The ROSAT X-ray Spectra of BL Lacertae Objects

9 Pages·0.33 MB·English

by  Paolo Padovani

Mon.Not.R.Astron.Soc.000,000–000(1996) Printed1February2008 (MNplainTEXmacrosv1.6)
The ROSAT X-ray spectra of BL Lacertae objects
Paolo Padovani1 and Paolo Giommi2
1 Dipartimento di Fisica, II Universit`a di Roma “TorVergata”, Via della Ricerca Scientifica1, I-00133 Roma, Italy
2 SAX, ScientificData Center, ASI, Viale Regina Margherita 202, I-00198 Roma, Italy
6
9 Accepted1995October11.Received1995July31
9
1
ABSTRACT
n
We study the X-ray spectra of 85 BL Lacertae objects using the hardness ratios as
a
J given in the WGA catalogue of ROSAT sources. This sample includes all WGA BL
Lacs with high-quality data and comprises about 50 per cent of presently known BL
7
1 Lacs. We find that BL Lacs have energy power-law spectral indices between 0 and
3 with a mean value αx ∼ 1.4. Significant differences, however, are present between

1 high-energy cutoff BL Lacs (HBLs), normally selected in the X-ray band, and low-
v energy cutoff BL Lacs (LBLs), generally found in radio surveys. HBLs have steeper
9 X-rayspectralslopes(αx ∼1.5)wellcorrelatedwithαox,andanti-correlatedwiththe
8 X-ray-to-radiofluxratioandcutofffrequency,withconvexoverallbroad-bandspectra.
0 LBLs, on the other hand, have flatter X-ray spectra (αx ∼ 1.1) and concave optical-
1
X-ray continuum. We interpret these results in terms of different mechanisms being
0
responsible for the X-ray emission in the two classes,namely synchrotronand inverse
6
ComptonforHBLsandLBLsrespectively.Theobserveddifferencesareconsistentwith
9
/ the hypothesisthatHBLs andLBLsarepoweredbyessentiallythe samenon-thermal
h engines differing mainly in their synchrotroncutoff energy.
p
- Key words: galaxies:active – BL Lacertae objects: general – X-rays: galaxies
o
r
t
s
a
1 INTRODUCTION was clearly not satisfactory, as it was not based on intrin-
:
v sicphysicalpropertiesbutsolely ontheselection band,and
Xi BL Lacertae objects constitute a relatively rare class of ac- didnotcharacterizeuniquelyasource.Recentall-skyX-ray
tive galactic nuclei (AGN), distinguished by their almost surveys (Laurent-Muehleisen et al. 1993; Brinkmann et al.
r
a complete lack of emission lines. At present, only about two 1995; Perlman et al. 1995a), in fact, include many BL Lacs
hundred BL Lacs are known (Padovani & Giommi 1995b) previously selected at radio frequencies, which could now
ascompared,forexample,tothethousandsofquasarslisted then be classified both as XBLs or RBLs.
in the most recent AGN catalogues (V´eron-Cetty & V´eron Ithasbeenknownforsometime(e.g.,Ledden&O’Dell
1993; Hewitt & Burbidge 1993). 1985)thatthebroad-bandspectraofthetwoclassesarealso
Despite their small numbers and other somewhat ex- different (with the likely exception of the radio to millime-
tremeandnon-typicalproperties(radiocore-dominance,su- tre band: Gear 1993). Recently, Giommi, Ansari & Micol
perluminalmotion,rapid variability,highpolarization: e.g., (1995a)havestudiedthemultifrequencyspectrumofalarge
Kollgaard 1994; Urry & Padovani 1995) BL Lacs are nev- sampleofBLLacs,arguingthattheremaybeonlyonepop-
ertheless an important AGN subgroup, as the broad-band ulation of objects, characterized by a wide range of energy
emission in these objects is most likely completely domi- cutoffs(oralternativelyenergypeaksinaνfν vs.ν plot):at
natedbynon-thermalprocesses.Theirmultifrequencyspec- infrared-opticalenergiesformostRBLs,atultraviolet/X-ray
tra are in fact fairly smooth over a wide frequency range, orhigherenergiesfor mostXBLs,with thelatterconstitut-
suggestive of a common emission process, and lack the ing only ∼ 10 per cent of all BL Lacs. The X-ray-to-radio
thermal features characteristic of the other, more common, flux ratios (fx/fr) for the two classes are also different, as
AGN,like dust emission in the infrared and (possibly) disk a result of thedifferent break frequencies. Note that within
emission in the ultraviolet band (e.g., Bregman 1990). thishypothesisthereisacontinuumofcutoffenergies, with
Historically,BLLacertaeobjectshavebeendividedinto different selection bands picking up objects with different
RBLs (radio-selected BL Lacs) and XBLs (X-ray-selected spectra.
BLLacs),dependingonthediscoveryband.Thetwoclasses We (Giommi & Padovani 1994; Padovani & Giommi
showed in fact somewhat different properties, RBLs being 1995a)havethenproposedadivisionbasedonfx/fr (asthe
more extreme, for example, in their optical polarization, break frequencyat presentcan bederiveddirectly from the
core-dominance, and variability properties (e.g., Jannuzi, spectraonlyforasmallnumberofobjects:seeSect.3),clas-
Smith&Elston1994;Perlman&Stocke1993).Thisdivision sifying as HBLs (or high-energy cutoff BL Lacs) and LBLs
2 P. Padovani and P. Giommi
(orlow-energycutoffBLLacs)sourcesrespectivelyaboveor according to the X-ray-to-radio flux ratio, as proposed by
below thevalue of fx/fr ∼10−11 (where X-ray fluxescover Padovani & Giommi (1995a), with theresult that theaver-
the0.3–3.5keVrangeandareinunitsofergcm−2 s−1 and age spectrum of the (3) HBLs was steeper than that of the
radio fluxes refer to 5 GHz and are expressed in janskys). (9) LBLs. Urryet al. (1996) reached similar conclusions.
