See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/231081610 Simultaneous Swift and REM monitoring of the blazar PKS 0537-441 in 2005 ARTICLE in THE ASTROPHYSICAL JOURNAL · DECEMBER 2008 Impact Factor: 5.99 · DOI: 10.1086/518469 · Source: arXiv CITATIONS 14 READS 35 27 AUTHORS, INCLUDING: Gianpiero Tagliaferri National Institute of Astrophysics 939 PUBLICATIONS 14,375 CITATIONS SEE PROFILE S. D. Vergani National Institute of Astrophysics 149 PUBLICATIONS 1,395 CITATIONS SEE PROFILE Emilio Molinari National Institute of Astrophysics 251 PUBLICATIONS 1,933 CITATIONS SEE PROFILE Lucio Angelo Antonelli National Institute of Astrophysics 747 PUBLICATIONS 8,914 CITATIONS SEE PROFILE Available from: Fabrizio Vitali Retrieved on: 05 February 2016
SimultaneousSwift and REM monitoring of the blazar PKS 0537–441 in2005
E. Pian1, P. Romano2,3, A. Treves4, G. Ghisellini2, S. Covino2, A. Cucchiara5, A. Dolcini4,G. Tagliaferri2, C. Markwardt6, S. Campana2, G. Chincarini2,3, N. Gehrels6, P. Giommi7,
L. Maraschi8, S.D. Vergani9,10, F.M. Zerbi2, E. Molinari2, V. Testa11, G. Tosti12, F. Vitali11,L.A. Antonelli11, P. Conconi2, G. Malaspina2, L. Nicastro13, E. Palazzi13, E.J.A. Meurs9,
L. Norci10
ABSTRACTThe blazar PKS 0537–441 has been observed with all instruments of theSwift satellite between the
end of 2004 and November 2005. The BAT monitored it recurrently for a total of 2.7 Ms, and the XRTand UVOT pointed it on seven occasions for a total of 67 ks, making it one of the AGNs best monitored bySwift. The automatic optical and near-infrared telescope REM hasmonitored simultaneously the sourceat all times. In January-February 2005 PKS 0537–441 has beendetected at its brightest in optical andX-rays: more than a factor of 2 brighter in X-rays and about a factor 60 brighter in the optical thanobserved in December 2004. The July 2005 observation recorded a fainter X-ray state, albeit still brighterthan the historical average. The simultaneous optical state, monitored by bothSwift UVOT and REM,is high, and in the VRI bands it is comparable to what was recorded in early January 2005, before theoutburst. In November 2005, the source subsided both in X-rays and optical to a quiescent state, havingdecreased by factors of∼4 and∼60 with respect to the January-February 2005 outburst, respectively. Ourmonitoring shows an overall well correlated optical and X-ray decay, with no measurable time lag largerthan about 1 month. On the shorter time scales (days or hours), there is no obvious correlation betweenX-ray and optical variations, but the former tend to be more pronounced, opposite to what is observedon monthly time scales. The widely different amplitude of the long term variability in optical and X-raysis very unusual and makes this observation a unique case study for blazar activity. The spectral energydistributions are interpreted in terms of the synchrotron and inverse Compton mechanisms within a jetwhere the plasma radiates via internal shocks and the dissipation depends on the distance of the emittingregion from the central engine.
Multiwavelength variability is the most effective di-agnostic tool of the properties of extragalactic jets andof their central engines. Due to the orientation of theirjets - nearly aligned to our line of sight - blazars al-low a better insight into their inner regions than otherradio loud Active Galactic Nuclei (AGN) do, becausethe intrinsic flux variability is magnified by relativis-tic effects. Past observations of blazars have identifiedthe active emission mechanisms (synchrotron processat frequencies up to the soft, and occasionally hard,X-rays and inverse Compton scattering at higher ener-gies, Ulrich, Maraschi & Urry 1997; Pian et al. 1998;Tagliaferri et al. 2003; Krawczynski et al. 2004; Der-mer & Atoyan 2004; Błazejowski et al. 2005; Sokolov& Marscher 2005; Aharonian et al. 2006; Albert etal. 2006; Kato, Kusunose & Takahara 2006; Massaroet al. 2006; Raiteri et al. 2006). Yet, the structure ofthe jet, the mechanisms of the energy transfer from thecentral engine to the emitting particles, and the dissi-pation processes along the jet are not clear. Intensivemonitorings and good coverage at all frequencies arenecessary to explore the multiwavelength variability ofblazars to its full extent and to understand how the jetinteracts with other circumnuclear components to pro-duce the radiation. TheSwift satellite (Gehrels et al.2004), with its easy and flexible scheduling, can be op-timally employed for the observation of bright blazars(Giommi et al. 2006; Sambruna et al. 2006; Tramacereet al. 2006).
