 5.223 x 10v5HE2

= 1.364 x lo-“Hy2

(keV),

which Ex is (5)

where H and y are magnetic field strength in Gauss, and Lorentz factor of electron, respectively and E in GeV. The model was applied to observed light curves shown in Fig.2. The light curves were calculated by using the following power law type injection spectrum. Q(E,t)

= IcE-~{~ - (t/r)“}

(0 5 t I T),

Q(E,t)

= 0 (T 5 t)

(6)

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F. Makino

The normalization k, spectral index cy, injection duration r, a parameter which expresses the decreasing rate of injection n, and synchrotron cooling time l/(bE) were determined by fitting the light curves. The results are shown in Fig.2 by solid curves superposed on the light curves. Obtained parameters are 7 = 5.4 x lo%, n = 10.9, (Y = 3.44, T(E) = consl. = 1.76 x 105s and electron cooling time=1.75 x 105s (at converted X-ray energy of 1.15 keV). Magnetic field strength obtained from electron cooling time is O.O58O-‘/3 gauss, where D is the kinematical Doppler factor.

3.3

Acceleration

of electrons

The calculated light curves did not reproduce very well the rising phase of the outburst as seen in Fig.3. The synchrotron cooling model produced spectral steepening in all ways, while the observed light curves showed hardening in rising phase. It is shown here that Fermi acceleration of the electrons will reproduce variability properties in the rising phase. We replace -bE2 in es.(l) by function g(E) which expresses energy loss and/or gain rate. A general solution for arbitrary functions g(E) and T(E) can be derived and by using this general solution, the following two well known spectra are obtained for constant injection of mono energetic electrons, Q(E, t) = T(E) has been assumed A6(E- Eo), where A, and 6(E) are a constant and a delta function, respectively. to be constant T. N, = A($-&-’

for g(E)

P - bE ,+1~~~-+-1

Ne= A(p-

= /3E

for g(E)

bEo

(7)

= ,!?E - bE2

If we use T, CY,and electron cooling time obtained in the decay phase, acceleration efficiency p is determined to be 2.32 x 10m6/s, which is smaller than the inverse of electron cooling time 5.22 x 10e6/s. In other wards, magnetic field in an accelerating region should be smaller than the radiating region obtained in the previous subsection. If we assume linearly increasing injection of low energy electrons of energy Eu into accelerating region, Q(E, t) = K6( E - Eo)t

K = const,

Ne = h’(g)-&-‘(t

and

g(E)

= /3E,

(9)

E - $jog_.)

(10)

is derived. If synchrotron cooling is not negligible, the power law in eq. (10) should be replaced by eq. (8). In this model, the flux increases linearly with time and spectrum flattens as time elapses. The spectral index of electrons is expressed by 1 1 d(log Ne) = ’ + @ + /3t log (E/Eo) o = - d(log E) This solution mimics the observed flux increase and the spectral hardening in the rise of the outburst. Furthermore, extrapolation of the electron spectrum eq.(lO) intersects the zero flux level at energy dependent time, to, which is given by to = 5og P

-5 Eo

This can also be seen qualitatively in the observed light curves in Fig.2. The linearly time of the burst in the hard band is delayed from that of the soft band.

(12) extrapolated

start

X-Ray Variability in BL Lac Objects

727

Figure 2: X-ray light curves of PKS 2155-304 in two energy bands and hardness. Solid curves are fitted light curves by synchrotron cooling model, which mainly describe declining part of the light curves. The rise of the outburst is qualitatively interpreted by Fermi acceleration model(see text).

4

DISCUSSION

X-rays of LBLs are produced by inverse Compton scattering of internal or external soft photons. Therefore, X-ray variability is controlled by the variability of high energy electrons and soft photons. It is not impossible to interpret X-ray spectrum of LBLs by SSC model, in which excess y-rays are attributed Correlated variation of X-ray flux with possible seed to other origins such as interaction of protons. approximately IR to optical has been observed from BL Lacertae and 3C 279. However, the origin of r-rays and its relation to X-ray production is still uncertain. However, strongly correlated flux variation between X-rays and GeV y-rays was observed from 3C 279, which may support external Compton origin (Wherle et al. 1998). Contrasting to LBLs, HBLs clearly showed an X-ray variability characteristic to synchrotron cooling. The declining part of the light curve can be reproduced by this model and the cooling time, escape time, and injection function were determined by fitting the light curve. Because all these time scales are similar to each other, this may suggest an important thing concerning the acceleration of electrons. The escape time was obtained precisely for the first time which makes it possible to derive an acceleration efficiency. The rise of the outburst was described by linearly increasing the injection of low energy electrons into an accelerating domain. The hard lag of the rise of the outburst is thought to be an evidence of the acceleration of electrons. Characteristic variability of X-rays is useful for the study of acceleration and Complicated variation of X-ray flux and the inverse cordeceleration of electrons in BL Lac objects. relation between flux and index observed occasio! nally can be interpreted by superposition of various outbursts. Soft lag was reproduced in the synchrotron cooling model, if injection of electrons was gradually terminated. In the Fermi acceleration model, the acceleration efficiency p has to decrease slowly at the end of the rise of the outburst to produce soft lag. The decrase of the acceleration efficiency /3 makes the spectrum soft. The softening of the spectrum started before the peak of the flux can also seen in Fig.3. Kirk et al. (1998) studied acceleration of particles in biazar by use of same formulae and obtained similar conclusion.

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