High energy gamma-ray properties of the FR I radio galaxy NGC 1275

0.21 MB·English

by  Anthony M. Brown

Mon.Not.R.Astron.Soc.000,1–7(2011) Printed17January2011 (MNLATEXstylefilev2.2)
High energy γ-ray properties of the FR I radio galaxy
NGC 1275
1
Anthony M. Brown1⋆† and Jenni Adams1
1
0 1Department of Physics and Astronomy, University of Canterbury, Christchurch, 8140, NewZealand
2

n
a Accepted 2011January13.Received2011January12.Inoriginalform2010December15
J

3
1 ABSTRACT

] We report on our study of the high-energy γ−ray emission from the FR I radio
E galaxy NGC 1275, based on two years of observations with the Fermi-LAT detector.
H Previous Fermi studies of NGC 1275 had found evidence for spectral and flux vari-
. ability on monthly timescales during the first year of Fermi-LAT observations. This
h
variabilityisalsoseeninthelargertwoyeardataset,duringwhichweobservealarge
p
- γ−ray flare (June-August 2010). The increased photon statistics from this large flare
o have allowed the discovery of flux variability from NGC 1275 on the timescales of
r days. The largest flux variation observed during this flare being a factor of ∼ 3 from
t
s one day to the next and a resultant e-folding risetime of 1.51±0.2 days. The two
a
year averaged E >100 MeV γ−ray spectrum is adequately described by a power-law
[
spectrum, with a photon index, Γ, of 2.09±0.02, and a resultant integrated flux of
1 F =(2.2±0.1)×10−7 ph cm−2s−1. While no hysteresis was observedin the photon
γ
v index−flux (Γ vs F ) parameterspace,there was obvious‘harder-when-brighter’be-
γ γ
7 haviour observed during the large γ−ray flare. Furthermore, during this large flare,
8
NGC 1275 appeared to migrate from the FR I radio galaxy to the BL Lac object
6
region of the photon index−luminosity (Γ vs L ) paramater space. In this paper we
2 γ γ
present details of our Fermi-LAT analysis of NGC 1275, including a brief discussion
.
1 on its implications for γ−ray blazar sources.
0
1 Key words: galaxies: active – galaxies: individual (NGC 1275) – galaxies: jets –
1 gamma rays.
:
v
i
X
r
a 1 INTRODUCTION objects within theunified model of AGN (Fanaroff & Riley
(1974) and Urry & Padovani (1995)), though there is evi-
NGC1275, also knownasPerseusAand3C84,isanearby
dencethattheAGNunificationschememaybeoversimpli-
activegalaxylocatedatthecenterofthePerseuscluster,at
fied (e.g., Kharb et al. (2010)).
a redshift of z=0.0179 (Strauss et al. (1992)). Classified as
a FR I radio galaxy, NGC 1275 is extremely radio bright,
exhibiting clear structure of a compact central source and Surroundedbyextendedfilamentstructures,NGC2175
an extended jet (e.g. Vermeulenet al. (1994), Asada et al. hashistoricallybeenofgreatinterestduetobothitscentral
(2006)). VLBI and VOSP observations have shown NGC locationinthePerseusCluster,withthePerseusclusterbe-
1275’s jets to have an instrinsic velocity of 0.5 ± 0.09c ingthebrightestclusteratX-rayenergies(eg.Fabian et al.
and an orientation angle ≈ 30−55◦ (Walker et al. (1994) (2003)),andthepossible‘feed-back’roleNGC1275playsin
& Asada et al. (2009)). With a central supermassive black thePerseus cluster(eg.Gallagher (2009)).Evidencefor the
hole mass of 3.4 × 108 M⊙ (Wilman et al. (2005)), NGC ‘feed-back’ role of NGC 1275 can be seen in both ROSAT
1275 appears toexhibit jet precession (Dunnet al. (2006)), and Chandra observations where cavity, or bubble, struc-
which has been interpreted as a possible indication that tures are visible (Bohringer et al. (1993) and Fabian et al.