The blazar PKS 0537–441 (z = 0.896) is a brightemitter at all frequencies from radio to gamma-rays.In the latter band it has been observed many timesby EGRET and detected in different states (Treves etal. 1993; Hartman et al. 1999; Pian et al. 2002).The source was targeted for long term optical and NIRmonitoring with the automatic optical/near-IR 60cmtelescope Rapid Eye Mount (REM, Zerbi et al. 2001;Chincarini et al. 2003; Covino et al. 2004) in Decem-ber 2004 - March 2005, when the blazar exhibited aflare with a time scale of about a month (Dolcini et al.2005). At that time,Swift had observed PKS 0537–441 for calibration purposes. On 25 June 2005, theRXTE All Sky Monitor recorded a high X-ray (2-10keV) state of the source, with a flux of (13± 5) milli-Crab. The REM monitoring in the optical also revealedthe blazar to be active (Covino et al. 2005). Based onthese alerts, we requested observations of PKS 0537–441 as a Target of Opportunity withSwift. A first ob-
servation was scheduled in July 2005. A secondSwiftvisit took place in November 2005, in order to moni-tor the long-term behavior of the source after the June2005 outburst. We report here the results of allSwiftobservations of PKS 0537–441, and of the simultane-ous REM observations in July and November 2005.
2. Data acquisition, reduction and analysis
2.1. X-ray observations
2.1.1. Swift/BAT
PKS 0537–441 was often in the field of view of theSwift Burst Alert Telescope (BAT, 15–150 keV) fromDecember 2004 to November 2005. The BAT datawere analysed using the standard BAT analysis soft-ware distributed within FTOOLS v6.0.5. Although theblazar is not detected in individualSwift orbits by BAT,averaging the BAT signal during all periods of obser-vation results in significant flux detection. A spec-trum of the integrated data set was extracted and fit-ted to a single power-lawFν ∝ ν
−β with spectral indexβ = 0.5 ± 0.5 (reducedχ2 = 1.03). The flux in the15-150 keV band is (3.2+0.9
−2.3) × 10−11 erg cm−2 s−1.
2.1.2. Swift/XRT
The blazar was observed with the X-Ray Telescope(XRT, 0.2-10 keV, Burrows et al. 2005) in Decem-ber 2004-February 2005, July 2005, and November2005. The monitoring is organized in seven obser-vations, four of which were obtained during theSwiftXRT calibration phase. The XRT data were first pro-cessed by theSwift Data Center at NASA/GSFC intoLevel 1 products (calibrated and quality-flagged eventlists). Then they were further processed with the latestHeasoft release1 (v6.0.5) to produce the final cleanedevent lists. In particular, we ran the taskxrtpipeline(v0.10.3) applying standard filtering and screening cri-teria, i.e., we cut out temporal intervals during whichthe CCD temperature was higher than−47 ◦C, and weremoved hot and flickering pixels which are presentbecause the CCD is operating at a temperature higherthan the design temperature of−100 ◦C due to a fail-ure in the active cooling system. An on-board eventthreshold of∼0.2 keV was also applied to the centralpixel, which has been proven to reduce most of thebackground due to either the bright Earth limb or the
CCD dark current (which depends on the CCD tem-perature). Given the low rate of PKS 0537–441 dur-ing the observing campaign (< 0.5 counts s−1 in the0.2−10 keV range), we only considered photon count-ing data for our analysis (PC; see Hill et al. 2004 fora full description of read-out modes) and further se-lected XRT grades 0–12, (according toSwift nomen-clature; Burrows et al. 2005). A summary of the XRTobservations is reported in Table 1.
PKS 0537–441 was detected in the XRT data at thecoordinates RA(J2000)= 05h38m50.s38, Dec(J2000)=−44◦05′09.′′1, with an estimated uncertainty of 3.′′5arcseconds radius (90% containment). This positiontakes into account the correction for the misalignmentbetween the telescope and the satellite optical axis(Moretti et al. 2006), and is consistent with the sourcecatalog position (ICRS coordinates are RA(J2000)=
05h38m50.s36, Dec(J2000)= −44◦05′08.′′94).
