Is the High-Energy Emission from Centaurus A. Compton-Scattered Jet Radiation? E. O. Hulburt Center for Space Research ABSTRACT

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1 I the High-Energy Emiion from Centauru A Compton-Scattered Jet Radiation? J. G. Skibo 1, C. D. Dermer and R. L. Kinzer E. O. Hulburt Center for Space Reearch Naval Reearch Laboratory Code 7650, Wahington, DC ABSTRACT We conider whether the hard X-ray and oft gamma-ray emiion from Centauru A i beamed radiation from the active nucleu which i Compton-cattered into our lineof-ight. We derive the pectrum and degree of polarization of cattered radiation when incident beamed radiation i cattered from a cold (kt << m e c 2 ) electron cloud moving with bulk relativitic motion along the jet axi, and calculate reult for an unpolarized, highly-beamed incident power-law photon ource. The pectra of the cattered radiation exhibit a cut o at gamma-ray energie due to electron recoil. The cut o energy depend on the oberver' viewing angle and the bulk Lorentz factor of the cattering medium. We t the OSSE data from Centauru A with thi model and nd that if the catterer are not moving relativitically, then the angle the jet make with repect to our line-of-ight i We predict a high degree of polarization of the cattered radiation below 300 kev. Future meaurement with X-ray and gamma-ray polarimeter could be ued to contrain or rule out uch a cenario. Subject heading: galaxie: individual galaxie: active galaxie: jet gamma ray: theory radiation mechanim: nonthermal polarization 1 NRC/NRL Reident Reearch Aociate

2 1. INTRODUCTION Centauru A (NGC 5128), at a ditance of 3-5 Mpc (z=0.0008), i the brightet extragalactic ource at photon energie 100 kev and one of the nearet active galaxie (ee Ebneter & Balick 1983 for a review). The radio emiion from Cen A diplay a twin-jet tructure with radio luminoity erg 1, uggeting that Cen A i a Fanaro-Riley Cla 1 (FR1) radio galaxy (Fanaro & Riley 1974). If FR1 galaxie are the parent population of BL Lac object, a propoed in one verion (Browne 1989; Padovani & Urry 1990) of the AGN unication cenario (ee Antonucci 1993), Cen A would be claied a a BL Lac object if it jet were coincident with our line-of-ight. Evidence for jet emiion in Cen A come from obervation of a one-ided X-ray jet near the nucleu (Feigelon et al. 1981) and, more recently, from obervation (Morganti et al. 1991, 1992) of emiion line lament aligned along the direction of the X-ray jet and coincident with the radio jet out to ditance kpc from the nucleu. From meaurement of [OIII] line emiion, Morganti et al. conclude that the ux of the ionizing radiation in the 2-6 kev band i 200 time greater in the jet direction than along our line-of-ight, conitent with the hypothei that the emiion i intrinically beamed rather than collimated by obcuring material. Cen A ha recently been oberved at hard X-ray and gamma-ray energie with the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Obervatory (CGRO; ee Johnon et al. 1993, Kinzer et al. 1994). Thi intrument operate in the energy range 50 kev < < 10 MeV. The pectra oberved from Cen A are much harder than typical Seyfert pectra (Maiack et al. 1993; Johnon et al. 1994), but Cen A ha not been detected at > 100 MeV energie (R. C. Hartman, private communication, 1993), a range where many blazar how trong gamma-ray emiion (Hartman et al. 1992; Fichtel et al. 1993). In fact, Cen A i only weakly detected with the Compton Telecope on CGRO in the MeV energy band by combining data from 3 oberving period (Collmar et al. 1993). A broken power-law give an acceptable t to the OSSE Cen A data 1