NGC 1275 is the result of a merger between two galaxies (2006)). These bubbles appear to be spatially coincident
(Liu & Chen (2007)). As a FR I radio galaxy, NGC 1275 withtheradiostructureextendingoutfromthecentralAGN
is believed to belong to the parent population of BL Lac engine.
⋆ E-mail:[email protected] NGC 1275 is also of interest due to its close proxim-
† Correspondingauthor ity to Earth affording us an excellent opportunity to study
2 Anthony M Brown
thephysicsofrelativisiticoutflows1.NGC1275wasfirstde-
tected as a source of high energy γ-rays by the Fermi-LAT
detector(Abdoet al.(2009a)).Duringthefirstfourmonths
ofall-sky-survey observations,takenfromAugusttoDecem-
ber 2008, NGC 1275 was found to have an average flux of
Fγ = (2.10 ±0.23) ×10−7 ph cm−2 s−1 above 100 MeV
and apower law photonindex of Γ=2.17±0.05. Whileno
variability was observed during these four monthsof obser-
vations,theFermi detectionofaγ−rayfluxabovetheupper
limit of F>100MeV < 3.72×10−8 ph cm−2 s−1 set by the
EGRET detector implied variability over larger timescales
of years or decades (Reimer et al. (2003)).
More recently, Kataoka et al. have found evidence for
monthly variations in the E > 100 MeV γ−ray flux from
NGC1275duringthefirstyearofFermi-LATobservations.
These observations also showed spectral variations during
periodsofhigherfluxwithahysteresisbehaviourintheflux-
photon indexparameter space (Kataoka et al. (2010)).It is
worth noting that during the first year of operations, NGC Figure 1. 100 MeV − 200 GeV γ-ray image centered on NGC
1275wascontinuallypresentintheFermi brightsourcecat- 1275(RA=49.951◦,DEC=41.512◦)withaROIof8◦,smoothed
witha3◦ Gaussian.
alogue (Abdoet al. (2009d) & Abdoet al. (2009e)).
At TeV γ-ray energies, upper limits for NGC 1275
above 100 GeV have been set by both the VERITAS
from the Earth and rocks ∼ 32◦ north and south of its or-
Cherenkov telescope array (Acciari et al. (2009)) and the
bitalplane.Thisrockingmotion,coupledwithFermi-LAT’s
MAGIC Cherenkov telescope array (Aleksic et al. (2010)).
largeeffectivearea,allowsFermi toscantheentireγ-raysky
These upper limits showed a deviation from extrapolating
every 3 hours(Ritz (2007)).
thepowerlawspectrumatMeV/GeVenergiesdowntoTeV
The observations used here comprise of all all-sky-
energies. However duringthepreparation of this paper, the
survey data taken between 4 August 2008 and 30 Septem-
MAGIC collaboration has recently announced the detec-
ber 2010, equating to a MET interval of 239557417 to
tion of NGC 1275 above 100 GeV with a significance of
307553159. After applying a zenith cut of 105◦ to elimi-
5.2σ at approximately 1% of the Crab flux at TeV energies
nate γ-ray photons from the Earth’s limb, all ‘diffuse’ class
(Mariotti et al. (2010)).
events were considered in the 100 MeV to 200 GeV en-
Thispaperinvestigatesthehigh-energyγ−rayfluxand
ergy range. Throughout thisanalysis, ScienceTools version
spectralpropertiesofNGC1275duringthefirsttwoyearsof
v9r15p2wereusedinconjunction with instrumentresponse
Fermi-LATobservations.Inparticular,theincreasedphoton
functions (IRF) P6 V3. Furthermore, a ΛCDM cosmology
statistics associated with a large γ-ray flare event, allow us
to probe flux and spectral variability from NGC 1275 with was adopted, with a Hubble constant of H0 = 71 km s−1
unprecedented temporal resolution. In §2 we describe the Mpc−1, Ωm =0.27 and ΩΛ =0.73.