We have also shown that the hypothesis that HBLs (that Itisclear thatatleast someofthedifferentresultscan
is, so far, most XBLs) are the minor component of the BL beexplainedbypoorstatisticsandtheuseoftheRBL/XBL
Lacpopulationwithhigh-energycutoffscanexplainmostof classification, whichaswehavediscussedaboveisnotphys-
theirpropertiesatleastaswellasthealternative(andmore ical. However, it is also probably true that the differences
diffuse) explanation for the existence of two classes of BL between the X-ray spectra of HBLs and LBLs are not that
Lacs, which posits that XBLsare intrinsically more numer- strong and requirebetter statistics to be singled out.
ous as they are viewed significantly off their beaming axis ThepurposeofthispaperistoanalyzetheX-rayspec-
(e.g., Stocke et al. 1985; Maraschi et al. 1986; Padovani & tra of all BL Lacs observed (as pointed or serendipitous
Urry1990). sources) by ROSAT to check for any differences between
A study of thepuzzling distribution of BL Lacs on the HBLs and LBLs, coupling for the first time a large statis-
αro,αox plane (Padovani & Giommi 1995a) has led us to ticswithwhatwethinkisameaningfuldistinctionbetween
suggest that the observed bimodality can also be explained the two classes. Our data have been taken from the WGA
by an energy break ranging from high (X-ray) to low (far- catalogue (White, Giommi & Angelini 1994), a large list of
infrared) energiesfor HBLsandLBLsrespectively.Thisas- X-raysourcesgeneratedfromalltheROSATPSPCpointed
sumes a different origin of the X-ray emission for HBLs observations,withwhichisnowpossibletostudytheX-ray
andLBLs:namely,anextensionofthesynchrotronemission properties of large numbers of objects in an homogeneous
likelyresponsibleforthelowerenergycontinuuminthefor- and relatively simple way. The selection of the objects was
merandinverseCompton(SSC:synchrotronself-Compton) donebycross-correlatingthefirstrevisionoftheWGAcata-
emission in the latter. This would be consistent with the logue (restrictingourselvestosourceswith qualityflag≥5,
peakoftheemittedpowerbeingatlowerenergiesforLBLs, whichexcludesproblematicdetections)withourrecentcat-
suggestive of an extra component takingover at higher fre- alogue of BL Lacs (Padovani & Giommi 1995b). This pro-
quencies(seeBregman 1990 and references therein).A cru- duced 163 observations of 85 distinct BL Lacs (58 HBLs
cialtestofourhypothesis(andingeneralofanyothermodel and 27 LBLs: see Sect. 2), which correspond to ≃ 46 per
wishing to explain the existence of two BL Lac classes) is centofconfirmedBLLacspresentlyknown.Thisrepresents
thentheanalysisofthespectralshapeoftheX-rayemission the largest number of BL Lacs for which homogeneous X-
ofalargesampleofBLLacsincludingbothHBLsandLBLs. rayspectralinformation isavailableandthelargest BLLac
IntheX-rayband,infact,Comptonemissionisexpectedto sample ever studied at X-ray frequencies (cf. Ciliegi et al.
be flatter than the synchrotron component (e.g., Ghisellini 1993whocollectedinformationonthespectraof42BLLacs
& Maraschi 1989). observed with various X-raytelescopes).
ThestudyofX-rayspectralindicesofBLLacshasgiven The structure of the paper is as follows: Section 2 de-
various and sometimes contradicting results, even consider- scribes the observational data and the data analysis, Sec-
ingonlythemostrecentpapersdealingwithpossiblediffer- tion 3 studies the X-ray spectral properties of BL Lacs
ences between the two classes (references to earlier papers while Section 4 discusses our results and presents our con-
can be found in Sambruna et al. 1994). Sambruna et al. clusions.Throughoutthispaperspectralindicesarewritten
(1994) have analyzed the EXOSAT spectra of 21 BL Lacs fν ∝ν−α.
(only five of which were RBLs) and found similar spectral
indices in the LE (0.04−2.0 keV) and ME (1−20 keV)
bands for the two classes, with indications of a steepening
2 DATA ANALYSIS
athigherenergiesforXBLs.Ciliegi,Bassani&Caroli(1995)
havestudiedtheX-rayspectraof42BLLacs(mostlyRBLs) Spectral indices for the WGA BL Lacs were obtained from
observedwithvarious(pre-ROSAT)X-raytelescopesindif- the count rates given in the catalogue in the 0.1−0.4 keV
ferent energy ranges and found that XBLs and RBLs have range (soft band: S), 0.4−0.86 keV range (mid band: M)
similar(withintheratherlargeerrors)spectralindicesinthe and 0.87 −2.0 keV range (hard band: H) combined into
soft(0.2−4keV)andpossiblyalsointhehard(2−10keV) hardness (HR) and softness (SR) ratios (see Giommi et
band.However,therewereindicationsthatXBLsandRBLs al. 1995b). Briefly, the count rates were combined to con-
havedifferentglobalX-rayspectra,withtheformerandthe struct one SR = S/M and two HRs, HR1 = H/M and
latter steepening and flattening, respectively, at higher en- HR2 =H/(M+S). Initially we converted thehardness ra-
ergies. Urry et al. (1996; see also Sambruna 1994) have ex- tiosintoenergyspectralindicesbothassumingGalactic NH
amined more recent ROSAT spectral data for 32 out of the derivedfrom21cmmeasurements(Starketal.1992;Shafer
34 radio-selected BL Lacs in the 1 Jy sample (Stickel et al. etal.,privatecommunication)andwithNHderivedfromthe
1991). They find that 1 Jy RBLs have spectra similar to softness ratio through an iterative procedure. The spectral
those of the Einstein Observatory Extended Medium Sen- indices derived from the two approaches were found to be
sitivity Survey (EMSS) XBLs (Perlman et al. 1995b), with similar andwellcorrelated inthe0.4−2.0keVrange,while
an indication for the subsample with Einstein IPC data of considerable scatter was present when the total ROSAT
aflatteningofthespectraathigher(>1keV)energies.Co- bandwasconsidered,withasizeablenumberofobjectshav-
∼
mastri, Molendi & Ghisellini (1995) analyzed the ROSAT ing αx,NH significantly different from αx,Galactic NH, sugges-
spectra of twelve 1 Jy BL Lacs (i.e. a subsample of the tiveofachangeofthespectrumatlowerenergies.Inthispa-
objects studied by Urry et al. 1996) dividing the objects perwewilladoptasX-rayspectralindexthevalueobtained
X-ray spectra of BL Lacs 3
with NH fixed to the Galactic value, which is better deter-
mined than theNH derivedfrom thesoftness ratios, and in
the0.4−2.0 keV range,i.e. themid tohard band(spectral 2.5
indices in the whole ROSAT range were used for the com-
parison between the values obtained with our method and
those derived by other authors from detailed spectral fits: 2
see below).
