Monte-Carlo Applications for Partially Polarized Inverse External-Compton Scattering (MAPPIES) II -- Application to the UV/Soft X-ray Excess in Blazar Spectra
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Monte-Carlo Applications for Partially Polarized Inverse External-Compton Scattering (MAPPIES)II - Application to the UV/Soft X-ray Excess in Blazar Spectra
Lent´e Dreyer and Markus B¨ottcher Centre for Space Research, North-West University, Potchefstroom 2531, South Africa (Accepted January 27, 2021)
Submitted to ApJABSTRACTThe spectral energy distributions (SEDs) of some blazars exhibit an ultraviolet (UV) and/or soft X-ray excess, which can be modelled with different radiation mechanisms. Polarization measurements ofthe UV/X-ray emission from blazars may provide new and unique information about the astrophysicalenvironment of blazar jets, and could thus help to distinguish between different emission scenarios. Inthis paper, a new Monte-Carlo code – MAPPIES (Monte-Carlo Applications for Partially PolarizedInverse External-Compton Scattering) – for polarization-dependent Compton scattering is used tosimulate the polarization signatures in a model where the UV/soft X-ray excess arises from the bulkCompton process. Predictions of the expected polarization signatures of Compton emission from thesoft X-ray excess in the SED of AO 0235+164, and the UV excess in the SED of 3C 279 are made forupcoming and proposed polarimetry missions.
Keywords:
BL Lacertae objects: general – galaxies: active – galaxies: jets – gamma rays: galaxies– polarization – radiation mechanisms: non-thermal – relativistic processes – scattering –X-rays: galaxies INTRODUCTIONActive galactic nuclei (AGNs) are some of the mostluminous objects in the universe. About 10% of AGNsare observed to host relativistic jets, which are consid-ered to be powerful emitters of radiation across the en-tire electromagnetic spectrum. Blazars are an extremeclass of AGNs – consisting of BL Lac objects and flat-spectrum radio quasars (FSRQs) – for which the ob-server’s line of sight is closely aligned to the jet axis(Urry & Padovani 1995; Padovani et al. 2018). Vari-ous properties of the radiation from blazars have beenstudied with multi-wavelength observations and spectralfitting. While FSRQs have strong optical emission lines(which indicates the presence of accretion-disk radiationand a dense broad line region (BLR)), BL Lac objectstypically do not have a luminous accretion-disk or broadlines (Giommi et al. 2012; Dermer & Giebels 2016). Thespectral energy distributions (SEDs) of blazars are dom-inated by non-thermal emission, and generally consistof two distinct components; the low-frequency (radiothrough ultraviolet (UV) or X-ray) component, and the
Corresponding author: Lent´e [email protected] high-frequency (X-ray and γ -ray) component. The rela-tivistic jets contain ultra-relativistic electrons that pro-duce soft photons from radio frequencies up to UV/X-rays through synchrotron emission, and photons up tovery high energies (VHE; E ≥
100 GeV) via inverse-Compton (IC) processes (i.e. leptonic models). Alter-natively, high-energy emission can be produced by syn-chrotron radiation of pair cascades, powered by hadronicprocesses and synchrotron emission of ultra-high-energyprotons and muons (i.e. hadronic models). Both lep-tonic models and hadronic models are generally able toproduce acceptable fits to blazar SEDs (B¨ottcher et al.2013).Polarization is an important observable that can beused to constrain the morphology and geometry of theemitting region, and to distinguish between variousemission mechanisms. Polarization measurements of theradio and optical emission from blazar jets have been thekey to understanding many diverse aspects of blazar jets(see e.g. B¨ottcher (2019); Trippe (2019); Zhang (2019)for recent reviews). The radio and optical emission ofblazars have moderate polarization degrees (PDs) up to(3 − a r X i v : . [ a s t r o - ph . H E ] J a n Dreyer and B¨ottcher confirming the dominant radiation mechanism for theradio and optical emission from blazars.The polarization of the UV, X-ray and γ -ray emissionhas so far been largely unexplored, although its scientificpotential has long been appreciated (see e.g. Krawczyn-ski et al. (2011); Andersson et al. (2015); Zhang (2017);Mignani et al. (2019); Rani et al. (2019) for reviews).For instance, the synchrotron origin