We extracted the source events in a circle with a ra-dius of 30 pixels (∼ 71′′), which corresponds to∼ 94%of the XRT PSF. To account for the background, thedata were also extracted within an annular region (radii55 and 95 pixels) centered on the source and devoid ofbackground sources.
The source and background spectra were extractedin the regions described above. Ancillary responsefiles were generated with the taskxrtmkarf, and ac-count for differences in extraction regions and PSFcorrections. We used the latest spectral redistributionmatrices (RMF, v008). The adopted energy range forspectral fitting is 0.3–10 keV, and all data were re-binned with a minimum of 20 counts per energy binto allowχ2 fitting within XSPEC (v11.3.2). The onlyexception was the observation of 23 December 2004,when the number of counts was limited (∼ 140) andCash (1979) statistics was appropriate, therefore un-grouped data were used instead.
The spectra do not exhibit significant features, ei-ther in absorption or emission, superimposed on thepower-law continuum (see a representative spectrumin Figure 1). We considered an absorbed power-lawmodel, with the neutral hydrogen column kept fixedto its Galactic value (2.91× 1020 cm−2, Murphy et al.1996), and the spectral index left as a free parameter.The fit results are reported in Table 1.
2.2. Optical observations
The Swift UltraViolet-Optical Telescope (UVOT,Roming et al. 2005) observed PKS 0537–441 in July
(only U,B and V filters) and November 2005 (all fil-ters) simultaneously with the XRT. The log of the ob-servations is reported in Table 2. UVOT data weretaken also in December 2004 and January 2005, butdue to early orbit checkout and calibration, they arenot reliable, and therefore we have not used them. Thedata analysis was performed using the “uvotsource”task included in the latestHeasoft software (see Sec-tion 2.1.2). We subtracted the background, and cor-rected for the coincidence loss effect (similar to thepileup for the XRT) in the case of a bright source. Themagnitudes were converted into fluxes using the latestin-flight flux calibration factors and zero-points.
REM acquired photometry of the AGN from De-cember 2004 to March 2005, and also in July and inNovember 2005, with various combinations of filters.The data reduction followed standard procedures (seeDolcini et al. 2005). The log of the July and Novem-ber 2005 observations is reported in Table 2, while thedetails of the previous REM observations have beenpresented in Dolcini et al. (2005).
3. Results
3.1. Multiwavelength light curves
With a totalSwift/XRT exposure of 67 ks, PKS 0537–441 is one of the blazars best monitored by this instru-ment. Figure 2 shows the observed (i.e., not correctedfor Galactic absorption), background-subtracted lightcurves extracted in the 0.2–1 keV and 1-10 keV en-ergy bands. For direct comparison with the X-rays,the merged UVOT and REM light curve in the V band(covered by both instruments) is also shown in Figure2. The UVOT V-band fluxes have been reduced to thecentral wavelength of the REM V-band observationsusing the power-lawFν ∝ ν
−β which best fits the opti-cal spectrum in July (β = 1.84± 0.04) and November2005 (β = 1.26± 0.05). The full transformation equa-tion is FUVOT,5505Å = (5505/5460)β × F0,UVOT−V ×
10−0.4VUVOT , whereF0,UVOT−V is the flux correspond-ing to zero UVOT V magnitude (equal to 3.19× 10−9
and 3.17 × 10−9 erg s−1 cm−2 Å−1 for the July andNovember 2005 measurements, respectively). TheREM and UVOT V-band data taken closest in time (12July 2005) differ by∼13%, the REM flux being lowerthan the UVOT flux. This difference may be intrinsic,since the REM and UVOT observations are about 6-7hours apart, however, it is within the sum of the sta-tistical uncertainties (see Table 2) and the systematicerrors due to flux transformation and calibration of the
3
two instruments (estimated to be no less than∼5%altogether).