The choice of this energy range stems from two facts:
first, our method assumes a single power-law; given the ev-
1.5
idence of a change of spectral shape at lower energies in
some sources, as described above (see also Comastri et al.
1995andUrryetal.1996),thisassumptionmightbeincor-
rect in the whole ROSAT range while it is probably more 1
acceptableinthenarrower0.4−2.0keVrange(asshownby
the similarity of the spectral indices obtained with Galac-
ticNH andNH derivedfrom thesoftnessratio);second,the
0.5
method used in the WGA catalogue to estimate the source
intensity uses the counts detected in a box whose size opti-
misesthesignaltonoise(S/N)ratio.Thissizeiscalculated
assuminganaveragepointspreadfunction(PSF)thatistoo 0
sharp for very soft photons. Forweak sources nearthefield 0 0.5 1 1.5 2 2.5
center the “optimum” box size is small and consequently
includes a small, energy-dependent, fraction of the source
photonscausinganunderestimationofthecountsinthesoft Figure 1. ROSAT band energy spectral indices of BL Lacs de-
band.Theeffectsofthedependenceofenergyresponsewith rivedfromourapproachbasedonthehardnessratios,αhr,com-
off-axis angle have been taken into account using different pared to the values obtained by other authors from a detailed
conversion matrices in five different off-axis ranges: 0−20, spectral fitting procedure of the same ROSAT data, αfit. The
solidlineindicatesthelocusofpointswithαhr=αfit.Errorbars
20−30, 30−40, 40−50, and 50−60 arcminutes.
represent1σerrorsforourvaluesand90percenterrorsforthose
Errors on the spectral indices (1σ) were derived from
obtainedbyother authors.
theuncertaintiesonthehardnessratios.Weincludedinour
analysisonlyobservationswithvaluesofhardnessratiosrel-
would represent more the typical X-ray spectrum of 1E
ativetotheiruncertainties>3.5,whichcorrespondsroughly
1415+2557 than that of BL Lacs as a class. Therefore, al-
to1σ errors on αx ∼<0.5 (Giommi et al. 1995b). Allobjects though we plot in the figures all the 163 X-ray spectral in-
in this study are “bona fide” BL Lacs: we excluded eight
dices, in all statistical tests we have kept only one observa-
sources classified as uncertain BL Lacs or BL Lac candi-
tion (and therefore one spectral index) per object, selected
dates (Padovani & Giommi 1995b). We are then left with
on a one-to-one basis taking the best combination of offset
163 observations of 85 BL Lacs.
from the field center and S/N ratio. When more than two
Figure 1 compares the X-ray energy spectral indices
observations with comparable offset and S/N ratios were
derived from our method in the whole ROSAT band (i.e.
available, we selected as most representative the one with
obtained from HR2) with those obtained by other authors
(Perlmanetal.1995b;Maraschietal.1995;Urryetal.1996) αx nearest tothe mean value for that source.
throughadetailedspectralfittingprocedure,usingthesame
ROSAT data for 44 BL Lacs. The comparison is between
spectralindices derivedunderthesameassumption regard- 3 THE X-RAY SPECTRA
ingtheabsorbingcolumndensity,i.e.eitherassumingGalac-
3.1 αx versus fx/fr
ticNH (Stark et al. 1992; Shaferet al., privatecommunica-
tion) or with NH derived from the softness ratio. Figure 1 The main purposeof this paper is to study theX-ray spec-
showsthatmostobjectsclusteraroundthelineαx,hr =αx,fit tral indicesof HBLsand LBLs. Tothisaim,it is important
with a linear relationship between the spectral indices de- to define the value of the fx/fr ratio which divides the two
rived with the two methods. Overall the agreement is then classes. (We prefer the use of broad-band X-ray fluxes in
very good, especially considering the underestimate of soft theX-ray-to-radiofluxratiosinsteadof,forexample,1keV
photons in the WGA data. The mean values for the two fluxes,becausethelatteraremoredependentontheprecise
derivations are αx,hr = 1.18±0.08 and αx,fit = 1.24±0.08 valueofαx andthereforerequirespectralfitting,whichisat
(here and in the following we give the error of the mean), presentnotpossibleforallBLLacs.)InPadovani&Giommi
which clearly indicates the statistical robustness of our ap- (1995a),basedontheenergydistributionsgivenbyGiommi
proach. etal.(1995a),wesuggestedaratiofx/fr ∼10−11.Itshould
AfewsourcesinthePSPCfieldoffamousobjectshave benoted,though,thatveryfewBLLacsinthecompilation
many X-ray observations: for example, 1E 1415+2557 (in byGiommietal.(1995a)had10−11.5 ∼<fx/fr ∼<10−10.5,be-
the field of NGC 5548) has been observed in 29 different cause thisintermediate zone isnot favoured eitherbyradio
occasions! Inclusion of all these data in statistical studies orX-rayselection andisthereforepoorlypopulatedbothin
(means, correlations, Kolmogorov-Smirnov [KS] tests etc.) X-rayandradiosurveys.Thedividingfx/fr valuewasthen
would clearly bias them: the resulting mean, for example, not well determined.
4 P. Padovani and P. Giommi
58 HBLs (BL Lacs with fx/fr ≥ 10−11.5) are steeper than
thoseofthe27LBLs(BLLacswithfx/fr <10−11.5).Infact,
hαx,HBLi=1.52±0.06whilehαx,LBLi=1.06±0.09,different
at the 99.99 per cent level according to a Student’s t-test.