of the X-ray emis-sion from high-synchrotron-peaked (HSP) blazars maybe confirmed with X-ray polarimetry (e.g. Krawczynski(2012)). High-energy polarimetry may also be able todistinguish between leptonic and hadronic emission sce-narios for the origin of the high-frequency componentin blazar SEDs, since hadronic models typically pre-dict higher degrees of X-ray and γ -ray polarization thanleptonic models (Zhang & B¨ottcher 2013; Paliya et al.2018; Zhang et al. 2019a). In leptonic models, the high-energy emission can be partially polarized, dependingon the source of the seed photon field for Compton scat-tering. While external-Compton emission is expectedto be unpolarized, Compton scattering of the polarizedlow-frequency synchrotron emission (i.e. synchrotron-self-Compton; SSC) is expected to be polarized with apolarization degree (PD) that is about half of the polar-ization of the synchrotron emission (e.g. Chakrabortyet al. (2015)).In addition to the two characteristic-broad, non-thermal components described above, blazar SEDssometimes exhibit an infrared (IR) bump, optical/UVbump (called the Big Blue Bump (BBB)), and/or softX-ray excess (e.g. Masnou et al. (1992); Grandi et al.(1997); Haardt et al. (1998); Pian et al. (1999); Rai-teri et al. (2005); Palma et al. (2011); Ackermann et al.(2012); Paliya et al. (2015); Pal et al. (2020); see An-tonucci (2002); Perlman et al. (2008) for reviews). Var-ious radiation mechanisms have been proposed for theorigin of the UV/soft X-ray excess in the SEDs, whichinclude the following: • Thermal emission from the accretion-disk (e.g.Pian et al. (1999); Blaes et al. (2001); Paliya et al.(2015); Pal et al. (2020)). • A higher than galactic dust-to-gas ratio to thesource, resulting in an over-estimation of theneutral-hydrogen column density and, therefore,an over-correction for the corresponding photo-electric absorption at low X-ray energies (in thiscase, the excess would not actually be physical;e.g. Ravasio et al. (2003)). • A distinct synchrotron component from a differ-ent region than the low-frequency component in amulti-zone construction (e.g. Paltani et al. (1998);Ostorero et al. (2004); Raiteri et al. (2005)). • Synchrotron emission from VHE γ -ray inducedpair cascades in blazar environments (Rous-tazadeh & B¨ottcher 2012). • Reduced radiative cooling of the highest-energyelectrons in a Compton-dominated blazar, dueto the Klein-Nishina suppression of the Comptoncross section (e.g. Ravasio et al. (2003); Moderskiet al. (2005)). • A signature of IC scattering of external radiationfields by a thermal, non-relativistic population ofelectrons (i.e. the bulk Compton effect; e.g. Sikoraet al. (1994, 1997); B(cid:32)la˙zejowski et al. (2000); Ack-ermann et al. (2012); Baring et al. (2017)).Polarization measurements of the UV and soft X-rayemission may yield new and unique information aboutthese spectral features in the SEDs, thus distinguish-ing between different emission scenarios. Given thesepromising prospects, numerous polarimetry missions an-ticipating to deliver polarization measurements of theUV/X-ray emission from astrophysical sources (includ-ing blazars) are at various stages of planning, design,and operation. It is, therefore, important to considerpredictions of the polarization signatures for differentemission scenarios that may be able to explain the ori-gin of the UV/soft X-ray excess in blazar spectra.In this paper, a new Monte-Carlo code – MAPPIES(Monte-Carlo Applications for Partially Polarized In-verse External-Compton Scattering; Dreyer & B¨ottcher(2020)) – is used to simulate the polarization signaturesin a model where the UV/soft X-ray excess arises dueto Compton scattering of external fields by thermal elec-trons contained in the blazar jet, as proposed by Baringet al. (2017) for the BL Lac object AO 0235+164. Twoblazar case studies are presented: The BL Lac objectAO 0235+164, and the FSRQ 3C 279. An overview ofthe soft X-ray excess in the SED of AO 0235+164 andthe BBB in the SED of 3C 279 is given in Section 2and Section 3, respectively. The model setup and pa-rameters considered for the simulations are described inSection 4, followed by the results in Section 5, and theconclusions in Section 6. THE SOFT X-RAY EXCESS IN THE SED OFAO 0235+164The BL Lac object AO 0235+164 (redshift z = 0 . APPIES II - Application to the UV/soft X-ray excess in Blazar Spectra ( ν/ Hz) ∼ . − .