Fermi-LATobservationsand dataanalysis routines usedin
this study. The results of the analysis are given in §3, with
the discussion in the context of AGN unification and TeV
γ-rayemission,givenin§4.Theconclusionsaregivenin§5. 3 RESULTS
3.1 Sky Map
The 100 MeV < E < 200 GeV sky map of NGC 1275 can
2 FERMI-LAT OBSERVATIONS be seen in Figure 1. Centered on NGC 1275 (RA=49.951◦,
The LAT detector aboard Fermi, described in detail in DEC=41.512◦), with a radius of interest (ROI) of 8◦, Fig-
Atwood et al. (2009), is a pair-conversion telescope, cover- ure 1 has also been smoothed with a 3◦ Gaussian func-
ingaphotonenergyrangefrombelow20MeVtoabove300 tion. The brightest source in this field of view is located
GeV. With a large field of view, ≃ 2.4 sr, improved angu- at RA=49.951◦, DEC=41.533◦, and as such, is positionally
lar resolution, ∼ 0.6◦ at 1 GeV, and large effective area, coincident with NGC 1275. It should be noted that the ex-
∼ 8000 cm2 on axis at 1 GeV, Fermi-LAT provides an or- tended structure towards the upper left of the field of view
derofmagnitudeimprovementinperformancecomparedto is believed to be diffuse γ-ray emission from the Galactic
previous γ-ray missions. plane (Abdoet al. (2009a)).
Since the 4th of August 2008, the vast majority of ob-
servations completed by Fermi havebeen performed in all-
sky-survey mode.Inall-sky-survey modeFermi pointsaway
3.2 Lightcurve
ToinvestigatetemporalvariabilityinNGC1275’sγ-rayflux,
1 AssumingH0=71kms−1Mpc−1,NGC1275hasaluminosity an unbinned maximum likelihood estimator, GTLIKE was
distanceof75.6Mpc.Atthisdistance1arcsecondcorrespondsto appliedtoall800MeV<E <200GeVdiffuseclassevents,
324parsecs. taking a ROI of 1◦ to minimise the contribution of soft γ-
High energy γ-ray properties of the FR I radio galaxy NGC 1275 3
1.2×10-7
1.0×10-7
8.0×10-8
)
1
-s
2
-m
c 6.0×10-8
h
p
(
x
u
Fl
4.0×10-8
2.0×10-8
0.0×100
54600 54700 54800 54900 55000 55100 55200 55300 55400 55500
MJD
Figure 2. The 2 year lightcurve of 800 MeV - 200 GeV γ-ray flux from NGC 1275 binned in week intervals, as detected by Fermi-LAT
withaROIof1◦.
rays from the Galactic ridge (see Section 3.1)2. Likewise, 3.3 Spectrum
only considering events with E > 800 MeV allows us to
To study NGC 1275’s average γ-ray spectrum during the
remove the majority of the contribution from the Galactic
two years of observation GTLIKE was applied to the 100
diffuseemission.TheP6 V3 DIFFUSEIRFwasapplied,as-
MeV < E < 200 GeV continuum, assuming a power law
sumingapowerlawspectrum,withanindexofΓ=2.17,as
sourcemodelwithadditionalGalacticandExtragalacticdif-
described in Abdoet al. (2009a). The resultant lightcurve
fusebackgroundcomponents.TakingaROIof8◦,weutilised
canbeseeninFigure2withatemporalresolutionofaweek
themostrecentGalactic (gll iem v02.fit)andExtragalactic
and statistical errors only. Figure 2 shows clear evidence
for variability on timescales of weeks, with a χ2 = 386 for diffuse(isotropic iem v02.txt)models3,with thenormalisa-
tion of these models free to vary.