The optical V-band and X-ray light curves arehighly correlated, however, the V-band flux varieswith much higher amplitude. To the initial factor of60 optical variation detected with REM between endof December 2004 and early February 2005 – notedand discussed in Dolcini et al. (2005) – corresponds avariation of only a factor∼2 of both soft (0.2-1 keV)and hard (1-10 keV) X-ray flux (Fig. 2). Thereafter,the flux decays nearly monotonically up to November2005 both in optical and X-rays, with overall ampli-tudes of factors of∼60 and∼4, respectively. Thevariability indices of the X-ray light curves, defined asthe ratios between the flux standard deviation aroundthe mean flux and the mean flux itself (σ/ < f >), are0.375 and 0.423 for the hard and soft X-rays, respec-tively, consistently lower than the optical variabilityindex, 1.434 (the variability indices have been com-puted from the original datasets, i.e. before apply-ing the temporal binning adopted in Figure 2). Thetime behavior of the hardness ratio between the bands0.2-1 keV and 1-10 keV shows no clear long termtrend: the spectrum hardens up to summer 2005 andsoftens thereafter, but only with marginal significance(Fig. 3a).
In Figure 4 portions of the light curves are reportedin smaller time intervals. The X-ray flux presents analmost fully resolved flare on 27-28 January 2005 withsomewhat higher total amplitude in the soft than in thehard band (factors of∼2 and∼1.5, respectively, seeFig. 3b and Fig. 4a). The correlated optical and X-ray behavior on short time scales (days to hours) hasno precise character: while the limited simultaneousX-ray and optical sampling in July 2005 shows a wellcorrelated decay in the 2 bands, with the X-ray fluxdeclining faster than the optical flux (Fig. 4b), the fac-tor of 2 X-ray variations in November 2005 have nocounterpart in UV-optical, where flickering of at most∼10% is observed (Fig. 4c,d).
Obviously, the better long term sampling availableat optical wavelengths favours the detection of daytime scale variations in optical with respect to the X-rays. This is relevant when attemping to determine apossible time lag between the optical and X-ray lightcurves. While we can constrain the occurrence of theoptical maximum (formally observed on 5 February2005) within the time window 3-12 February 2005,the X-ray light curve maximum is much less well con-strained. Fig. 4a indicates that the X-ray observed
maximum occurred between 27 and 28 January 2005.If this is the absolute peak of the X-ray light curve, andit is correlated with that in the optical, then it has pre-ceded the optical maximum by at least one week in theobserver frame. However, given the sampling of theX-ray light curve, we cannot exclude more intense andunobserved flares preceding or following the observedX-ray maximum by time intervals of up to∼1 month.Therefore, this is our upper limit on the time lag of thecorrelation between the X-ray and optical light curves.
3.2. Broad-band spectrum
In Figure 5 we report the broad-band spectral en-ergy distributions of PKS 0537–441 at three epochsduring ourSwift and REM campaign, representativeof three different emission states: 24-25 February, 12July and 24 November 2005 for the bright, intermedi-ate and low state, respectively. The spectral energy dis-tribution of 24 November 2005 has been selected be-cause theSwift/UVOT observations made on that daycover the near-UV wavelengths (1930-2600 Å), unlikethose of 17 November, that are limited to the UBV fil-ters (Table 2). However, no strictly simultaneous REMdata are available on 24 November. The REM data of20 and 30 November 2005 have been used instead, andinterpolated at the date of 24 November. We excludethat possible variability between 20 and 30 Novem-ber 2005 may significantly affect the reliability of theREM fluxes obtained through interpolation: no largevariability is observed in this period (see Table 2);the UVOT and REM V-band points are consistent (seeFig. 5); we have verified that the shape of the near-IR-to-near-UV spectrum of 24 November 2005 is similarto that of 17 November 2005 (constructed with data si-multaneous within 1 day), in the common wavelengthrange (3400–16000 Å).
Whenever more than one UVOT or REM measure-ment is available at a given date and filter, we take theflux average. The associated error is the standard de-viation when three or more data points are averaged.When only two measurements are available, the erroris the larger of the two individual errors, or the fluxdifference, whichever is larger. The X-ray data arecorrected for photoelectric absorption by the Galac-tic neutral hydrogen as described in Section 2.1.2,and the near-infrared to ultraviolet data are correctedfor Galactic dust absorption withE(B − V) = 0.037(Schlegel, Finkbeiner, & Davis 1998), using the ex-tinction law of Cardelli, Clayton, & Mathis (1989).For comparison, we have reported also the historical
4
multiwavelength spectra obtained in 1991-1992 and1995 (see Pian et al. 2002) and the non-simultaneousIRAS, ISO, HST and BeppoSAX data taken at variousepochs (Padovani et al. 2006; Pian et al. 2002).