1. While thermal emissionfrom the disk might be able to explain the bump, Raiteriet al. (2006) proposed that synchrotron emission from aregion that is closer to the black hole than where the low-frequency component originates, could be another pos-sible mechanism. Additionally, Ostorero et al. (2004)showed that the soft X-ray excess can be obtained inthe ambit of the rotating helical jet model (e.g. Villata& Raiteri (1999)) by admitting a synchrotron contribu-tion to the X-ray radiation. A soft X-ray excess withinthe frequency range log ( ν/ Hz) ∼ . − . Swift
X-ray Telescope (
Swift -XRT; Bur-rows et al. (2005)) and the Rossi X-ray Timing Explorer(RXTE; Rothschild et al. (1998)) presented a soft X-rayexcess in the SED. Since the X-ray spectrum was toosoft to be attributed to the SSC component, IC scatter-ing off cold electrons in the jet was considered to be apossibility.Baring et al. (2017) employed the multi-wavelengthobservations of Ackermann et al. (2012) and modelledthe soft X-ray excess as a bulk Compton component thatresults from an external radiation field scattering off athermal population of shock-heated electrons containedin the blazar jet (see Figure 1). Monte-Carlo simula-tions of the diffusive shock acceleration (DSA) processby Summerlin & Baring (2012) were coupled with theradiation transfer modules of B¨ottcher et al. (2013), andthe fit of the soft X-ray feature through the bulk Comp-ton process aided in fixing the thermal-to-non-thermalparticle ratio in the jet, thus tightly constraining theparticle diffusion parameters in the DSA process. Aqualitative prediction for this scenario is that the ther-mal Comptonization process should lead to significantpolarization in the soft X-ray spectral component. Inwhat follows, the model of Baring et al. (2017) will beconsidered in order to test this prediction for the softX-ray excess in the SED of AO 0235+164, using theMAPPIES code. THE BBB IN THE SED OF 3C 279The FSRQ 3C 279 (redshift z = 0 . γ -ray sources in the sky, and the first blazarto be detected by the Energetic Gamma Ray Experi-ment Telescope (EGRET; Kanbach et al. (1989); Hart-man et al. (1992)). Multi-wavelength variability andradio/optical polarimetry suggest that the broadbandspectrum of 3C 279 at radio to UV frequencies is pro-duced by synchrotron radiation (e.g. Maraschi et al. Figure 1.
The multi-wavelength spectrum (points) span-ning the radio, optical, X-ray, and γ -ray bands, togetherwith model fits from Baring et al. (2017), for the 2008 Oc-tober high-state Fermi-LAT observation of AO 0235+164(data from Ackermann et al. (2012)). The blue butterfly block represents Swift-XRT data. The broad-band modelconsists of a synchrotron component (dashed green curve)up to the optical band, a two order SSC contribution inthe optical, X-rays and γ -rays (dashed orange curve), andexternal Compton (EC) emission, including a bulk Comp-ton feature (dotted black curve) between log ( ν/ Hz) ∼ . ( ν/ Hz) ∼ .