3 (Kinzer et al. 1994), but provide no phyical bai for the pectral oftening in the hard X-ray/oft gamma-ray regime. In thi Letter, we examine whether the high-energy emiion from Cen A i cattered jet radiation, ince Compton cattering of beamed radiation can produce cut o in photon pectra (Dermer 1993) due to the kinematic recoil of electron. The precie value of the oberved cut o energy i a function of the cattering angle and bulk motion of the electron. Thu an incident power-law beam of photon will, after cattering, retain it power-law form in the Thompon regime ( m e c 2 in the ret frame of the electron), but will be cut o above a certain energy in the Klein-Nihina regime ( m e c 2 ). In addition, the cattered radiation can be highly polarized, depending on the cattering angle and initial photon polarization. 2. ANALYSIS Conider the Compton cattering of an unpolarized beam of gamma ray photon by a cold (kt m e c 2 ) cloud moving relativitically along the axi of the beam, which we take to be the poitive z-axi. Let dn (; ) repreent the rate that photon are emitted dtdd from the central ource (photon 1 r 1 MeV 1 ). In the ingle cattering approximation ( 1) the photon ux (photon 1 cm 2 MeV 1 ) cattered into the direction 0 with repect to the z-axi and received by an oberver located at ditance r from the cattering cloud i given by the expreion ( ; 0 ) = 1 r 2 [(1 co 0 )] 2 I d N e ( ) j d d j d C d ( ; ) dn 0 dt d d ( ; ): (1) Here Ne ( ) i the electron column denity in the direction () and d C d ( ; ) i the 0 dierential Compton cro ection (cm 2 ter 1 ) for the cattering of a photon with initial direction and energy through an angle into the direction 0 with energy. The tarred ymbol repreent quantitie in the ret frame of the cattering cloud which i moving with velocity c ( = 1=p 1 2 ) along the z-axi in the tationary frame. 2

4 For a photon of energy and direction (; ), the relevant tranformation are =, co = (co )=(1 co ), and = (1 co ). In equation (1) the factor 2 (1 co 0 ) 2 relate the cattered emiion in the ret frame of the cloud to the emiion received by the oberver (cf. eq. [4.97b] of Rybicki & Lightman 1979). The cattered photon energy i given by the well-known formula = 1 + m e c 2 (1 co ) ; (2) where co = co co 0 +co in in 0. It can be een upon inpection of equation (2) that the recoil of the electron impoe a kinematical cut o in the emergent photon pectrum at an energy in the ret frame of the catterer given by c = m ec 2 : (3) 1 co In thi frame the cut o i only a function of the cattering angle. However, in the tationary frame, thi cut o occur at an energy given by c = m e c 2 (1 + )(1 co ) : (4) Hence, the cut o in the pectrum i both a function of the cattering angle and the bulk Lorentz factor of the cattering cloud. We conider two cae for the incident photon number intenity: (1) a mono-directional power-law beam along the poitive z-axi, given by dn 1 1 (; ) = dtdd 2 k 1 (co 1); (5) and (2) a power-law photon ource which i iotropic in a frame moving relativitically along the poitive z-axi with velocity 0 c. The emiion in the tationary frame in cae 2 i beamed by virtue of relativitic motion and we have dn 2 k (; ) = dtdd 2 2+ : (6) [ 0 (1 0 co )] 3