The 2005 optical spectra, spanning a factor of∼50in normalization, bracket the historical optical obser-vations. They are described by single power-laws andare steeper at higher states. The near-IR flux varieswith lower amplitude. At the lowest state of November2005, we note a large discrepancy between the opticaland infrared fluxes: the H-band flux exceeds by a fac-tor of ∼4 the extrapolation of the optical spectrum tothe H-band wavelengths. The flatness of the Novemberoptical-UV spectrum and the spectral discontinuity be-tween the optical and near-IR wavebands suggest thatin the lower states different emission components playa role in shaping the spectrum. In particular, radiationproduced by the accretion disk may partially accountfor the optical-UV spectrum. This behavior is reminis-cent of that seen in 3C 279 (Pian et al. 1999).
The X-ray fluxes detected by XRT encompass boththe BeppoSAX and ROSAT states. The steadiness ofthe XRT spectral slope over time, as opposed to a fac-tor 4 variation in the normalization, is remarkable. TheX-ray spectral shape is also very similar to that of theBeppoSAX spectrum, which covers an energy rangesimilar to that covered by XRT.
The BAT spectrum is dominated by the episodesof more intense activity of January-February and July2005, and despite its large uncertainty gives a goodestimate of the spectral shape at hard X-rays in highstate.
4. Discussion
The character of the multiwavelength variability de-tected by our monitoring is extremely unusual: the out-burst of PKS 0537–441, jointly monitored by XRT andREM from its rise in December 2004 - January 2005to its long decay ended in November 2005, has a re-markably higher amplitude at optical (factor of∼60)than at X-ray frequencies (factor of∼4). These ob-viously represent only lower limits to the variability,the intrinsic amplitude of which may be contaminatedby a constant component that is more relevant in X-rays than in the optical. The optical spectra suggestthe presence of an underlying thermal optical compo-nent in low state (Section 3.2), presumably only mod-estly variable. However, trying to assess whether - andhow significantly - this dilutes the intrinsic multiwave-
length variability of the non-thermal flux is prone tomany uncertainties. The sampling of our monitoringindicates that the long–term decay is monotonic (Fig.2), but small flares are present on day time scales (Fig.4).
The fact that the optical–UV flux variability has amuch larger amplitude than the simultaneous X–rayflux variability, may at first sight be surprising. In asimple synchrotron self–Compton scenario, where theoptical emission is due to synchrotron radiation andthe X-rays are due to inverse Compton scattering off
the synchrotron photons, one would expect to observethe opposite if the changing parameter is the densityof the emitting particles. In fact, the self–Comptonemissivity scales with the square of the particle den-sity, while the synchrotron emissivity varies linearlywith it. If instead the varying parameter is the mag-netic field, we expect that both the synchrotron and thesynchrotron self-Compton fluxes vary with the sameamplitude.
On the other hand, in models producing the highenergy emission by upscattering of radiation producedoutside the jet (”external” Compton) both the syn-chrotron and the inverse Compton fluxes vary linearlywith the particle density. In these models, a variationof the magnetic field could produce a variation of thesynchrotron flux leaving almost unchanged the inverseCompton flux.
Note also that the X-ray and optical spectra in thesemodels derive from very different portions of the rela-tivistic electron distribution: the optical emission orig-inates from electrons above the spectral break, whilethe X-rays are produced via inverse Compton scatter-ing of synchrotron or external photons by electrons ofmuch lower energies.
We applied a simple, one–zone, homogeneous syn-chrotron self-Compton plus external inverse Comptonmodel to the different states of PKS 0537–441. Themodel is described in Ghisellini, Celotti & Costamante(2002). The general assumptions are the following:
• The source is a cylinder of cross sectional radiusR = ψz, wherez is the distance from the apex ofthe jet, assumed to be a cone of semi-apertureangleψ. The width∆R′, as measured in the co-moving frame, is assumed to be equal toR;
• the magnetic fieldB is homogeneous and tan-gled;
• the blob moves with a bulk Lorentz factorΓ and
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the viewing angle isθ (we assumed hereθ = 3◦);
• the external radiation is produced at a fixed ra-dius, which can be identified with the radius ofthe broad line region. We assume that 10% ofthe disk luminosity is reprocessed by the broadline region. These assumptions should be takenwith care, because, beside the contribution ofthe broad line region, other processes can con-tribute to the external radiation (scattering byionized intercloud plasma, synchrotron radia-tion “mirrored” by the clouds and/or the wallsof the jet, reprocessing by a molecular torus;Sikora, Begelman, & Rees 1994; Błazejowski etal. 2000; Ghisellini & Madau 1996; and finally,direct radiation from the accretion disk, Dermer& Schlickeiser 1993; see also Celotti, Ghisellini& Fabian 2007);
• the particle distribution is the result of injectionand cooling. We calculate the random Lorentzfactor γcool at which the particles cool in onelight crossing time. If the particles are injectedbetweenγinj andγmax with a power law distri-bution of slopes, in the “fast cooling” regime(γcool < γinj), we have an emitting particle distri-butionN(γ) ∝ γ−p betweenγinj andγmax (wherep = s + 1 is the injection slope increased byone unit), andN(γ) ∝ γ−2 betweenγcool andγinj
(Ghisellini et al. 2002; Ghisellini et al. 1998;Ghisellini 1989). In the models presented here,this is always the case, since the adopted param-eters (see Table 3) imply that the radiative cool-ing (synchrotron, synchrotron self-Compton andexternal Compton) is fast, and guarantee thatγcool is always smaller thanγin j and close tounity (after a light crossing time).