4. The orange curve is the total modelspectrum (which includes a very small correction for γγ ab-sorption by the extra-galactic background light). Taken fromBaring et al. (2017). (1994); Hartman et al. (1996)). The UV-optical con-tinuum usually has a steep power-law spectrum, due toradiation losses of the relativistic electrons in the jet,revealing its non-thermal origin. The source was moni-tored by Pian et al. (1999) with the International Ultra-violet Explorer (IUE; Nichols & Linsky (1996)) – com-bining the UV data with observations from the ROent-gen SATellite (ROSAT;Truemper (1993)) and EGRET– during its low state, thus allowing the detection of anexcess in the UV regime at log ( ν/ Hz) ∼ . γ -ray flare observedfrom 3C 279 was presented by Paliya et al. (2015), andthe modeling of the low-activity state showed a slight turnover at log ( ν/ Hz) ∼ . − . Dreyer and B¨ottcher
Figure 2.
The SED of 3C 279 during low-activity states. Si-multaneous data from the Small and Moderate Aperture Re-search Telescope System (SMARTS), Swift XRT, and Fermi-LAT are shown with red circles. The green and orangedashed curves correspond to synchrotron and SSC emis-sion, respectively. The orange solid curve is the sum of allthe radiative mechanisms (the thermal contributions fromthe torus, accretion-disk, and X-ray corona, as well as theexternal-Compton disk, BLR, and the dusty torus compo-nents are not shown in the figure). From Paliya et al. (2015). to unpolarized accretion-disk emission towards the UV,will result in a decrease of the PD throughout the op-tical - UV regime. Additionally, the radiation emittedby electron-scattering–dominated accretion-disks is ex-pected to be considerably polarized, perpendicular tothe disk axis, with a strong angle dependence (up to aPD ∼ .
7% for an edge-on disk; e.g. Chandrasekhar(1960)). However, assuming that the jet of a blazarpropagates along the symmetry axis of the accretiondisk, the high-energy emission region in the jet will havea perfect face-on view of the disk, in which case thereis no net polarization in the accretion-disk emission dueto the azimuthal symmetry (Smith et al. 2004).Synchrotron emission from VHE γ -ray induced paircascades in blazar environments, was suggested by Rous-tazadeh & B¨ottcher (2012) as an alternative contribu-tion to the BBB. This cascade emission may peak inthe UV/soft X-ray range for sufficiently strong magneticfields, and can resemble the BBB in the UV regime. Theexternal radiation field was modelled as isotropic blackbody radiation with a temperature of kT rad ∼ . γ -ray induced pair cascades can reproduce theBBB in 3C 279, peaking at log ( ν/ Hz) ∼ . − . Table 1.
The model parameters for the BL Lac object AO0235+164.
Parameter description
ValueRedshift of the source, z . jet . d L . × cmTemperature of the dusty torus, kT rad . U DT . · cm − Effective size of the dusty torus R eff a × cm a See main text for the definition of the effective size
Note —The model parameters correspond to those of Bar-ing et al. (2017). The seed photon field is assumed to beIR emission from the dusty torus, and the electron en-ergy distribution is drawn from the electron fit spectrumof Baring et al. (2017) in order to simulated the IC scat-tering off thermal shock-heated electrons in the jet of theblazar. 4.
MODEL SETUPThe MAPPIES code is a newly developed Monte-Carlo code for polarization-dependent Compton scat-tering of external fields in jet dominated astrophysicalsources (Dreyer & B¨ottcher 2020). An external radia-tion field (originating in the AGN rest frame) scatters offan arbitrary (thermal and non-thermal) electron distri-bution, assumed to be isotropic in the co-moving frameof the emission region that moves along the jet witha bulk Lorentz factor Γ jet . The polarization signaturesare calculated using the Stokes formalism (Stokes 1851),and the polarization-dependent Compton scattering ofthe seed photons are calculated following Monte-Carlomethods by Matt et al. (1996). The code is used to sim-ulate the polarization signatures in a model where theUV/Soft X-ray excess in the SEDs of blazars is due tobulk Compton emission. In this section, we describe themodel setup for the two case studies of AO 0235+164and 3C 279. In particular, a description of the seedphoton fields and electron energy distributions is given.4.1.