5 Synchrotron-elf Compton model (e.g., Bloom & Marcher 1993) and model involving the Compton cattering of external photon (Dermer & Schlickeier 1993; Sikora, Begelman, & Ree 1994) both produce beaming pattern characterized by equation (8). The contant k 1 and k 2 in equation (5) and (6) are eaily normalized to the total jet luminoity or photon number. In the ret frame of the cattering cloud, the expreion for the received radiation are dn1 dt d d ( ; ) = k 1 (co 1) 2 [(1 + co )] ; (7) dn 2 dt d d ( ; ) = f 0 [1 + ( 0 ) co 0 ]g (2+) k 2 : (8) The cattered photon ux i obtained by ubtituting equation (7) or (8) into equation (1) and uing the expreion d C d ( ; ) = r in 2 (9) where r 0 = e 2 =m e c 2 = 2: cm 2 i the claical electron radiu (e.g. Rybicki & Lightman 1979). For the mono-directional power law beam (cae 1) we have 1 ( ; 0 ) = N er 2 0 k 1 (1 ) 2r 2 2 (1 co 0 ) 2 + in 2 0 ; (10) where i obtained by inverting equation (2) with = 0. For cae 2 the cattered ux i 2 ( ; 0 ) = N e r 2 0 k 2 2r 2 2 (1 co 0 ) 2 I d + in 2 (11): f 0 [1 + ( 0 ) co 0 ]g (2+) ; In deriving equation (11), we have aumed that the cattering region ha contant electron column denity N e and i much larger than the angular extent of the beam. Thi reult i correct in a regime where the light-travel time through the cloud i mall in comparion with the time cale of variation of the central ource. A relativitically correct formalim of the time-dependent ytem i in preparation by the author. 4

6 In Figure 1, we plot the cattered photon pectra multiplied by 2 for variou cattering angle and bulk Lorentz factor. The pectral index of the incident photon pectrum i et equal to 1.6, cloe to value meaured for Cen A at hard X-ray energie (e.g., Baity et al. 1981). The dotted curve repreent the ux obtained uing equation (10) (cae 1) and the olid curve were obtained uing equation (11) (cae 2). In cae 2, we plot the quantity 2m e c 2 r 2 2 =N e r0 2 k 2 and take 0 = 10. The normalization of the ux in cae 1 wa adjuted to make the low-energy uxe for cae 1 and cae 2 coincide. Figure 1. Scattered photon ux multiplied by 2 a a function of energy for variou cattering angle. The dierent panel correpond to bulk Lorentz factor of the cattering cloud of = 1; 3. The dotted (olid) curve repreent cae 1 (cae 2) a decribed in the text. 5

7 For cae 1, the mono-directional beam, the high-energy cut o c are very harp due to the kinematic contraint expreed by equation (6). At c, the pectra are power law with pectral indice equal to. Slight pectral oftening are apparent at < c. For cae 2, the cut o are le harp than in cae 1 due to the angular extent of the incident beam. Thi eect i enhanced for larger bulk Lorentz factor of the cattering cloud. For = 0, the incident beam become iotropic (ee eq. [8]). In general, the cattered radiation will be partially polarized with the direction of the electric eld vector perpendicular to the jet axi. The degree of polarization a a function of cattering angle i given by the expreion (e.g. McMater 1961): ( ; ) = p Q2 + U 2 + V 2 I = in 2 + in 2 ; (12) where I, Q, U and V are the Stoke' parameter (e.g. Rybicki & Lightman 1979) of the cattered radiation. Equation (12) i valid for a mono-directional beam. To obtain the degree of polarization of a Compton cattered beam in cae 2 it i neceary to average thi expreion in the tarred ytem over the angular extent of the incident beam weighted by the dierential cattering rate dn dt d d, which i proportional to the integrand in equation (11). Thi follow from the additivity of the Stoke' parameter and the fact that U = V = 0 for Compton cattering of an unpolarized beam by unpolarized electron. In addition, the degree of polarization i a Lorentz invariant quantity in the ene that ( ; ) = ( ; ). Hence, we obtain the expreion h( ; 0 )i = h ( ; 0)i = H d dn dt d ( d ; ) H d dn : (13) dt d d In Figure 2, the degree of polarization i plotted a a function of energy for variou cattering angle and bulk Lorentz factor. Here, a in Figure 1, we et = 1:6 and 0 = 10. For a tationary cattering cloud (panel a), the degree of polarization i greatet for Thomon cattering ( m e c 2 ) through 90. The maximum polarization for a cloud 6