112 d.o.f for a constant flux fit. Figure 2 also reveals that
Weobtain thefollowing best-fit power law function for
several major flaring events have occurred during the two
NGC 1275:
yearobserving period. Thefirst flare duringMarch to June
2009(54850<MJD<54950)haspreviouslybeendiscussed dN =(2.43±0.09)×10−9( E )−2.09±0.02
in Kataoka et al. (2010). However, a larger flaring event is dE 100 MeV
visible at the end of the observing period June to Septem-
ber 2010 (MJD > 55350), with a maximum weekly flux of −2 −1 −1
photons cm s MeV (1)
(9.03±1.22)×10−8 ph cm−2s−1, increasing by a factor of
2.3 from theprevious week. which equates to an integrated flux, in the 100 MeV − 200
GeV energy range, of
FE>100MeV =(2.2±0.1)×10−7 photonscm−2s−1 (2)
only taking statistical errors into account. From this power
2 ItshouldbenotedthatthelowerlimitontheROIsizeissetby
theangularresolutionperformanceoftheLATinstrument.At800
MeV, 68% of events are reconstructed within 1◦ (Atwoodetal. 3 Itshouldbenotedthattheisotropiciem v02.txtmodelfilealso
(2009)). modelsanyresidualinstrumentalbackground.
4 Anthony M Brown
10-8
1.8×10-7
10-9 1.6×10-7
1.4×10-7
1.2×10-7
10-10 -2-1m s) 1.0×10-7
-1eV) 10-11 Flux (ph c 68..00××1100--88
M
1 4.0×10-8
-2-m s 10-12 2.0×10-8
c 0.0×100
h
p -2.0×10-8
ux ( 10-13 55360 55370 55380 MJD 55390 55400 55410
Fl
Figure4.Lightcurveof800MeV-200GeVγ-rayphotonsfrom
NGC 1275 binned with daily temporal resolution, from June to
10-14 August2010,asdetected byFermi-LATwithaROIof1◦.
10-15 powerlawandexponentialcut-offpowerlaw fit.Assuch,it
is not possible to differentiate between the two models for
theaveraged two year spectrum.
10-16
102 103 104 105
Energy (MeV)
4 DISCUSSION
Figure 3. The 2 year averaged LAT spectrum, E > 100 MeV,
of NGC 1275. The solid line represents the power law function
4.1 Flux Variability
defined inEquation 1, with the dashed linerepresenting the ex-
ponential cut-off power law fit. With similar χ2 fits, it is not To investigate the major flare further, we rebinned the 800
possibletodifferentiatebetweenthetwomodels. MeV<E <200GeVfluxforJune−August2010withdaily
resolution,theresultsofwhichcanbeseeninFigure4.The
most rapid flux variation is seen at 55373 < MJD < 55376
law,thepredictedγ-raycountwas6844.1,withateststatis-
where the flux increased by a factor of ∼8 over a 3 day
tic4 of TS = 9459.9, corresponding to a ∼ 97σ detection.
period.
The Galactic diffuse background was found to have a nor-
To characterise the timescales of this flare event, we
malisation of 1.07±0.01 and a predicted count of 94496,
evalulate the time for an exponential flux increase or de-
while the Extragalactic diffuse background was found to
crease, referred to as the e−folding time, which is defined
haveanormalisation of0.77±0.02 andapredictedcountof
by:
22118.5.Thepredicteddiffusebackgroundcountscalculated
fromthetwoyeardatasetareconsistentwiththeseprevious F(t)=F(to)exp(τ−1(t−to)) (3)
works. However the photon index of NGC 1275 calculated
where τ is the characteristic e−folding timescale and F(t)
over the two year data set, Γ = (2.09±0.02), appears to
beslightly harderthosepreviously publishedin Abdoet al. and F(to) are the fluxes at time t and to respectively. The
variability observed during 55372<MJD <55380 equates
(2009a), Γ = (2.17±0.04), from four months of observa-
tions and Γ = (2.13±0.02), from one year of observations to an e−folding rise time, τrise, of 1.51±0.2 days and a
subsequente−foldingdecaytime,τ ,of2.54±0.31days.