The seed photon fields
The primary seed photon fields for IC models of AGNscan either be external emission from the BLR and/ordusty torus (see e.g. Sikora et al. (1994); B(cid:32)la˙zejowskiet al. (2000); Ghisellini & Tavecchio (2008)), or directaccretion-disk emission (see e.g. Dermer & Schlickeiser(1993); B¨ottcher et al. (1997); Dermer & Schlickeiser(2002)). The MAPPIES code, therefore, draws the seedphotons from either an isotropic, single-temperature
APPIES II - Application to the UV/soft X-ray excess in Blazar Spectra Table 2.
The model parameters for the FSRQ 3C 279.
Parameter description
ValueRedshift of the source, z . jet . d L . × cmBlack hole mass, M BH × M (cid:12) Inner radius of the accretion-disk, R inAD R G Outer radius of the accretion-disk, R outAD R G Accretion disk luminosity, L AD × erg · s − Height of the emission region abovethe central black hole, h . × cm Note —The parameters correspond to that of Paliya et al.(2015) for 3C 279 in low-state. The seed photons are assumedto come directly from the disk, and the electron energy dis-tribution is drawn from the electron fit spectrum of Baringet al. (2017) in order to simulated the IC scattering off ther-mal shock-heated electrons in the jet of the blazar.In the table above, M (cid:12) ∼ . × g is the solar mass, and R G = GM BH /c is the gravitational radius of the black hole(with G ∼ . × − cm · g − · s − the gravitational constantand c ∼ × cm · s − the speed of light). black body distribution (corresponding to external emis-sion from the BLR and/or dusty torus), or from a multi-temperature accretion-disk spectrum. In the first case,the radiation energy density U AGNDT appears uniform inthe AGN rest frame, provided that the emission region islocated inside the characteristic scale of the BLR/dustytorus. The angular distribution in the AGN rest framevaries over angular scales ∆ θ (cid:29) Γ − so that the angu-lar distribution in the co-moving frame is dominated byrelativistic aberration rather than intrinsic anisotropy(B¨ottcher et al. 2013). The radiation energy density U DT is thus boosted to a highly anisotropic field in theemission frame with U DT ∼ Γ U AGNDT .Figure 3 shows the seed photon spectra of AO0235+164 (orange dotted curve) and 3C 279 (blue dot-ted curve). The seed photon field for AO 0235+164is assumed to be IR emission from the dusty torus( kT rad ∼ . L DT = (4 πR c ) × U DT , where R eff is an effectivesize. The dusty torus is expected to have a character-istic radius of R DT ∼ a few pc. However, due to thetorus-like geometry, its effectively IR emitting surface(as seen from a distant observer) is significantly smallerthan 4 πR . We therefore introduce an effective size ofthe dusty torus, R eff ∼ × cm, which is expectedto be about an order of magnitude smaller than R DT . Figure 3.
The total SEDs for AO 0235+164 (solid or-ange curve) and 3C 279 (solid blue curve). The seed pho-ton spectra are determined by the model parameters, andare assumed to be IR emission from the dusty torus for AO0235+164 (dotted orange curve) and direct disk emission for3C 279 (dotted orange curve). Compton scattering off ther-mal electrons in the jet (as shown in Figure 4) results in softX-ray radiation for AO 0235+164 (dashed orange curve) andUV radiation for 3C 279 (dashed blue curve).