8 with bulk relativitic motion i found in the direction co =, which follow from the invariance of the degree of polarization. Figure 2. The degree of polarization of the cattered radiation a a function of energy for the variou cattering angle. The dierent panel correpond to bulk Lorentz factor of the cattering cloud of = 1; 3. The dotted (olid) curve repreent cae 1 (cae 2) a decribed in the text. The dahed curve in panel a i the predicted degree of polarization for Cen A. The nite angular extent of the beam in cae 2 ha the eect of increaing the degree of polarization relative to the mono-directional cae in the Klein-Nihina regime. Thi i more pronounced for larger bulk Lorentz factor of the cattering cloud. The bump jut below the high-energy cut o in panel (b) i the reult of the increaingly limited domain 7

9 of the angular integration with increaing photon energy impoed by the Klein-Nihina cro ection in equation (13). 3. RESULTS AND DISCUSSION We conider the cae where the cattering cloud i at ret ( = 1, 1), and t the pectrum given by equation (10) for a mono-directional beam to the data obtained with OSSE for Cen A during October 1991 (Kinzer et al. 1994). Thi i done by folding the model photon ux through the full OSSE repone matrix and conducting the 2 earch in count pace. There are three free parameter in thi t: the overall normalization, N e r 2 0 k 1=2r 2 ; the pectral index of the incident beam, ; and the cattering angle, 0. The reult are hown in Fig. 3 where the curve correpond to the bet t with a reduced 2 of for 158 degree of freedom, giving a probability of to obtain thi much or more catter in the data. The parameter for thi t are = 1:68 0:03, 0 = 61 5 and N e r 2 0 k 1=2r 2 = (9:8 1:3) 10 4 (photon 1 cm 2 MeV 1 ). The polarization of the gamma ray emiion i hown a a function of energy for thi inclination angle by the dahed curve in panel (a) of Figure 2. The emiion i approximately 60% polarized for < 300 kev, but the polarization fall rapidly to zero above thi energy. Thu we nd that if the cattering cloud i at ret, the jet in Cen A i directed at an angle of 61 5 with repect to our viewing direction. Studie of the ditribution of HII region in NGC 5128 imply that the direction of our line-of-ight i oriented by 73 3 (Graham 1979) or 72 2 (Dufour et al. 1979) with repect to the normal to the plane of the dik of the HII region and, preumably, Cen A' galaxy. If one can aume that the jet axi i perpendicular to thi plane, then the reult of thee tudie are within 2 of our inferred angle. If the cloud i moving relativitically, a might be uggeted by time variability in the OSSE Cen A data on time cale < 0.5 day (Kinzer et al. 1994), then the deduced angle could be much dierent. For example, if = 2, then a imilar analyi produce a t with an inclination angle 35. Nonrelativitic catterer could be provided, however, by material conning the jet, high-energy particle decelerated to 8

10 nonrelativitic energie by Compton drag or, in the two-ow model (e.g., Sol, Pelletier, & Aeo 1989), by a background electron-proton plama which upport a pair jet. We now how that if the X-ray and gamma-ray ux from Cen A i cattered jet radiation, then the meaured ux i conitent with optical determination of the ource energetic and the aumption that the cattering cloud i optically thin. Morganti et al. (1991), from their analyi of the [OIII] line emiion, conclude that the intenity of ionizing radiation in the jet i at leat photon 1 r 1, and i 200 time more intene in the jet direction than along our line-of-ight. Uing the pectrum derived from their model and equation (5), we obtain k 1 = b photon 1 MeV 0:68, where b i the angular extent of the ionizing beam and r = 3:7 Mpc, the value ued by Morganti et al. (1991). From the normalization in the t we obtain an electron column denity of N e = b cm 2. Thi correpond to a Thompon optical depth T = 0:07 1 b, in agreement with a imple etimate obtained by letting the ratio of cattered ux to the ux of the primary jet radiation T b =4 1=200. Hence, the optically-thin approximation i adequate if the opening angle of the beam i > 10, and we ee that the oberved > 1 kev X-ray ux from Cen A could be entirely cattered jet radiation. Throughout thi analyi we aumed that the polarization of the incident beam wa negligible along the jet axi, a uggeted by ome model where the jet emiion i upcatterd dik radiation (e.g., Begelman & Sikora 1987). The polarization propertie of blazar jet emiion (e.g., Mrk 421) can be directly meaured to improve thi aumption, but we note that except for pecialized cattering geometrie, the degree of polarization of the cattered radiation i, in general, greater than that determined auming an unpolarized incident beam. Hence the degree of polarization we obtain from an initially unpolarized beam probably give a lower limit. The preence of accretion dik emiion from a Seyfert nucleu in Cen A could, however, lower the degree of polarization, but the average pectrum of Seyfert galaxie i very oft at > 100 kev (Johnon et al. 1994). 9