(Kataoka et al. (2010)). decay
With observed flux variability on timescales of days, it
The best-fit power law function, given by Equation 1,
ismostlikely thattheobservedγ-rayvariability from NGC
can be seen in Figure 3, where the data points in Figure 3
1275 is associated with relativistic outflows in the form of
was obtained by applying GTLIKE separately to ten loga-
jets. This implies that relativistic effects such as Doppler
rithmic energy bands in the range 100 MeV to 102.4 GeV.
boosting of radiation density and temporal contraction of
In an attempt to account for the small deviation between
variability timescales will be observed. Taking the Doppler
the model and data above E = 20 GeV, the 100 MeV −
factoroftherelativisticjetintoconsideration,causalityim-
102.4GeVspectrumwasalsofitwithanexponentialcut-off
bpeoswte-firtlaswho(wdnNi/ndFEig∝urEe−3.ΓH×owexepv(e−r,Eit/sEhcouut−ldofbfe))n,owteitdhththaet pfalcietsort5h,aδt,ains reemlaistseidontoretghieonγ,−wriatyhvraardiiaubsi,liRty,,atnvadra, bDyo:ppler
the χ2 and resultant fit probability is similiar for both the R6ctvarδ(1+z)−1 (4)
Takingtvar tobethee-foldingrisetime,τrise=1.51±
4 Theteststatistic,TS,isdefinedastwicethedifferencebetween
thelog-likelihoodoftwodifferentmodels(2[logL−logL0]where
LandL0aredefinedasthelikelihoodwhenthesourceisincluded 5 δ=(Γ(1−βcosθ))−1 whereΓisthebulkLorentzfactorofthe
ornotrespectively)Mattoxetal.(1996). jet,β=v/candθ istheangletothelineofsight.
High energy γ-ray properties of the FR I radio galaxy NGC 1275 5
0.2daysconstrainstheemissionregiontoRδ−163.84×1013 2.2
meters.
By convolvingthislimit with that invokedbytheopti-
2.15 AA
caldepthoftheγ-rays,τγγ 61,weareabletoestimatethe
minimum Doppler factor, δmin (Dondi& Ghisellini (1995)
&Mattox et al.(1997)).Sincenomultiwavelength observa- 2.1
(tXCio-rhnausyrwaflzeuorxevateantkdaesnl.pde(cu2t0rri0un3mg))tv.haeIltuflesashr:oeαupl=edri0bo.ed65,nwaonetedadsFs5tukhmeaVte=aRrXc4hTµivEJay-l Photon Index 2.05 BB
ASM data was used to confirm that the 2−10 keV X-ray
2 DD CC
fluxlevelduringtheChurazovetal.observationswassimilar
tothe2−10keVfluxlevelduringtheflareperiodanalysed
here. 1.95
For tvar = 1.51 days and 10 GeV photons we obtain a
miminpilmiesuma mDaoxpipmleurmfaecmtoirssoifonδmriengi≈on2.raAdiDusopopfle∼rf8a×cto1r01o3fm2 1.9 1 1.5 2 2.5 3 3.5 4 4.5
or 2×10−3 pc,strongly suggesting that theinnerregion of Flux [10-7 ph cm-2 s-1]
NGC 1275 is the source of the high-energy γ-ray emission. Figure5.Fluxversusphotonindexparameterspaceforthefour
We note that a Doppler factor of 2 is consistent with epochs ‘A’, ‘B’, ‘C’ & ‘D’, assuming a power law spectrum, for
those derived for NGC 1275 via independent methods (eg. thelargeflareMJD>55200.
Lahteenmaki & Valtaoja(1999)),aswellasbeingconsistent
withtheDopplerfactorderivedforotherFermi detectedra-
quencypeakedBLLacobjects(HBL)(Abdoet al.(2010b)).
dio galaxies such as Cen A and M87 (Abdoet al. (2010c),
Furthermore,astudyoftheE >100MeVpropertiesofTeV-
Abdoet al.(2009b)).ItisworthmentioningthataDoppler
brightAGNhasfoundthatthemajorityofTeVAGNexhibit
factor of 2 is also consistent with broad-band modelling of
hard spectrum, with Γ < 2 (Abdoet al. (2009c)). At its
IntermediatefrequencyBLLacobjects(IBL),whicharebe-
peakflux,NGC1275hadaphotonindexofΓ=1.98±0.04,
lievedtobethebeamedcounterpartsofFRIradiogalaxies
hardeningfromtheΓ=2.13±0.06observedduringits‘qui-
within theunified model of AGN (eg. Reyers(2009)).
escent’ state. This would imply that during the flare event,
NGC 1275 migrated from a misaligned IBL to a HBL class
4.2 Spectral Variability of AGN. This migration has some interesting implications
for detecting NGC 1275 at higher energies. Kataoka et al.
