Gamma-ray excess from a stacked sample of high- and intermediate-frequency peaked blazars observed with the MAGIC telescope

0.26 MB·English

by  MAGIC Collaboration

Gamma-ray excess from a stacked sample of high- and
intermediate-frequency peaked blazars observed with the MAGIC
telescope
1
1
J. Aleksi´ca, L. A. Antonellib, P. Antoranzc, M. Backesd, C. Baixerase, J. A. Barriof,
0
2 D. Bastierig, J. Becerra Gonz´alezh, W. Bednareki, A. Berdyuginj, K. Bergerj,
n E. Bernardinik, A. Bilandl, O. Blancha, R. K. Bockm,g, G. Bonnolib, P. Bordasn, D. Borla
a Tridonm, V. Bosch-Ramonn, D. Bosef,I. Braunl, T. Bretzo, D. Britzgerm, M. Camaraf,
J
E. Carmonam, A. Carosib, P. Colinm, S. Commichaul, J. L. Contrerasf, J. Cortinaa,
4
1 M. T. Costadoh,p, S. Covinob, F. Dazziq,∗, A. De Angelisq, E. De Cea del Pozor, R. De los
Reyesf,∗∗∗, B. De Lottoq, M. De Mariaq, F. De Sabataq, C. Delgado Mendezh,∗∗, M. Doertd,
]
E A. Dom´ınguezs, D. Dominis Prestert, D. Dornerl, M. Dorog, D. Elsaessero, M. Errandoa,
H D. Ferenct, M. V. Fonsecaf, L. Fonte,R. J. Garc´ıa Lo´pezh,p, M. Garczarczykh, M. Gaugh,
. N. Godinovict, D. Hadaschr, A. Herreroh,p, D. Hildebrandl, D. H¨ohne-Mo¨ncho,+, J. Hosem,
h
p D. Hrupect, C. C. Hsum, T. Joglerm, S. Klepsera, T. Kr¨ahenbu¨hll, D. Kranichl, A. La
o- Barberab, A. Lailleu, E. Leonardoc, E. Lindforsj, S. Lombardig,F. Longoq, M. Lo´pezg,
r E. Lorenzl,m, P. Majumdark, G. Manevav, N. Mankuzhiyilq, K. Mannheimo, L. Maraschib,
t
s M. Mariottig, M. Mart´ıneza, D. Mazina, M. Meuccic, J. M. Mirandac, R. Mirzoyanm,
a
[ H. Miyamotom, J. Mold´onn, M. Moless, A. Moralejoa, D. Nietof, K. Nilssonj, J. Ninkovicm,

3 R. Oritom, I. Oyaf, S. Paianog, R. Paolettic, J. M. Paredesn, S. Partinic, M. Pasanenj,
v D. Pascolig, F. Paussl,R. G. Pegnac, M. A. Perez-Torress, M. Persicq,w, L. Peruzzog,
1
F. Pradas, E. Prandinig, N. Puchadesa, I. Puljakt, I. Reichardta, W. Rhoded, M. Rib´on,
5
9 J. Ricox,a, M. Rissil, S. Ru¨gamero, A. Saggiong, T. Y. Saitom, M. Salvatib,
2 M. S´anchez-Condes, K. Sataleckak, V. Scalzottog,V. Scapinq, C. Schultzg, T. Schweizerm,
.
2 M. Shaydukm, S. N. Shorey, A. Sierpowska-Bartosiki, A. Sillanpa¨¨aj, J. Sitarekm,i,
0
D. Sobczynskai, F. Spaniero, S. Spirob, A. Stamerrac, B. Steinkem, J. C. Struebigo,
0
1 T. Surict, L. Takaloj, F. Tavecchiob, P. Temnikovv, T. Terzict, D. Tescaroa, M. Teshimam,
v: O. Tibollao, D. F. Torresx,r, H. Vankovv, R. M. Wagnerm, Q. Weitzell, V. Zabalzan,
i F. Zandanels, R. Zanina
X
r
a
ABSTRACT
Between2004and2009asampleof28X-rayselectedhigh-andintermediate-frequencypeaked
blazars with a X-ray flux larger than 2µJy at 1keV in the redshift range from 0.018 to 0.361
was observed with the MAGIC telescope at energies above 100GeV. Seven among them were
detected and the results of these observations are discussed elsewhere. Here we concentrate on
the remaining 21 blazars which were not detected during this observation campaignand present
the 3sigma (99.7%) confidence upper limits on their flux. The individual flux upper limits lie
between1.6%and13.6%oftheintegralfluxfromtheCrabNebula. Applyingastackingmethod
tothesampleofnon-detectionswithatotalof394.1hoursexposuretime,wefindevidenceforan
excesswithacumulativesignificanceof4.9standarddeviations. Itisnotdominatedbyindividual
objects or flares, but increases linearly with the observation time as for a constant source with
an integral flux level of ∼1.5% of that observed from the Crab Nebula above 150GeV.