3
The two αx distributions, shown in Fig. 3, are also differ-
ent at the 99.9 per cent level according to a KS test. (Had
the dividing line been drawn at the value fx/fr = 10−11,
against the evidence presented in Fig. 2, one would have
2
found hαx,HBLi = 1.43±0.05 and hαx,LBLi = 1.27±0.11,
withthetwodistributionsdifferentatthe95.4percentlevel
according to a KS test.)
AnothernotablefeatureofFig.2istheanti-correlation
1 betweenαx andfx/fr forHBLs,significantatthe99.99per
centlevel(Spearmanrank-ordertest),independentlyof the
precise value of thedividing fx/fr ratio, and not present in
LBLs. A linear fit to the HBL data gives αx = −(0.41±
0 0.07)log(fx/fr)−(2.65±0.66). In HBLs, in other words,
the smaller the X-ray to radio flux ratio, (i.e., the steeper
the effective radio-X-ray spectral index), the steeper is the
X-rayspectral index,which points toa connection between
-13 -12 -11 -10 -9 the radio and X-ray bands and/or to the effects of a break
in the energy distribution (see Sect. 3.3).
Figure 2. The X-ray spectral index of BL Lacs versus their
X-ray-to-radioflux ratio (with X-rayfluxes inthe 0.3 – 3.5 keV 3.2 αx versus αox
range in units of erg cm−2 s−1 and radio fluxes at 5 GHz ex- A similar and related connection for HBLs exists between
pressedinjanskys).Errorbarsrepresent1σ errors.
theopticaland X-raybands.Figure4plotstheX-rayspec-
tralslopeversusαox,theeffectiveoptical-X-rayspectralin-
InFig.2weplotαxversusfx/frforalltheobjectsinour dex evaluated between the rest-frame frequencies of 5000
sample, with radio data from Padovani & Giommi (1995b ˚A and 1 keV. Optical fluxes were derived from the V mag-
andreferencestherein)andX-rayfluxesinthe0.3–3.5keV nitudes collected by Padovani & Giommi (1995b), correct-
band derived from the Galactic NH and αx values. (Ide- ingforGalacticabsorptionfollowingGiommietal.(1995a),
ally the radio and X-ray observations should be simultane- while 1 keV fluxes have been obtained from the ROSAT
ous;althoughvariabilityintheradioisnotverystrong,this counts and the derived spectral indices and have also been
undoubtedly introduces some “noise” in our plots.) Within correctedforGalacticabsorption.Thek-correctionhasbeen
our hypothesis on the difference between HBLs and LBLs derived assuming an optical index αo = 1.05 for LBLs and
(Padovani&Giommi1995a),describedintheIntroduction, αo =0.65forHBLs(Falomo,Scarpa&Bersanelli1994).For
the X-ray emission of HBLs should be dominated by syn- k-correctionpurposesobjectswithnoredshifthavebeenas-
chrotronemission.HBLsnearthetransitionregion,inwhich signed the mean redshift of the corresponding sub-sample,
thebreakisjustbelowtheX-raybandandwhereweseethe i.e. zmean,LBL =0.60 and zmean,HBL =0.25. Note that opti-
steeptailofthesynchrotronemissionbutnotyettheinverse cal fluxes are not simultaneous with X-ray data (although
Compton component, should then be characterized by the for most of the 1 Jy BL Lacs in our sample they should be
steepest X-ray spectra (see also Sect. 3.3). Figure 2 shows representative of their “typical” optical state: see Padovani
this to happen for 10−11.5 ∼< fx/fr ∼< 10−11, where all ob- &Giommi1995b),whichwillcertainlyintroduceascatter:a
jects have αx ∼> 1.5 and reach values up to ∼ 3. A better variation∆V inmagnitude,infact,translatesintoachange
dividinglinebetweenthetwoclassesisthenfx/fr ∼10−11.5, ∆αox =0.15∆V.
which is the value at which there is a discontinuity in αx. From the data presented in Fig. 4 we infer that: 1. for
We stress that our results (and those obtained in Padovani HBLstheX-rayspectralindexiscorrelated withαox at the
&Giommi1995a)areunaffectedbytheprecisevalueofthis 99.99 percentlevel(Spearman rank-ordertest) with abest
parameter.(Oneobjecthasαx ∼3butfx/fr ∼10−12.This fit αx = (1.38±0.20)αox −(0.01±0.22); 2. no correlation
is 1WGA J1202.1+444, which appears three times in the is present for LBLs; 3. the two classes separate extremely
WGA catalogue and only in its faintest state crosses the well on this plane, that is all HBLs apart from one have
HBL/LBL boundary to become an LBL with an unusually αx ≥ αox (within the errors), while the majority of LBLs
steepX-rayspectrum,albeitwithlargeerrors,followingthe haveαx <αox.
αx −fx anti-correlation normally observed in HBLs [e.g., This separation is better appreciated in Fig. 5, which
Giommi et al. 1990; Sambruna et al. 1994]. We note how- plots αx−αox versus fx/fr. This plane can be divided into
ever that a decrease in radio flux by a factor of 2−3 as four quadrants, out of which one is basically unpopulated
compared to the value we used, which we cannot rule out, and anotheroneis only marginally populated. Theremain-
would havekeptthis object within theHBL limits.) ing ones are the HBL–convex-spectrum quadrant (fx/fr ≥
The mean value of the X-ray spectral index for the 85 10−11.5 and αx > αox) and the LBL–concave-spectrum
BL Lacs under study is hαxi = 1.37±0.05. Figure 2, how- quadrant (fx/fr < 10−11.5 and αx < αox). The only HBL
ever, already shows that the X-ray spectral indices of the with possibly concave spectrum is MS0122.1+0903, which
X-ray spectra of BL Lacs 5
fourdifferentobjects:oneis1WGAJ1202.1+444, discussed
above,whiletheotherthreehavefx/fr valueswithin afac-
35 tor of two of the HBL/LBL dividing line, which, as far as
weknow,couldhavewellbeencrossed hadtherebeenade-
creaseinradiofluxbyafactorof2(comparedtothevalues
30
we used) at thetime of theROSAT observation.