The contribution of direct accretion-disk emission to theradiation density in the emission-region rest frame, isobtained as U AD ∼ . × − erg · cm − from Equa-tion 9 of Ghisellini & Madau (1996) for the combinationof free parameters considered by Baring et al. (2017),and is negligible compared to that of isotropic IR emis-sion from the dusty torus, where U DT ∼ Γ U AGNDT =0 . · cm − (with U AGNDT ∼ × − erg · cm − andΓ jet = 35). The emission region is assumed to be muchcloser to a more luminous accretion-disk for 3C 279 thanin the case of AO 0235+164. The seed photons for 3C279 are thus drawn from a multi-temperature accretion-disk spectrum (dotted blue curve), with a disk lumi-nosity of L AD = 1 . × erg · s − and a disk radius R AD = R outAD − R inAD ∼ . × cm (where R outAD and R inAD are the outer and inner radius of the disk, respec-tively). 4.2. The electron energy distribution
Generally, the effects of Compton scattering dependon the electron energy distributions. DSA at relativis-tic shocks is thought to be an important accelerationmechanism in blazar jets, which may produce the non-thermal particles that emit the broad-band continuumdetected from the jets. Baring et al. (2017) employedthe results from Monte-Carlo simulations of DSA at rel-ativistic shocks by Summerlin & Baring (2012), in or-der to model the particle acceleration at blazar shocks.The simulations captured the connection between thethermal component and the power-law tail of the non-
Dreyer and B¨ottcher thermal electrons in the blazar jet. The soft X-ray ex-cess of AO 0235+164, modelled as IC scattering off thethermal, non-relativistic shock heated electrons, tightlyconstrained the energy dependence of the diffusion coef-ficients for the electrons. For more details on the modelparameters and the electron energy distribution result-ing in the SED fit, see Baring et al. (2017).
Figure 4.
The thermal and non-thermal electron distribu-tion (with Lorentz factors of γ ; drawn from the electron fitspectrum of Baring et al. (2017)) as a function of the dimen-sionless electron momentum γβ . In the interpretation of the soft X-ray excess as a bulkCompton signature, a measurable degree of polarizationin the frequency range in which the bulk Compton radia-tion dominates is expected, as it results from anisotropicCompton scattering off thermal, non-relativistic elec-trons. The MAPPIES code allows the quantificationof this prediction and thereby, in comparison with fu-ture X-ray polarimetry data, the quantification of thepresence of a pool of thermal electrons in the emissionregion of AO 0235+164. The electron energy distribu-tion is thus drawn from the electron spectrum used inthe SED fit by Baring et al. (2017), shown in Figure 4.The thermal-to-nonthermal particle ratio (comparableto that in the case of AO 0235+164) may result fromshock acceleration of electrons in 3C 279 as well. There-fore, as an illustrative test case, we use the same elec-tron distribution for 3C 279 as AO 0235+164 in orderto study the potential bulk Comptonization effects.4.3.
Polarized-dependent Compton scattering
A variety of physical phenomena may alter the po-larization state of the observed radiation, including theinfluence of the magnetic fields, general relativity, andthe emission mechanisms. Synchrotron radiation of rel-ativistic charged particles in ordered magnetic fieldsis expected to be both linearly and circularly polar-ized (Westfold 1959; Rybicki & Lightman 1979), while Compton scattering off relativistic electrons will reducethe degree of polarization to about half of the seed pho-ton field’s polarization (Bonometto et al. 1970). Forboth blazar case studies, the seed photons are assumedto be unpolarized. Due to the polarization dependenceof the Klein-Nishina cross section, polarized photonsscatter preferentially in a direction perpendicular totheir electrical field vector, and the electric field vec-tors of the scattered photons tend to align with the seedphotons’ electric field (Matt et al. 1996). Polarizationcan therefore be induced when non-relativistic electronsscatter an anisotropic photon field, even if the seed pho-tons are unpolarized.The polarization signatures of the scattered photonsare obtained by summing the photons’ contributionto the Stokes parameters in a specified direction afterthe simulation is complete. Circular polarization maybe generated either as an intrinsic component of Syn-chrotron radiation (i.e. if the seed photons are circu-lar polarized Synchrotron emission) or via Faraday con-version of linear polarization into circular polarizationdriven by some internal Faraday rotation (see e.g. War-dle et al. (1998); Homan et al. (2009); MacDonald &Marscher (2018); Boehm et al. (2019)). Since the seedphoton fields are assumed to be unpolarized externalradiation, and high-energy polarization will not be af-fected by Faraday rotation due to the λ (where λ isthe wavelength) dependence of this effect, we only con-sider the results of linear polarization. The results ofcircular polarization are unessential for making predic-tions of the expected polarization signatures for upcom-ing polarimetry missions, since all existing or proposedhigh-energy polarimeters only measure linear polariza-tion. Due to the photon-counting nature of the X-rayand γ -ray observatories (and thus, polarimeters), to ourknowledge, there is fundamentally no way to measurecircular polarization in X-rays or γ -rays. RESULTS AND DISCUSSIONIn this section, the polarization signatures of IC scat-tering for the two blazar case studies will be discussed.The results of Compton scattering off a thermal pop-ulation of shock-heated electrons contained in the jetof the blazar (as shown in Figure 4), are given for AO0235+164 and 3C 279 for the combination of free pa-rameters listed in Table 1 and Table 2, respectively.5.1.