1
Subject headings: BL Lacertae objects: general — gamma rays: observations
1. INTRODUCTION 2200m a.s.l. The observations refered to in this
study were obtained during the years 2004 - 2009
MAGIC (Major Atmospheric Gamma-ray
when MAGIC was stilla single-dishtelescope. Its
Imaging Cherenkov) is currently a system of two 234m2tessellatedparabolicmirrorallowsobserva-
17m telescopes located atop the Roque de los tionsofVHE(VeryHighEnergy)γ-raysbetween
Muchachos on the Canary Island of La Palma at
∼50GeV and 10TeV.
One key goal of the MAGIC telescope project
aIFAE, Edifici Cn., Campus UAB, E-08193 Bellaterra,
is to determine the properties of extragalactic
Spain
bINAF National Institute for Astrophysics, I-00136 VHE sources, among which the high-frequency
Rome,Italy peaked BL Lacertae objects are the most numer-
cUniversit`a di Siena, and INFN Pisa, I-53100 Siena, ous. Blazarsareasubclassofradio-loudAGNand
Italy belong to the most extreme and powerful objects
dTechnischeUniversit¨atDortmund,D-44221Dortmund, in the universe. They are characterizedby a non-
Germany
thermal broad-band continuum emission which is
eUniversitat Aut`onoma de Barcelona, E-08193 Bel-
highly variable on time scales from years down
laterra,Spain
fUniversidadComplutense,E-28040Madrid,Spain to minutes (Albert et al. 2007; Aharonian et al.
gUniversit`adiPadovaandINFN,I-35131Padova,Italy 2007a).
hInst. de Astrof´ısica de Canarias, E-38200 La Laguna, The spectral energy distribution (SED) of
Tenerife,Spain blazars is characterized by two bumps in a νF
ν
iUniversityofL ´od´z,PL-90236Lodz, Poland representation. The first component peaks at
jTuorlaObservatory,UniversityofTurku,FI-21500Pi- energies between IR and hard X-rays, and is
ikki¨o,Finland
assumed to originate from leptonic synchrotron
kDeutsches Elektronen-Synchrotron (DESY), D-15738
radiation. The maximum of the second peak
Zeuthen, Germany
lETHZurich,CH-8093Switzerland lies in the γ-ray energy regime. The origin of
mMax-Planck-Institut fu¨r Physik, D-80805 Mu¨nchen, this peak can be explained by different and par-
Germany tially concurring models either relying on inverse
nUniversitat de Barcelona (ICC/IEEC), E-08028 Compton scattering of electrons (Maraschi et al.
Barcelona,Spain 1992; Dermer & Schlickeiser 1993; Sikora et al.
oUniversit¨atWu¨rzburg,D-97074Wu¨rzburg,Germany
1994) or proposing hadronic interactions inside
pDepto. de Astrofisica, Universidad, E-38206 La La-
the jet (Mannheim 1993; Muecke & Protheroe
guna,Tenerife,Spain
2001). In the case the synchrotron peak oc-
qUniversit`adiUdine,andINFNTrieste,I-33100Udine,
Italy curs at energies above ∼ 1016.5Hz, (according
rInstitut de Ci`encies de l’Espai (IEEC-CSIC), E-08193 to Nieppola et al. 2006) these blazars are called
Bellaterra,Spain HBLs (high-frequency peaked BL Lacertae ob-
sInst. de Astrof´ısica de Andaluc´ıa (CSIC), E-18080 jects) and for peak energies of ∼ 1014.5−16.5Hz
Granada,Spain
IBLs (intermediate BL Lacertae objects).
tCroatian MAGIC Consortium, Institute R. Boskovic,
AsofApril2010,altogether29blazarswerees-
UniversityofRijekaandUniversityofSplit,HR-10000Za-
greb,Croatia tablished as VHE sources (24 of them HBLs in-
uUniversityofCalifornia,Davis,CA-95616-8677,USA cludingM87as’misaligned’blazar)1,comparedto
vInst. for Nucl. Research and Nucl. Energy, BG-1784 six HBLs, when the MAGIC telescope began its
Sofia,Bulgaria regular observationsin December 2004. The sam-
wINAF/Osservatorio Astronomico and INFN, I-34143
plepresentedherecomprises21X-rayselectedob-
Trieste,Italy
jects which were not detected in the VHE regime
xICREA,E-08010Barcelona,Spain
prior to the MAGIC observations. Nine of the
yUniversit`adiPisa,andINFNPisa,I-56126Pisa,Italy
*supportedbyINFNPadova objects were already observed between December
**now at: Centro de Investigaciones Energ´eticas, 2004 and February 2006 and the upper limits of
Medioambientales yTecnolo´gicas these observations are reported in Albert et al.