Based on the above assumptions, the modellingof the spectral energy distribution yields the resultsshown in Fig. 5, where we report model curvesfor three states of the source during our 2005 cam-paign. We have also modeled under similar assump-tions previous multiwavelength energy distributionsof PKS 0537–441, presented in Pian et al. (2002),including data in the MeV–GeV domain from CGRO-EGRET (Fig. 6). The radiation processes at workare synchrotron at radio-to-UV frequencies, and in-verse Compton scattering off both synchrotron pho-tons (self-Compton) and external photons, dominat-ing at X-ray and gamma-ray frequencies, respectively.
The input parameters used for these models are listedin the upper part of Table 3. In the lower part we re-port some interesting output parameters, namely theDoppler factorδ and the power carried by the jet inthe form of magnetic field (LB), cold protons (Lp), rel-ativistic electrons (Le), and produced radiation (Lrad),defined as:
LB = πR2Γ2c UB
Lp = πR2Γ2c∫
N(γ)mpc2dγ
Le = πR2Γ2c∫
N(γ)γmec2dγ
Lrad = πR2Γ2c U ′rad (1)
whereUB andU ′rad are the magnetic and the radiationenergy density measured in the comoving frame, re-spectively.
Usually, when applying a single–zone synchrotronand inverse Compton model, the choice of the inputparameters is not unique, especially when the peak ofthe inverse Compton component is not observed, as inour case. However, we were guided in our choice bythe knowledge of the total luminosity of the broad lines(∼ 5×1044 erg s−1, Pian, Falomo, & Treves 2005), andthe requirement that the spectra observed here in thelow optical–UV state are unusually flat because theyare “contaminated” by the thermal accretion disk com-ponent. We can then infer the luminosity of this com-ponent (which we have fixed to 1.8 × 1046 erg s−1).Note also that in the low state of November 2005 theflux in theH filter suggests that the synchrotron spec-trum on these occasions is very steep. The other re-quirements we have applied, which help us in choos-ing the input parameters, include minimizing the totalpower budget and describing the different states of thesource with a minimal change of the power carried bythe jet. The latter point is crucial, because it allows adirect test once high energy observations – as will beperformed by GLAST – will be available. In fact itis possible to obtain reasonably good fits by allowingthe jet power to vary by a large amount from state tostate, being larger in high states. This would howevercorrespond to very different spectra and fluxes in theMeV–GeV band.
Remarkably, the chosen parameters correspondclosely to the expectations of the jet radiation modelproposed by Katarzynski & Ghisellini (2007): dra-matic variations in specific frequency bands can beproduced by relativistic jets carrying the same amount
6
of energy in bulk relativistic and Poynting flux form.In fact, if blobs having the same bulk kinetic energyΓMc2 (whereM is the mass of the blob) dissipate afraction of their kinetic energy at different locationsalong the jet, and if the distance of the emitting re-gion from the origin of the jet is directly related toΓ,then slow blobs will dissipate closer to the jet apex,when the blob is more compact, and embedded in alarger magnetic field. In this case the external Comp-ton scattering is reduced because the external radiationenergy density as seen in the comoving frame (∝ Γ2) islower, while the synchrotron radiation (∝ B2) is likelyto be enhanced, since we expect larger values of themagnetic field closer to the apex of the jet.