Both Abdoet al. (2009a) and Kataoka et al. (2010) found
(2010)suggestedthatitisthe>GeV fluxthatisimportant
evidence for spectral variability in the E > 100 MeV
when triggering TeV observations of Fermi-LAT sources.
spectrumofNGC1275. Furthermore,Kataoka et al.(2010)
Thissuggestion appearstobeavalid onegiventhatduring
found evidence for hysteresis behaviour in the flux versus
NGC 1275’s bright flare, when the spectrum was hardest,
photon index parameter space during the April−May 2009
that is, when the GeV γ-ray flux was at its greatest, TeV
flare. To investigate whether this hysteresis behaviour is
γ-rayemissionwasdetectedbytheMAGICCherenkovtele-
present in the larger flare observed during June−August
scope array (Mariotti et al. (2010)).
2010, the MJD>55200 data was binned into four sepa-
rate epochs corresponding to periods before, during the
rise, at the peak and the decay of the flare event. These
4.3 High energy properties of FRI radio galaxies
epochs, denoted as A, B, C and D, correspond to the MJD
intervals (55200<MJD<55300), (55300<MJD<55372), Now that three near FR I radio galaxies, Cen A, M87
(55372<MJD<55421) and (55421<MJD<55468) respec- and NGC 1275, have been detected by both Fermi-LAT
tively. The unbinned maximum likelihood estimator was andgroundbasedCherenkovexperiments,somepreliminary
applied to each epoch, assuming a power law and the same comparisons can bemade (see Table 1).
diffuse emission models as before. The photon index and While the MeV/GeV flux for NGC 1275 and Cen A
subsequentfluxlevels for epoch A,B, C and D can beseen are similiar, the photon indices are quite different (Γ =
in Figure 5. 1.98 ± 0.04 compared to Γ = 2.75 ± 0.04) (Abdoet al.
While there is no evidence for hysteresis during the (2010e)). Meanwhile, M87 has a MeV/GeV flux that is an
larger July−August 2010 flare, the flux level is clearly cor- order of magnitude smaller than both NGC 1275 and Cen
related with the photon index, such that the photon in- A,butaphotonindexthatisharderthanCenAwhilebeing
dex becomes harder at higher flux levels (Γ ∝ −0.06 × softer thanNGC1275 (Abdoet al. (2010e)).Whilethereis
FE>100MeV).This‘harder-when-brighter’behaviouriswhat no obvious correlation between these observational charac-
one would expect in a simple acceleration and cooling sce- teristics, it is interesting to note that while M87 and NGC
nario (Kirk et al. (1998)) and hasbeen observed previously 1275 are knownvariable sources, Cen A does not appearto
with Fermi observations of other AGN; eg. PKS 1502+106 vary at E >100 MeV.
and PKS 1510-089 (Abdoet al. (2010a) & Abdoet al. All of these FR I radio galaxies have been predicted
(2010d)). to be strong sources of MeV−TeV γ-rays by a variety of
Morphological studies of Fermi detected blazars have emission models. One such model attributes the high en-
found the photon index to harden from 2.46 for Flat Spec- ergy emission to pulsar-like particle acceleration in a ro-
trum Radio Quasars to 2.13 for IBL to 1.86 for High fre- tating magnetosphere around the supermassive black hole
6 Anthony M Brown
a power law, with Γ = 2.09 ±0.02, though a power law
Table 1.Summaryofhighenergycharacteristics fornearbyFR
I radio galaxies. The Cen A and M87 MeV/GeV characteris- with exponential cut-off cannot be ruled out. This photon
ticsarefromAbdoetal.(2010e),withtheNGC1275MeV/GeV index is slightly harder than those of Kataoka et al. (2010)
characteristics taken during the June−August 2010 flare when and Abdo et al. (2009a), with thedifference most likely at-
TeV emission was detected. The MeV/GeV flux is in units of tributedtotheincrease in activityin thefull twoyeardata
photons cm−2 s−1.TheTeVfluxvalueforNGC1275wascalcu- set compared to theone yearand four month data sets.