***nowat: Max-Planck-Institutfu¨rKernphysik,D-69029
Heidelberg,Germany 1cf. http://wwwmagic.mppmu.mpg.de/∼rwagner/sources/
+correspondence: [email protected] foranup-to-datelist.
2
(2008a). As therehavebeenimprovementswithin strong attenuation of γ-rays by limiting the red-
the MAGIC analysis, the data of these objects shift range. According to Kneiske & Dole (2010),
werere-analyzedandthenewresultsarepresented at a redshift of z = 0.4, the expected cutoff en-
inthiswork. Sincenosignificantdetectionwasat- ergy lies well above 200GeV, allowing MAGIC to
tained, upper limits on a 3σ (99.7%) confidence observe still with its highest sensitivity. There-
level will be presented. fore all objects with a maximum redshift z = 0.4
None of the observed sources showed any vari- where considered. The energy threshold of the
ability on diurnal timescales in the VHE regime. observations increases with the zenith distance θ.
Assumingapositivedetectioninthecaseofaflar- Accounting for this effect, the selection of sources
ing state, the observations presented here provide with higher θ (30◦ < θ < 45◦)during culmination
a means of investigating the baseline emission of should be limited to z < 0.15. The increasing ef-
these objects. Therefore, a stacking method ap- fect of EBL absorption should, however, imprint
pliedtotheblazarsamplecanrevealsuchanemis- itself by a net steepening on the spectrum of the
sionbelow the sensitivity limitfor eachindividual stacked excess.
object. Together with VERITAS (Benbow 2009) All criteria are described in detail below. They
this is the second stacking analysis which turns have been chosen to enhance the probability to
out to be successful in the VHE γ-ray regime. detect the sources,hence we selected objects with
Former experiments like HEGRA failed in de- high fluxes and inverse Compton peaks as well as
tecting a significant signal in a stacking analysis allowingforthelowestpossibleenergiestobemea-
due to their limited sensitivity (cf. for instance sured with MAGIC.
Mannheim et al. 1996). Compared to Albert et al. (2008a), the selec-
InSection2theselectioncriteriafortheobjects tion criteria have been extended. The reason is
will be presented. The observations and the data the enhancement of the sample by taking a wider
analysistechniquearedescribedinSection3. The redshift or zenith distance range into accountand
analysis results are shown in Section 4. Finally, a including sources whose fitted synchrotron peak
discussionofthe results andinherentimplications flux is high enough even if they show a lower X-
can be found in Section 5. ray flux level at 1keV. The sample is divided into
four parts:
2. BLAZAR SAMPLE
• I. X-ray selected HBLs obtained from
Weselectedblazarsfromthecompilationsfrom Donato et al.(2001)andCostamante & Ghisellini
Donato et al.(2001)andCostamante & Ghisellini (2002): (i) redshift z < 0.4, (ii) X-ray flux
(2002). Additionally, some objects were chosen F (1keV) > 2µJy, and (iii) zenith dis-
x
based on the synchrotron peak luminosity from tance θ < 30◦ during culmination. As-
Nieppola et al.(2006)andonefromthesedentary suming the same luminosities at 1keV as
survey by Giommi et al. (2005). at 200GeV (following the argumentation
The main selection criteria are the measured of Stecker et al. 1996)), the X-ray flux
X-ray flux at 1keV and the distance of the ob- Fx(1keV) = 2µJy corresponds to a γ-ray
jects. According to Stecker et al. (1996), the syn- fluxat200GeVof∼4.8·10−12ergcm−2s−1.
chrotron flux in the X-ray regime is connected to Thiscriterionappliesto15sourcesincluding
the flux in the VHE regime by ninesourcesalreadyobservedduringcycle1
ofregularMAGICobservations. Thesources
νXFX ∼νTeVFTeV , (1) are listed in Table 1.
assuming comparable synchrotron and Compton • II. TwoHBLsobtainedfromthesamecom-
peak luminosities. Therefore objects with high pilations taking a wider range in declina-
X-ray fluxes are promising candidates for TeV tion and a lower maximum of the redshift
emission. As the absorption of γ-rays within into account: 1ES 0033+59.5 and RXS
the extragalactic background light (EBL, see e.g. J1136.5+6737. Selection criteria: (i) red-
Kneiske & Dole 2010) is energy dependent, it is shift z < 0.15, (ii) X-ray flux F (1keV) >
x
particularlyimportantintheVHEregimetoavoid 2µJy, and (iii) θ <45◦ during culmination.