11 Figure 3. The gamma ray emiion from Cen A a detected by OSSE during October The curve repreent the bet t Compton cattered pectrum for an ideal beam (Cae 1) cattered by a cold tationary ( = 1) electron cloud. 4. CONCLUSIONS We have examined whether the hard X-ray and oft gamma-ray emiion oberved from Cen A could be jet radiation cattered into our line-of-ight by an electron cattering cloud. Model for jet formation in AGN (e.g., Begelman, Blandford, & Ree 1984; Dermer & Schlickeier 1992) invoke the exitence of outowing plama, o thi proce hould operate at ome level. Making the implet aumption that the cattering cloud i at ret, we obtain a good t to the OSSE data for Cen A during the 1991 obervation. Moreover, we ee that gamma-ray obervation provide a new method for the determination of the angle between the jet and viewing direction which doe not involve aumption regarding the orientation of the jet and the plane of the galaxy. Thi could be crucially important to the interpretation of radio obervation of uperluminal ource, where the peed of the 10

12 jet are inferred indirectly ince thi angle i not known. Unfortunately, thi method i not without it problem: it can only be applied to the brightet ource with the preent generation of gamma-ray telecope. Moreover, the cattering ga may not be cold and could have ome bulk relativitic motion which, a we have een, coniderably change the inferred jet angle. There i, furthermore, no guarantee that the oberved radiation i not direct jet radiation nor central ource emiion. Polarization meaurement in the hard X-ray/oft gamma-ray regime would be extremely important in aeing whether the emiion from Cen A i direct or reproceed radiation, ince the polarization propertie of direct blazar jet emiion and cattered radiation hould be very dierent. We encourage hard X-ray and gamma-ray polarization meaurement to tet thee idea. Acknowledgement: We thank Jim Kurfe for comment on the manucript, and Kellie McNaron-Brown for aitance with the IGORE oftware. REFERENCES Antonucci, R. 1993, ARA&A, 31, 473 Baity, W. A. et al. 1981, ApJ, 244, 429 Begelman, M. C., Blandford, R. D. & Ree, M. J. 1984, Rev. Mod. Phy., 56, 1984 Begelman, M. C. & Sikora, M. 1987, ApJ, 322, 650 Bloom, S. D. & Marcher, A. P. 1993, in The Compton Gamma-Ray Obervatory, ed. M. Friedlander, N. Gehrel & D. Macomb (New York: AIP), 578 Browne, I. W. A. 1989, in BL Lac Object, ed. L. Marachi, T. Maccacaro & M.-H. Ulrich (Springer-Verlag), 401 Collmar, W. et al. 1993, in The Compton Gamma-Ray Obervatory, ed. M. Friedlander, N. Gehrel & D. Macomb (New York: AIP), 483 Dermer, C. D. 1993, in Proceeding of the Fifth International Workhop on Neutrino Telecope, ed. M. Baldo Ceolin, p. 427 Dermer, C. D. & Schlickeier, R. 1992, Science, 257, 1642 Dermer, C. D. & Schlickeier, R. 1993, ApJ, 416,

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