ThedifferencebetweenHBLsandLBLsinhαx−αoxiis
25 striking,thisparameterbeingequalto0.40±0.04 forHBLs
and−0.33±0.09forLBLs(0.17±0.06forthewholesample).
This is simply an amplification of the different X-ray and
20
effective optical-X-ray spectral indices. While, as we have
seen,HBLshaveαx steeperthanLBLs,theoppositeistrue
15 asregardsαox (seeFig.4),forwhichhαox,HBLi=1.11±0.03
and hαox,LBLi=1.39±0.03.
Sambruna (1994; see also Maraschi et al. 1995) has
10
found a significant difference between the αox −αx distri-
butions of 1 Jy RBLs and EMSS XBLs, with mean values,
5 however,apparentlylessdifferentthanours.Thisdifference
can be explained by her smaller statistics and inclusion of
three1 Jy HBLs with theRBL sample.
0
-0.5 0 0.5 1 1.5 2 2.5 3 Perlman et al. (1995b) have compared the ROSAT X-
ray spectral indices of 22 EMSS BL Lacs (all HBLs) to the
slope of the tangent at 1 keV to a parabola fitted to the
Figure 3. The X-ray spectral index distribution of all BL Lacs radio, optical, and X-ray fluxes. They find that the X-ray
inoursample(dottedline),andofHBLs(fx/fr≥10−11.5:solid spectra havesimilar or slightly steeperslopes than thetan-
line) and LBLs (fx/fr < 10−11.5: dashed line) separately. Only gent slope in most cases, a somewhat different result from
one X-ray spectrum per object is used, as described in Sect. 2.
ours.Thismightbeduetoourlargersampleandalsototheir
ThedistributionsofHBLsandLBLsaredifferentatthe99.9per
use of the tangent slope instead of the effective optical-X-
centlevelaccordingtoaKStest.
ray index. Brunner et al. (1994) have studied the ROSAT
spectra of five 1 Jy BL Lacs and derived a mean value for
αx −αox consistent with zero, unlike our result for LBLs.
Despite their small statistics, exclusion of the HBL object
intheirsample(S50716+714),whichhasthelargest value
3
of αx−αox, makestheir mean valueconsistent with ours.
3.3 αx versus νbreak
2 The anti-correlation between αx and fx/fr and thecorrela-
tionbetweenαx andαox forHBLsarebothrelatedtowhat
we think is the most interesting correlation, that between
αx and νbreak.
1 In theory, one should infer the break frequency νbreak
fromtheobservedenergydistributionoftheobjects.Inprac-
tice,however,theavailablespectralcoverageisfarfromcom-
pleteandthemeasurementsaregenerallynon-simultaneous.
In particular, far-ultraviolet and far-infrared data are miss-
0
ing for the large majority of the sources, and these are the
bandswherethepeak of theemission is most likely located
forHBLsandLBLsrespectively(Giommietal.1995a).The
break frequencyhas thenbeen derived as follows: Padovani
0.8 1 1.2 1.4 1.6 1.8
&Giommi (1995a), usingatriple power-law approximation
tothespectra,haveshown thatthedistributionofBLLacs
Figure4.TheX-rayspectralindexversusαoxforBLLacs.Filled ontheαro,αox plane(whereαroistheeffectiveradio-optical
pointsindicateHBLs(fx/fr≥10−11.5),whileopensquaresindi- spectralindexevaluatedbetween therest-frame frequencies
cateLBLs (fx/fr <10−11.5). Thesolidlinerepresents the locus of5GHzand5000 ˚A)can beeasily explained byanenergy
ofpointshavingαx =αox.Errorbarsrepresent1σ errors. break migrating from high (X-ray) to low (far-infrared) en-
ergies (see their Fig. 12). Here we use their results to infer
is however consistent with having αx > αox at the ∼ 2σ νbreak fromtheαro andαox values.Theformerwerederived
level, especially considering that errors on αx−αox do not from the radio and optical data collected by Padovani &
include errors on αox. As regards the twelve LBL spectra Giommi (1995a), using optical indices and mean redshifts
with possibly convex optical-X-ray continuum, only four of (when necessary) as described above. As regards the radio
themhaveαx >αox at the∼>2σ level. These correspond to spectralindex,necessary for the k-correction of αro,for the
6 P. Padovani and P. Giommi
1.5
3
1
0.5 2
0
1
-0.5
-1
0
-1.5
-2
-13 -12 -11 -10 -9 12 13 14 15 16 17
Figure5.αxminusαoxversusfx/frforBLLacs.Thehorizontal Figure 6.The X-rayspectral index versus the logarithm of the
dashed line represents the locus of points with αx = αox, while breakfrequencyforBLLacs.Objectswithνbreak∼>1015Hzhave
theverticaldashedlinerepresentsthedividinglinebetweenLBLs fx/fr>10−11.5andare,bydefinition,HBLs,whiletheremaining
andHBLsatfx/fr=10−11.5.Errorbarsrepresent1σerrorsand objectsareLBLs.Errorbarsrepresent1σerrors.Seetextforthe
includeonlytheuncertainties onαx. derivationofνbreak.
BLLacslackingthisparameteritwasassumedequaltothe thelargestandsmallestspectralindex).Thismightsuggest
meanvalueforthetwosub-classes,thatisαr =0.2forHBLs thatLBLsdisplaya(weak)positivecorrelation,possiblydi-
and αr =−0.2 for LBLs. luted by the relatively small statistics and larger error bars
Operationally, we projected the points on the αro,αox (theLBL sampleis lessthanhalf thesize theHBL oneand
planeonthepredictedrelationship betweenthetwoparam- on average the LBLs studied in this work are weaker X-ray
eters derived by Padovani & Giommi (1995a), namely on sources than HBLs).