3
• III. Intermediate BL Lacs taken from order to get a well-balanced coverage inside the
Nieppola et al. (2006) with high peak lu- camera, the wobble position is changed regularly
minosities at the synchrotron peak. Se- to the opposite (with respect to the source po-
lection criteria: (i) redshift z < 0.4, (ii) sition). Signal and background events are then
synchrotron peak frequency ν > 2 · determined fromthe same showerimages with re-
peak
1015Hz, (iii) flux at the peak F > spect to the sourcepositionand to three symmet-
νpeak
10−11ergcm−2s−1, and (iv) zenith angle ric OFF positions, respectively, all at the same
θ <30◦duringculmination. Thisisvalidfor distance to the camera center. Part of the data
three sources: B2 1215+30,PKS 1424+240, of RX J0319.8+1845, 2E 1415.6+2557 and RX
and B3 2247+381. All of them can also J1725.0+1152 were taken in ON mode where the
be found in Donato et al. (2001) but with pointing position of the telescope is centered on
an X-ray flux at 1keV below 2µJy. B2 the object in the sky. For these observations ded-
1215+30 is listed there as LBL. In return icated OFF observations have been used for the
it is included in the TeV candidate list in background estimation.
Costamante & Ghisellini (2002). The data were processed with the software
package MARS (Bretz 2005) using an auto-
• IV. One HBL from the sedentary survey
mated analysis pipeline. Details can be found in
(Giommi et al. 2005) with the same selec-
Bretz & Wagner (2003), Bretz & Dorner (2008),
tion criteria as applied for point one of the
and Albert et al. (2008b). Furthermore, the ar-
sample: 1RXS J044127.8+150455.
rival time information of neighboring pixels was
taken into account (Aliu et al. 2009).
As several other blazars fulfilling these selection
criteria were already detected with MAGIC or For the separation of signal and background
otherVHEinstruments,apost-prioriselectionwas events, dynamic cuts on the distribution of im-
done using only the objects which were not yet age parameters are applied. The image parame-
detected in the VHE regime in advance of the ters are moments up to third order in the light
MAGIC observations leaving 21 objects as dis- distribution of the shower images (Hillas 1985).
cussed herein. All blazars in the MAGIC AGN The background suppression is done by means of
observationprogramthatfulfilltheseselectioncri- a parabolic cut in AREA (Riegel et al. 2005) and
teria either have been detected (or were known in a cut in ϑ2. The latter parameter is the squared
advance)ortheyarelistedhereasnon-detections. angular distance between the source position and
thereconstructedshowerorigindeterminedwitha
Table1listsallsourcesinthesamplewithrele-
refined DISP method (Lessard et al. 2001) taking
vant parameters. In case of multiple flux or spec-
into account the timing information of the show-
tral slope measurements the mean value is dis-
ers. Theϑ2-cutusedinthisanalysisisϑ2 <0.0196
played.
which is a somewhat smaller value than usually
used for the Crab Nebula, but provides a better
3. OBSERVATIONS AND DATA ANAL-
background rejection for weak point sources. The
YSIS TECHNIQUE
chosen value for ϑ2 corresponds to a signal region
The observations presented here were carried in the camera plane with a diameter of 2.8 cam-
out between December 2004 and April 2009 with era pixels. The optical point spread function of
atotalamountofobservationtime of490.0hours. the MAGIC telescope during the campaign was
After quality selection(removinglowquality data smallerthan16.0mmcorrespondingtoadiameter
runs from the analysis) 394.1 hours were used for of1.1pixels,wellwithinthisarea. Alargesample
the analysis or 18.8 hours per source on average. ofobjectsspanningalongtimeofobservationshas
Themainreasontodiscarddatafromtheanalysis to be treated with a robust analysis. The usage
is a low event rate after image cleaning which is of dynamic cuts provides such an analysis on the
primarily influenced by the weather conditions. expense of sensitivity (cf. Section 4.1).