ated as 1% of the Crab flux at TeV energies, with the TeV flux In general, the spectral variability of NGC 1275 ob-
unitsofcm−2 s−1.
served by Fermi raises interesting questions with regards
totheunificationmodelofAGN.Duringthelargeflare,the
γ-rayspectrumevolvedfrom IBLtoHBLvaluesforphoton
CenA M87 NGC1275
indices.Likewise,intheΓγ−Lγ parameterspace,NGC1275
redshift 0.0009 0.004 0.0179 migrated from the FR I radio galaxy to the BL Lac object
SMBHMass(M⊙) 5.5×107 3.2×109 3.4×108 region. Taking into consideration these two changes, it is
Flux(MeV/GeV) 2.14×10−7 2.45×10−8 4.1×10−7 nosurprisethatNGC1275hassubsequentlybedetectedat
Γ(MeV/GeV) 2.75 2.21 1.98 TeV energies, however, it raises the question as to why the
Flux(TeV) 2.45×10−13 1.5×10−12 ∼1×10−12 threenearFRIradiogalaxies,NGC1275,M87andCenA,
Γ(TeV) 2.7 2.6 −
alldetectedatbothMeV/GeVandTeVenergies,havesuch
logLγ (ergss−1) 41.13 41.67 44.62 different high energy properties.
and was successfully applied to the variable γ-ray flux of
ACKNOWLEDGMENTS
M87 (Neronov & Aharonian (2008) & Rieger & Aharonian
(2008)). In this model LIC ∝MBH is expected, where LIC This work is supported by theMarsden FundCouncil from
is the inverse Compton luminosity and MBH is the super- New Zealand Government funding, administered by the
massive black hole (SMBH) mass. For FR I radio galaxies, Royal Society of New Zealand. The authors would like to
the inverse Compton emission spans the MeV−TeV energy thankthereferee,RalphWijers,forhisusefulcommentsand
rangeandassuch,iftherotatingmagnetospheremodelwas suggestions. This work has made use of public Fermi data
applicable to this class of AGN, one would expect to see and RXTE-ASM data obtained from the High Energy As-
both the MeV/GeV and TeV γ-ray fluxes and luminosities trophysics Science Archive Research Center (HEASARC),
to be correlated to the SMBH mass. While this does ap- provided by NASAs Goddard Space Flight Center. This
peartoholdforthefluxatTeVenergies,itdoesnotforthe work has also made use of the NASA/IPAC Extragalactic
MeV/GeV flux. This would imply that the MeV/GeV and Database (NED), which is operated by the Jet Propulsion
TeVemission haveoriginsindifferentphysicalprocessesfor Laboratory,Caltech,undercontactwiththeNationalAero-
radio galaxies. nautics and Space Administration.
Finally, it is worth noting that FR I radio galaxies oc-
cupyadistinct region intheΓγ−Lγ parameterspacecom-
paredtoBLLacobjects(Abdoet al.(2010e)).Thisdivideis
REFERENCES
attributed to a difference in accretion rate (Ghisellini et al.