In this picture the key ingredient is the link be-tween the dissipation site and the bulk Lorentz factor atthat location: smaller Lorentz factors are required forsmaller distances between the jet apex and the dissipa-tion site. There are two scenarios: the blob could bestill accelerating when it dissipates, or else the dissipa-tion is the result of internal shocks. In Katarzynski &Ghisellini (2007) the second scenario is adopted, be-cause it provides the scalings needed to characterizecompletely the model. In this scenario (see its spe-cific application to blazars by Ghisellini 1999, Spadaet al. 2001; Guetta et al. 2004), faster blobs can catchup with slower ones at a distancez = Γ2∆z0 from thejet origin, wherez0 is the initial separation of the twoblobs, and∆ is their thickness.
Furthermore, in the Katarzynski & Ghisellini(2007) model, it is assumed that the blobs always carrythe same amount of bulk kinetic energy (ΓMc2 is thesame) and magnetic energy (ΓB2V ′ is the same, whereB andV ′ are the magnetic field and volume measuredin the comoving frame of the source, respectively). Asa result of the dissipation process, the fraction of theavailable energy transferred to the emitting electronsis the same (i.e., the efficiency is the same). Based onthese assumptions, we can assign for all the input pa-rameters their scalings withΓ: therefore, when fittingtwo or more different states of the same source withthis model, once we have chosen the parameters forone state we are left with only one free parameter, i.e.the bulk Lorentz factorΓ. We can relax this by allow-ing the particle distribution slopes to be changed, aswell as the fraction of the electron population whichis accelerated to relativistic energies. This does notviolate any strong requirement.
In Fig. 7 it is shown how the choice of the presentinput parameters compares with the Katarzynski &
−3. We also show (bottom panel)that the power carried by the jet is almost constant.The good agreement leads us to conclude that the vari-ations seen in this source are probably due to (small)variations in the bulk Lorentz factor, which inducesdissipation to occur at different locations along the jet.Consequently, the emitting regions have different radii,particle densities and magnetic fields. Note that vari-ations ofΓ between 10 and 15 can explain the entireobserved variability. In this respect, PKS 0537–441is very similar to 3C 454.3 during its large 2005 mul-tiwavelength flare (Fuhrmann et al. 2006; Pian et al.2006; Giommi et al. 2006). For that source, the 100–fold amplitude of the optical variability could be ac-counted for by changingΓ by a factor of less than 2(Katarzynski & Ghisellini 2007).
The knowledge of the spectrum of PKS 0537–441at MeV-GeV energies is crucial for fully constrainingthe models. Therefore, PKS 0537–441 qualifies as aprime candidate for further monitoring withSwift andfor simultaneous observations with INTEGRAL, AG-ILE and GLAST.
We thank P. Roming and S. Holland for assistancewith the UVOT data, C. Pagani for help with theSwiftscheduling, and the wholeSwift team for their supportof the mission. We acknowledge use of the quick-look results provided by theRXTE All Sky Moni-tor team. This research has made use of the Simbaddatabase. This work is supported at OABr by ASIgrant I/R/039/04 and at OATs by the contracts ASI IN-TEGRAL I/R/046/04 and ASI-INAF I/023/05/0.
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This 2-column preprint was prepared with the AAS LATEX macrosv5.2.
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Fig. 1.— ObservedSwift/XRT spectrum taken on 28 January 2005. The solid stepped curve represents the singleabsorbed power-law which best fits the spectrum (see Table 1 for spectral parameters).
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Fig. 2.— Swift/XRT background-subtracted light curves in the 1-10 keV (filled circles) and in the 0.2–1keV (opencircles) energy bands, and optical light curve (triangles), obtained from the merging of the UVOT V filter and REMV filter observations. The signal has been averaged within the Swift pointings for the X-ray data and with a timeresolution of 1 day for the optical data. The curves are not corrected for Galactic extinction, and are normalized totheir respective averages (0.136 cts s−1 in the 1-10 keV band, 0.084 cts s−1 in the 0.2-1 keV band, 6.58 mJy in theoptical band), computed on the time-binned datasets. The dotted horizontal lines indicate the average values of thethree light curves: for clarity, the 0.2-1 keV band and V-band light curves have been scaled up by additive constants1 and 2, respectively. Note that this upscaling implies thatthe flux ratios derived by direct inspection of the softX-ray (0.2-1 keV) and optical light curves do not correspondto the real ones, the fluxes having been increased byconstants 1 and 2, respectively. The maximum amplitudes of variability in optical and X-rays are a factor of∼4 and∼60, respectively.