that of HBLs for fx/fr ≥ 10−11.5 and on that of LBLs for
fx/fr <10−11.5 (seeFig. 12of Padovani&Giommi 1995a). 3.4 Interpretation
We then read off the corresponding values of the break fre-
quency. Although this method gives only an approximate Thenegativecorrelationbetweenαx andνbreak observedfor
estimate of νbreak, the values derived this way agree well HBLs is expected if the emission of these objects is due to
with those inferred from an inspection of the plots, for ex- thesynchrotronprocessallthewayuptotheX-rayband,as
ample, in Giommi et al. (1995a) for the (few) objects with shown in Fig. 7: when the break is near the X-ray band (∼
enough data to permit a direct determination of the break. 1017 Hz: dotted line) the X-ray spectrum will be relatively
Moreover, our values are typically consistent within ∼ 0.2 flat,butassoonasthepeakshiftstolowerenergies(∼1016
dex with the peak frequencies obtained by Comastri et al. Hz: dashed line) the steep tail of the synchrotron emission
(1995) from polynomial fits to the broad band spectra of moves in. When νbreak is at much lower energies (e.g., ∼
twelve1 Jy BL Lacs. 1014 Hz:solidline)sothatthesynchrotroncomponentisno
Figure 6 clearly shows the strong anti-correlation (sig- more dominant, Compton emission starts to be important:
nificant at the 99.99 per cent level: Spearman rank-order atthispointonehascrossedtheHBL/LBLdividingline.For
test) between X-ray spectral index and break frequency for evensmallerbreakfrequencies,twoeffectscompete:first,the
HBLs. A linear fit gives αx = −(0.63±0.09)logνbreak + smallerνbreak,thesmalleristheproductBγm2ax,e,whereBis
(11.87±1.50): i.e., the highest νbreak, the flattest αx. As themagneticfieldandγmax,eisthemaximumLorentzfactor
regards LBLs, here the situation is not clear: a fit to the oftheelectrons(e.g.,Rybicki&Lightman1979);thisshould
“compressed”dataasdoneforallthecorrelationsstudiedin notaffectthelowerenergies,therebyleavingunchangedthe
thispaper(i.e.,usingonlyonespectrumperobject,selected shape of the Compton component; second, a lower νbreak
asdescribedinSect.2)givesnocorrelation betweenαx and meansa weakersteep X-raysynchrotron component,which
νbreak for the 27 LBLs (Pcorr ≃ 61 per cent). However, shouldresultinaflatterspectruminobjectswithstillanon-
Fig. 6 shows an apparent positive correlation between the negligible synchrotron emission. The sum of the two effects
two parameters (αx ∝0.41logνbreak) when all (42) spectra is not easy to quantify but the fact that in LBLs theX-ray
are included, significant at the 99.6 per cent level (Spear- spectral index is not strongly dependent on νbreak suggests
man rank-order test; this reduces to 98.2 per cent, with that the Compton component might dominate completely
αx ∝ 0.26logνbreak, if one excludes the two outliers with theX-ray band in at least some objects.
X-ray spectra of BL Lacs 7
Thistiesinnicelywiththestudyoftheαoxindex,which
shows that the X-ray band connects smoothly to the opti-
caloneinHBLs, with asteepeningofthebroad-bandspec-
trum,supportingacommonemissionmechanismforthetwo
bands,whileinthegreatmajorityofLBLsαx isflatterthan
αox, indicative of a different component entering the X-ray
range. Also, the anti-correlation between αx and fx/fr in
HBLs is simply related to that between αx and νbreak as
objects with larger νbreak havealso larger values of fx/fr.
Inobjectswithverylowbreakfrequencies,forwhichthe
X-ray emission should be pure inverse Compton, the X-ray
spectral indices should be similar to those in the radio–far-
infrared region, obviously up to ν ∼< νbreak. Unfortunately,
veryfewBLLacshavebeendetectedbyIRASsothestatis-
ticsisquitepoor:wefindthatthe5GHz–60µspectralindex
for about ten 1 Jy BL Lacs is αri ∼ 0.3 (see also Giommi
etal.1995a),whichroughlycorrespondstothelowerbound
of αx for LBLs. Clearly a detailed study of the far infrared
spectraofLBLswouldbeextremelyvaluableinthisrespect:
thisproject will be carried out with ISO.
We predict that the observed anti-correlation between
αx and νbreak observed for HBLs cannot continue indefi-
nitely. Since the synchrotron spectrum steepens with fre-
quency(e.g.,Ghisellini&Maraschi1989),ifνbreak movesto Figure 7. Examples of multifrequency spectra of BL Lacs with
larger and larger values, the X-ray spectral index at min- different peak (or break) frequencies: ∼ 1017 Hz (∼ 0.4 keV:
imum will be as flat as the effective radio-optical spectral dotted line), ∼ 1016 Hz (dashed line), ∼ 1014 Hz (solid line).
The two parallel (dot-dashed) lines represent the ROSAT band.
index αro. In that case, then, one will have αx ∼ αox ∼
Note how the X-ray spectrum steepens with decreasing break
αro ∼0.3−0.5, the typical values for HBLs. Extrapolating
frequencyuntilComptonemissionbecomesthedominantprocess.
the observed anti-correlation, one should find αx ∼ 0.4 ∼ The normalizations of the different curves have been chosen to
constant for νbreak ∼> 10 keV. As this requires very high best illustrate the spectral differences inthe X-rayband and do
electron energies, it is not surprising that noBL Lac in our notreflectintrinsicproperties.
samplehassuchahighνbreak.Ifsuchobjects existtheyare
probably quite rare but should be detected by future hard- the WGA catalogue, for example.
X-raymissions. Basedontheadditionalinformationpresentedhere,we
Wenotethatthepaucityofobjectswithbreakfrequen- canbetterpinpointtheborderlinebetweenHBLsandLBLs,
ciesintheoptical/ultraviolet band(∼1014.5−1015.5 Hz)is andthereforeproposearefineddividinglinebetweenthetwo
simply aselection effect, as theseobjects haveintermediate classes at fx/fr ∼ 10−11.5, which corresponds to αrx ∼ 0.8
values of fx/fr which make them difficult to detect both in between1keVand5GHzandabreakfrequency∼1015 Hz
radio and X-ray surveys. Optical surveys, which in princi- (see Fig. 6). Our results agree with the hypothesis we put
ple do not discriminate against this sort of objects, are at forwardthatthecutoffenergyisthefundamentalparameter
presentveryinefficient(e.g.,Green,Schmidt&Liebert1986; giving rise to two different classes of BL Lacs (Giommi &
Jannuzi, Green & French 1993) and have so far led to the Padovani 1994; Padovani & Giommi 1995). Oddly enough,
selection of only about ten BL Lacs (Padovani & Giommi our results are also consistent with the other explanation
1995b). fortheexistenceoftwoBLLacclassesbasedonorientation.