The blazar SEDs
Figure 3 shows the SEDs of AO 0235+164 (orange)and 3C 279 (blue). The seed photon fields are shown indotted lines (determined by the model parameters; seeSection 4.1), the IC spectra are shown in dashed lines,and the total (seed + IC) SEDs are shown in solid lines.For AO 0235+164, the seed photon field is assumed to beIR emission from the dusty torus (dotted orange curve).The photon frequency increases by a factor of ∼ γ Γ APPIES II - Application to the UV/soft X-ray excess in Blazar Spectra jet = 35, and γ ∼
1) due to Compton scat-tering off thermal electrons, which results in soft X-rayradiation (dashed orange curve). The soft X-ray spec-trum peaks at log ( ν/ Hz) ∼ .
3, in agreement withthe detection of the soft X-ray excess in the SED ofAO 0235+164 by Ackermann et al. (2012), and the re-sults from Baring et al. (2017) (as shown in Figure 1).For 3C 279, essentially all the seed photons (direct diskemission; dotted blue curve) enter the emission regionfrom behind, since ( R AD /h ) ∼ . (cid:28) Γ − , causingthe photons to receive a negative Doppler boost into theemission region frame. Compton scattering of the directdisk emission results, therefore, in UV radiation (dashedblue curve) with frequencies log ( ν/ Hz) ∼ . − . ( ν/ Hz) ∼
15, which is consistent withthe detection of the UV excess by Pian et al. (1999) andPaliya et al. (2015), as well as the reconstructed BBB ofRoustazadeh & B¨ottcher (2012).5.2.
Compton polarization
The Compton polarization signatures are shown asa function of the photon frequency (in the observer’sframe) for viewing angles of Θ
AGN ∼ Γ − rad in Figure5. The Compton emission from AO 0235+164 (shown inorange) exhibits a significant PD (top panel) within thefrequency range log ( ν/ Hz) ∼ . − . ∼
48% at a frequencyof log ( ν/ Hz) ∼ .
4, and a PD of ∼
30% at the peakof the soft X-ray component (at log ( ν/ Hz) ∼ . X -ray Polarization Probe (XPP; Marshall et al. (2019)),the Enhanced X -ray Timing and Polarimetry Mission(eXTP; Zhang et al. (2019b)), the Imaging X -ray Po-larimetry Explorer (IXPE; Deininger et al. (2020)), andthe X -ray Polarimeter Experiment (POLIX; Paul et al.(2016)). The predicted PDs are listed in Table 3, andare expected to decrease with increasing frequency for allthe polarimeters considered except POLLUX, for whichthe PD is expected to increase with increasing frequency.For 3C 279, the Compton polarization (shown in blue)is detectable in the frequency range of log ( ν/ Hz) ∼ . − . ( ν/ Hz) ∼ . − .
5, with expected PDs of (18 − ∼
23% at the peak of theBBB (at log ( ν/ Hz) ∼ Figure 5.