(2009)).Duringthelargeγ-rayflarepresentedinthispaper, Abdo,A.A., et al. 2009a, ApJ, 699, 31
NGC 1275 migrated from the FR I radio galaxy to the BL Abdo,A.A., et al. 2009b, ApJ,707, 55
Lac region of the Γγ −Lγ parameter space with ∆Γ ∼ 0.2 Abdo,A.A., et al. 2009c, ApJ, 707, 1310
and ∆log L∼0.62. If indeed, the difference in Γγ−Lγ pa- Abdo,A.A., et al. 2009d, ApJ,700, 597
rameterspaceisduetoaccretionrate,thentheNGC1275’s Abdo,A.A., et al. 2009e, ApJS, 183, 46
flaring eventsof June−August2010 could possibly beasso- Abdo,A.A., et al. 2010a, ApJ, 710, 810
ciatedwithanincreaseinaccretion,andassociatedincrease Abdo,A.A., et al. 2010b, ApJ,710, 1271
in thematter ejected along therelativistic jets. Abdo,A.A., et al. 2010c, ApJ, 719, 1433
Abdo,A.A., et al. 2010d, ApJ,721, 721
Abdo,A.A., et al. 2010e, ApJ, 720, 912
Acciari, V.A., et al., 2009, ApJ, 706L, 275
5 CONCLUSIONS
Aleksic, J., et al., 2010, ApJ, 710, 634A
Wehavereported on ourstudiesof NGC1275 utilising two Angel, J.R.P & Stockman, H.S.,1980 ARA&A,18, 321
years of all-sky-survey data from the Fermi-LAT detector. Asada, K., et al. 2006 PASJ,58, 261
ThesestudieshavefoundNGC1275toexhibitE >100MeV Asada, K., et al. 2009 ASPC, 402, 91A
fluxvariabilityontimescalesofdays,withτrise=1.51±0.2 Atwood, W.B., et al. 2009, ApJ, 697, 1071
days, during a large γ-ray flare in June-August 2010. This Bohringer et al. 1993, MNRAS,254, L25
largeflarealsosawNGC1275exhibitingspectralevolution, Churazov, E., et al. 2003, ApJ, 590, 225C
with a ‘harder-when-brighter’trend. This large flare period DondiL. & Ghisellini G., 1995, MNRAS,273, 583
co-incided with the MAGIC collaboration announcing the Dunn, R.J.H., Fabian, A.C., & Sanders, J.S., 2006, MN-
detectionofTeVemission fromNGC1275,givingweightto RAS,366, 758
the belief that it is the >GeV flux that is important when Fabian, A.C., et al. 2003, MNRAS,344, L43
triggering observations at very high energies. Fabian, A.C., et al. 2006, MNRAS,366, 417
The LAT spectrum of NGC 1275 is best described by Fanaroff, B.L., & Riley,J.M. 1974, MNRAS,167, 31P
High energy γ-ray properties of the FR I radio galaxy NGC 1275 7
Gallagher, J.S. 2009, AN 220, 1040G
Ghiselleni, G. et al. 2009, MNRAS,396, L105
Kataoka, J. et al. 2010, ApJ, 715, 554
Kharb, P., Lister, M.L., & Cooper, N.J., et al. 2010, ApJ,
710, 764K
Kirk, J.G., Gieger, F.M., & Mastichiasdis, A. 1998, A&A,
333, 452
Lahteenmaki, A., & Valtaoja, E. 1999, ApJ,521, 493
Liu, F.K. & Chen, X.2007, ApJ,671, 1272
Mariotti, M., et al. 2010 Astronomers Telegram #2916
Mattox J.R., et al. 1996, ApJ,461, 396
Mattox J.R., et al. 1997, ApJ,476, 692
Neronov, A., & Aharonian, F.A. 2008, ApJ,671, 85
Reimer, O., et al. 2003, ApJ, 588, 155
Reyers, L. 2009, ICRC Proceedings, arXiv:0907.5175
Rieger, F.M. & Aharonian, F.A 2008, A&A,479, L5
Ritz, S. 2007, Overview of GLAST Mission and Opportu-
nities, Vol. 921 (AIP),3-7
Strauss, M., et al. 1992, ApJS,83, 29S
Urry,C.M. & Padovani, P.1995, PASP,54, 215
Vermeulen, R.C., et al. 1994, ApJ, 430, L41
Walker, R.C., Romney, J.D., & Benson, J.M., 1994, ApJ,
430, L45
Wilman, R.J., Edge, A.C., & Johnstone, R.M., 2005, MN-
RAS,359, 755