Ghisellini & Maraschi (1989), in fact, have shown that in
objectsatlargerangleswithrespecttothelineofsight(i.e.,
4 DISCUSSION AND CONCLUSIONS
XBLs/HBLs in their view) synchrotron emission should be
The data presented in this paper allow us to draw a con- the main radiation mechanism in the X-ray band,while in-
sistentpictureofHBLsand LBLs.Namely,thecorrelations verse Compton should be more important for more aligned
oftheX-rayspectralindiceswithαox,fx/fr,andbreakfre- sources (i.e., RBLs/LBLs), in agreement with our findings.
quency all point to a common, likely synchrotron, origin Themainproblemofthatmodel,however,apartfromother
fortheradio/optical/X-rayemissioninHBLs,withanover- ones discussed by Padovani & Giommi (1995a), is that a
all convex spectrum. On the other hand, as hinted by the change in viewing angle does not seem to be able to ex-
lower cutoff energies, in LBLs a second, harder, most likely plain thelarge differencein cutoffenergies between thetwo
inverse Compton, component appears in the X-ray band. classes, as exemplified by Fig. 4 of Ghisellini & Maraschi
The ROSAT spectral indices of the two classes are differ- (1989) (Sambruna 1994; Sambruna et al. 1995, in prepara-
ent,withHBLsdisplayingonaveragesteeperspectrawhich tion).
show a strong anti-correlation with break frequency, while Our results explain the peculiar spectral variability of
LBL spectra might show a positive dependence on this pa- XBLs (i.e., HBLs) and RBLs (i.e., LBLs). Various studies
rameter. These are yet other differences between the two have shown that in XBLs higher flux states are associated
BL Lac classes, which add to the previously known ones, with flatter spectra (e.g., Giommi et al. 1990; Sambruna et
and might be helpful in selecting BL Lac candidates from al.1994).LittleisknownaboutspectralvariabilityinRBLs
8 P. Padovani and P. Giommi
but Urry et al. (1996) have shown that the handful of 1
Jy BL Lacs with suitable data seem to show the opposite
trend, that is steeper spectra in brighter states (the excep-
tion,PKS2005−489, isanHBL).IfX-rayvariabilityinBL
Lacs is accompanied by a change in break frequency (e.g.,
George, Warwick & Bromage 1988: MKN 421; Sambruna
1994: S5 0716+714, PKS 2005−489), this behaviour is eas-
ilyexplained:whenanobjectbrightensandthebreakmoves
tohigherenergies,inHBLsthespectrumhardens,asshown
inFig.8forasinglesourceandinFigs.2and6fortheHBL
population. The opposite effect for RBLs can be explained
inthosecaseswheresynchrotronemissionintheX-rayband
isnot negligible in thehigh state: when thebreak movesto
higher energies, the steep synchrotron component moves in
the X-ray band therefore steepening the spectrum (Fig. 8).
Thiswouldbeinagreementwiththe(weak)correlation be-
tweenαxandνbreakforLBLs,discussedinSect.3.3(seeFig.
6).Figure8 also shows thataspectral variability similar to
that observed in the X-ray band for HBLs, that is flatter
spectraathigherstates, shouldbemirrored byLBLsinthe
infrared band,as in fact observed (e.g., Gear et al. 1986).
Ourresultsseemincontradiction withthoseofUrryet
al.(1996),whofoundthattheαxdistributionfor1JyRBLs
wasconsistentwiththatoftheEMSSXBLs.Asdiscussedin Figure 8.Theeffect of achange inbreakfrequencyon theflux
theIntroduction,however,mostpreviousworkonthestudy andspectralvariabilityofBLLacs:inHBLs(dashedlines),when
ofspectralindicesofBL Lacswaslimited bysmallerstatis- theobjectbrightens andthebreakmovestohigherenergies, the
X-ray spectrum hardens. In LBLs (solid lines), the opposite ef-
tics and the use of the RBL/XBL classification, which can
fectmightapply(i.e.,spectralsteepeningathigherfluxes)when
bemisleading. Inparticular,theresult ofUrryetal. (1996)
synchrotron emission in the X-ray band is not negligible in the
can be explained as follows: 1. three of the 1 Jy RBLs are
highstate.Incidentally,notealsothemuchlargerfluxvariability,
HBLs (S5 0716+714, MKN 501, PKS 2005−489), whose
at fixed radio variability, expected for LBLs in the optical band
inclusion in the RBL sample certainly shifted the αx dis- (paralleldottedlines),ascomparedtothatofHBLs,aseffectively
tribution to steeper values (the mean value of αx for these observed (e.g., Jannuzi et al. 1994). AsinFig. 7,the normaliza-
threeobjectsissignificantlydifferent,i.e.,steeper,thanthat tionsofthedifferentcurveshavebeenchosenonlyforillustrative
of the rest of the sample: see also Comastri et al. 1995 and purposesanddonotreflectintrinsicproperties.