The PD (top panel) and PA (bottom panel) as afunction of the photon frequency for AO 0235+164 (orange)and 3C 279 (blue). The dotted lines indicate negligible po-larization, and the frequency range in which other (electron-synchrotron) radiation components may dominate over theUV/soft X-ray components. The dashed lines indicate thepolarization of the Compton emission, and the polarizationof the total observed spectrum is shown with solid lines.
Figure 6.
The PD (top panel) and PA (bottom panel) as afunction of the scattered photon viewing angle (in the AGN-rest frame) for AO 0235+164 (orange) and 3C 279 (blue).The dotted lines indicate the viewing angle where the maxi-mum polarization occurs.
The Compton polarization signatures are shown as afunction of the scattered photon viewing angle Θ
AGN inFigure 6. In the emission region rest frame, most of the
Dreyer and B¨ottcher
Table 3.
Predictions of the expected PDs for future proposed polarimetry missions ina model where the UV/soft X-ray excess in the SEDs of AO 0235+164 and 3C 279 isdue to a bulk Compton feature.
Blazar Case Study Polarimeter Frequency range [log ( ν/ Hz)] PD [%]POLLUX 14 . − . − . AO 0235+164
REDSoX 16 . − . − . − . (cid:46) . − . (cid:46) . − . (cid:46) . − . (cid:46)
3C 279
POLLUX 14 . − . − Note —The PDs in the table above are estimated from the results shown in the toppanel of Figure 5, where the results for AO 0235+164 are shown in orange, and theresults for 3C 279 are shown in blue. The frequency range listed in the table refers tothe range where polarization could be detectable for the corresponding polarimetrymissions. seed photons move in the negative jet direction. Themaximum polarization, therefore, occurs at the rightangle, Θ
AGN ∼ ( π/
2) rad, in the electron rest frame,which is essentially the same in the emission region restframe. Boosting to the AGN rest frame, the maxi-mum PD occurs at Θ
AGN ∼ Γ − rad, with Γ jet = 35for AO 0235+164 (indicated with a orange dotted line)and Γ jet = 10 for 3C 279 (indicated with a blue dottedline). The polarization angle (PA; shown in the bottompanels of Figures 5 and 6) of the polarized fraction ofthe Compton emission assumes a constant value of PA= ( π/
2) rad for both blazar case studies, which refers topolarization perpendicular to the jet axis. SUMMARY AND CONCLUSIONIn this paper, the MAPPIES code is used to simulateIC scattering off a thermal population of shock-heatedelectrons contained in the blazar jet. Predictions of thepolarization signatures, in a model where the UV/softX-ray excess in blazar spectra is due to bulk Comp-tonization of external radiation fields, as proposed byBaring et al. (2017), are made for AO 0235+164 and3C 279. Compton scattering of external IR emissionfrom the dusty torus results in soft X-ray radiation forAO 0235+164, and Compton scattering of UV emissionfrom the accretion-disk results in UV radiation for 3C279 (as shown in Figure 3). The Compton X-ray spec-trum of AO 0235+164 agrees with the results of Baringet al. (2017), and the UV spectrum of 3C 279 is consis-tent with the BBB detected by e.g. Pian et al. (1999);Paliya et al. (2015) in the low state. Therefore, whilean isotropic IR radiation field (in the AGN rest frame) is required to reproduce the soft X-ray excess as bulkCompton emission in the SED of AO 0235+164, directdisk emission likely dominates the seed radiation fieldfor 3C 279, with the emission region closer to a moreluminous disk compared to that of AO 0235+164.The thermal Comptonization process involved in thebulk Compton feature leads to significant polarizationwithin the UV/soft X-ray excess in the SEDs of bothblazar case studies (PD (cid:46)
48% for AO 0235+164 andPD (cid:46)
23% for 3C 279; as shown in Figure 5). The max-imum PD occurs at viewing angles of Θ
AGN ∼ Γ − rad(shown in Figure 6), and the PA for the polarized frac-tion of the Compton emission assumes a constant valueof PA= ( π/