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Fig. 3.— Hardness ratios computed using the 1-10 keV and 0.2-1 keV count rates for (a) the whole monitoring; (b)the observation of January 2005.
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Fig. 4.— Multiwavelength light curves at various epochs during the 2005 campaign, in logarithmic scale, shifted influx by arbitrary additive constants: (a) 1-10 keV (filled circles) and 0.2-1 keV (open circles) XRT light curves inJanuary 2005; (b) XRT light curves in July 2005 (symbols as inpanel (a)), and simultaneous REM and UVOT V-band(filled triangles), REM R- (open diamonds) and I-band (crosses) light curves. Note the optical increase preceding theX-ray observation and the correlated X-ray and optical decay; (c) XRT light curves on 17 November 2005 (symbols asin panel (a)), and UVOT U- (open triangles), B- (stars) and V-band (filled triangles) light curves; (d) XRT light curvesin 24-25 November 2005 (symbols as in panel (a)), and UVOT light curves in the W2 (1930 Å, filled squares), W1(2600 Å, filled diamonds) and V filters (filled triangles).
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Fig. 5.— Observed spectral energy distributions of PKS 0537–441 on 24-25 February 2005 (small filled circles), 12July 2005 (filled squares) and 24 November 2005 (filled triangles). The big filled circles represent the BAT data. TheSwift/XRT data are reported along with the 1σ confidence ranges of their power-law fits. Systematic errorsof 5% and10% have been added in quadrature to the statistical uncertainties associated with the UVOT UBV filters and UV filtersdata points, respectively. For comparison, in lighter, open squares the multiwavelength data from previous epochs arereported (including CGRO-EGRET spectra), already discussed in Pian et al. (2002), and the non-simultaneous IRAS,ISO, HST-FOS and BeppoSAX data (Pian et al. 2002; Padovani etal. 2006). The 1σ confidence ranges of theEGRET spectra are reported as light dashed lines. The flux uncertainties are 1σ (in some cases they are smallerthan the symbol size). The X-ray, UV, optical and near-IR data are corrected for Galactic extinction (see text). Theoptical and near-IR magnitudes have been converted to fluxesfollowing Fukugita, Shimasaku, & Ichikawa (1995) andBersanelli, Bouchet, & Falomo (1991), respectively. Overplotted are the jet models (Katarzynski & Ghisellini 2007,see text) for the energy distributions of 24-25 February 2005 (solid curve), 12 July 2005 (dotted curve), 24 November2005 (dashed curve). The thermal component required to account for the observed optical-UV flux is also reported asa dashed curve.
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Fig. 6.— Historical spectral energy distributions of PKS 0537–441. The data are the same as those presented in Fig. 5as open squares. Here we distinguish them according to the observation epoch: the gamma-ray (CGRO-EGRET), softX-ray (ROSAT), UV (IUE), optical and millimetric data have been taken nearly simultaneously in 1991-1992 (filledsquares) and 1995 (filled circles). The far-infrared data taken by IRAS and ISO and the X-ray BeppoSAX data are notsimultaneous and are represented as open squares, open circles and open triangles, respectively (see Pian et al. 2002,and references therein; Padovani et al. 2006). As in Fig. 5, the data have been modelled according to Katarzynski &Ghisellini (2007). The model curves for the 1991-1992 and 1995 states are dotted and solid, respectively.
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Fig. 7.— Top panel: The logarithms of 3 quantities (”Q”) are reported as a function of the logarithm of the bulkLorentz factor: the size of the emitting sourceR15 in units of 1015 cm, the value of the magnetic fieldB in Gauss, andthe injected powerL′43 (in the comoving frame) in the form of relativistic particles, in units of 1043 erg s−1, as used forour modelling. The dashed lines represent the relationships predicted by the Katarzynski & Ghisellini (2007) model.The labelled dates identify the specific model/state of the source (see Table 3).Bottom panel: The power carried bythe jet in the form of magnetic field (LB), cold protons (Lp), relativistic electrons (Le) resulting from our modelling, asa function of the bulk Lorentz factor.
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T 1
Swift/XRT O a
Start time (UT) End time (UT) Exposure Mean Fluxb Counts Spectral index χ2red (d.o.f.)
N.—For all models we have assumed a viewing angleθ = 3◦ and a bolo-metric luminosity of the accretion diskLdisk = 1.8 × 1046 erg s−1. The energydensity of the external radiation and its peak frequencyνext are measured in theobserver frame.
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