Urryetal.1996);2.theEMSSBLLacs(allHBLs)haveval-
ues of fx/fr ∼>10−10.5 (Padovani & Giommi 1995a), partly our most important results, i.e. we find an anti-correlation
because, being a serendipitous survey, the EMSS excludes
between αx and νbreak, significant at the99.3 percent level
by definition well-know BL Lacs with considerable radio
(Spearman rank-ordertest),consistent with theonewe ob-
emission which have lower fx/fr values but more impor- tained for thewhole HBL sample.
tantly because objects with relatively low values of fx/fr Comastrietal.(1995) havefoundapositivecorrelation
have a small probability of being detected and large-area
betweenαxandνbreakintheirsampleoftwelve1JyBLLacs
X-raysurveysareneeded.(This interpretationis supported
(which includesnineLBLsandthreeHBLs).Thisisconsis-
by the fact that the Slew survey HBLs reach lower fx/fr tent with our results. If we consider all objects irrespective
values:seePerlmanetal.1995a).Giventheanti-correlation
oftheHBL/LBLclassification,infact,wealsodetectapos-
betweenαxandfx/fr(Fig.2),thisimpliesthatEMSSHBLs itive correlation between αx and νbreak, because on average
areboundtohaveαx flatterthanaverage,therebyreducing HBLshavesteeperspectra.Ourmuchbetterstatistics,how-
any difference between the two classes. In fact, if we com-
ever, allows us to examine this relation in more detail and
pareourLBLsamplewiththeEMSSHBLs,wealsofindno
to see that the dependence of αx on break frequency is dif-
significantdifferencebetweenthespectralindicesofthetwo
ferentforthetwoBLLacclasses,inparticularHBLsexhibit
samples.
a negative correlation between the two parameters.
One could argue that our sample is not complete in a
The main conclusions of this paper, which studies the
statistical sense. However,it isthelargest sample forwhich
ROSATX-rayspectraofabouthalftheknownBLLacs,can
homogeneous X-ray data are available and includes 50 per
be thussummarized:
cent of known BL Lacs. We needed a large enough sample
tomaximizethecoverageoftheparameterspacetobeable BLLacsarecharacterizedbyenergypower-lawspectral
to test for any differences in the X-ray spectra of the two indices ranging between ∼ 0 and 3, with an average value
BL Lac classes. Previous results, even those based on com- ∼ 1.4. The X-ray spectral properties of BL Lacs with high
plete(andthereforenotverylarge)samples,wereinfactnot (>1015Hz)frequencycutoffs(HBLs)aresignificantlydiffer-
∼
clear-cut. We are not aware of selection effects which could ent from those with lower frequency cutoffs (LBLs). HBLs
possibly alterourresults significantly.Furthermore,consid- have on average steeper X-ray spectral indices (αx ∼ 1.5)
ering only the complete sub-sample of 22 EMSS BL Lacs which correlates with the effective radio-X-ray and optical-
(all HBLs: Perlman et al. 1995b) we still reproduce one of X-rayspectralindices. Theoverall spectrumisconvexfrom
X-ray spectra of BL Lacs 9
radio to X-ray energies, and gets steeper with frequency. A MaraschiL.,FossatiG.,TagliaferriG.,TrevesA.,1995,ApJ,443,
strong anti-correlation is also present between X-ray spec- 578
tral index and break frequency, with sources having cutoffs Maraschi L., Ghisellini G., Tanzi E., Treves A., 1986, ApJ, 310,
atlargerenergiesdisplayingharderX-rayspectra.InLBLs, 325
Padovani P.,GiommiP.,1995a,ApJ,444,567
on the other hand, which have αx ∼ 1.1, the X-ray spec-
Padovani P.,GiommiP.,1995b,MNRAS,277,1477
trumshowsnocorrelationwiththeeffectivespectralindices
Padovani P.,UrryC.M.,1990,ApJ,356,75
andtheoptical-X-raycontinuumhasaconcaveshape.These
PerlmanE.S.etal.,1995a, ApJS,inpress
differences might also explain the peculiar X-ray spectral
PerlmanE.S.,StockeJ.T.,1993, ApJ,406,430
variability of HBLs and LBLs. Perlman E. S., Stocke J. T., Wang Q. D., Morris S. L., 1995b,
Thesefindingsstronglysupportthehypothesisthatthe ApJ,inpress
mainradiation mechanisminHBLsissynchrotronemission Rybicki G., Lightman A., 1979, Radiation Processes in Astro-
from theradio to theX-rayband,while in LBLs an inverse physics.Wiley,NewYork
ComptoncomponentdominatestheX-rayemission,andare SambrunaR.M.,1994,Ph.D.Thesis,SISSA
consistent with the view that the apparent dichotomy be- Sambruna R. M., Barr P., Giommi P., Maraschi L., Tagliaferri
G.,TrevesA.,1994,ApJ,434,468
tweenthetwoBLLacclassesismainlyduetodifferentcutoff
StarkA.A.,GammieC.F.,WilsonR.W.,BallyJ.,LinkeR.A.,
energies.Theywouldalsobeconsistentwiththepredictions
HeilesC.,HurwitzM.,1992,ApJS,79,77
of an inhomogeneous SSC model in which the main differ-
StickelM.,PadovaniP.,UrryC.M.,FriedJ.W.,Ku¨hrH.,1991,
ence is the orientation of the beaming axis, but this model
ApJ,374,431
appear not to be able to reproduce the observed large dif- Stocke J.T.,LiebertJ.,SchmidtG.,GioiaI.M.,Maccacaro T.,
ferences in cutoff energies. SchildR.E.,MaccagniD.,ArpH.C.,1985,ApJ,298,619
UrryC.M.,Padovani P.,1995,PASP,107,803
Urry C. M., Sambruna R. M., Worrall D. M., Kollgaard R. I.,
ACKNOWLEDGMENTS FeigelsonE.,PerlmanE.S.,StockeJ.T.,1996,ApJ,inpress
V´eron-CettyM.-P.,V´eronP.,1993, ACatalogueofQuasarsand
WethankFabrizioFiore,GabrieleGhisellini,andRitaSam- ActiveNuclei(6th ed.),ESOScientificReportn.13
bruna for useful discussions, and Eric Perlman and Meg WhiteN.E.,GiommiP.,AngeliniL.,1994,IAUC6100
Urry for providing us with results in advance of publica-
tion. This research has made use of the BROWSE pro- NOTE ADDED IN PROOF
gram developed bythe ESA/EXOSATObservatory and by
Afterthispaperwasacceptedforpublication,itcametoour
NASA/HEASARC. We acknowledge partial support from
knowledge that Lamer, Brunner & Staubert (1996), A&A,
ESA/ESRIN, Frascati, where part of this work was done,
inpress,haveindependentlyanalysedmost ofthedatapre-
whileP.G.was theESISseniorscientist andP.P.wasavis-
sented in this paper, reaching similar conclusions.
iting scientist.
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