Most of the data were taken in wobble mode. The statistical significance for any excess is
Inthismodethepointingpositionofthetelescope calculated from the ϑ2 distribution of signal
is displaced by 0.4◦ from the source position. In and background events making use of Eq. 17 in
4
Table 1
List of targets
Object Season z log(ν )a F b F c α c Cat.d Sel.
p νp X X
[µJy] crit.e
1ES 0033+595 08/2006– 07/2008 0.086f 18.9 2.0 5.66 – C†, N II
1ES 0120+340 08 – 09/2005 0.272 18.3 2.5 4.34 1.93 C, D†, G, N I
1ES 0229+200g 08 – 11/2006 0.140 19.5 1.6 2.88 – C†, N I,II
RX J0319.8+1845g 12/2004– 01/2006 0.190 17.0 0.4 1.76 2.07 D†, G, N I
1ES 0323+022 09 – 12/2005 0.147 19.9 6.3 3.24 2.46 C, D†, G, N I,II
1ES 0414+009g 12/2005– 01/2006 0.287 20.7 10.0 5.00 2.49 C, D†, G, N I
1RXS J044127.8+150455 10 – 12/2007 0.109 – – 4.74 2.10 G† IV
1ES 0647+250 02 – 03/2008 0.203f 18.3 3.2 6.01 2.47 C†, D, N I
1ES 0806+524g 10 – 12/2005 0.138 16.6 1.6 4.91 2.93 C, D†, N I,II
1ES 0927+500 12/2005– 02/2006 0.188 21.1 5.0 4.00 1.88 D†, G, N I
1ES 1011+496g 03 – 04/2006 0.212 16.7 1.3 2.15 2.49 C, D†, N I
1ES 1028+511 03/2007– 02/2008 0.361 18.6 1.3 4.42 2.50 C, D†, G, N I
RGB J1117+202 01/2007– 03/2008 0.140 – – 6.93 1.90 C†, D, G I,II
RX J1136.5+6737 02/2007 0.135 17.6 1.3 3.17 2.39 C, D†, G, N II
B2 1215+30 03/2007– 03/2008 0.237 15.6 1.3 1.59 2.65 C, D, N†h III
2E 1415.6+2557 04/2005– 04/2008 0.237 19.2 3.2 3.26 2.25 C, D†, G, N I
PKS 1424+240g 05/2006– 02/2007 0.160f 15.7 1.0 1.37 2.98 D, N†h III
RX J1725.0+1152 04/2005– 04/2009 0.018f 15.8 2.0 3.60 2.65 C, D†, N I,II
1ES 1727+502 05/2006– 05/2007 0.055 17.4 1.3 3.36 2.61 C, D†, N I,II
1ES 1741+196 07/2006– 04/2007 0.083 17.9 1.0 1.92i 2.04 C, D†, N I,II
B3 2247+381 08 – 09/2006 0.119 15.6 1.0 0.60 2.51 D, N†h III
aFitted peak frequency from Nieppola et al. (2006) in units of log(Hz).
bFlux at peak frequency extracted from Nieppola et al. (2006) in units of 10−11ergcm−2s−1.
cFlux and photon spectral index at 1keV.
dCompilation where the object appears (C: Costamante & Ghisellini (2002); D: Donato et al. (2001); N:
Nieppola et al. (2006); G: Giommi et al. (2005)). The catalog from which the object was selected is marked
with a dagger.
eSelection criteria which are met by the object.
fTentative redshift.
gKnown VHE blazar (as of April 2010) due to a detection after the MAGIC observation period.
hThe objects chosen from Nieppola et al. (2006) are also listed in Donato et al. (2001), but with a X-ray
flux lower than 2µJy.
iMean X-ray flux of multiple measurement in Donato et al. (2001) below 2µJy.
Note.—ListofobjectsinthesampleofX-rayselectedblazarswiththeirobservationtimewindows,redshifts
and X-ray measurements.
5
Li & Ma (1983). quality selection is t = 19.2h. The individual
exp
Concerning the stacking method as described valuesaswellasthecombinedresultcanbefound
in Sections 4.4 and 4.5, the ϑ2 distributions have intable2. Theenergyspectrumcanbefittedwith
been summed up to retrieve the stackedϑ2 signal a logparabola(accordingto Eq.2 inAlbert et al.
plot. The differentialenergyspectrumis thencal- 2008c) accounting for the flattening of the spec-
culatedfromallexcesseventsusingaveragevalues trum towards the inverse Compton peak:
fortheeffectivecollectionareaandaMonteCarlo
correction factor (spill-over correction), weighted dN E [a+blog10(E/300GeV)]
=f · (2)
each with the exposure time t . The same dE 0 (cid:18)300GeV(cid:19)
exp
methodhasbeenappliedtoadatasetoftheCrab
Nebula(cf.Section4.1)demonstratingitsfeasibil- with f0 =(5.37±0.11)·10−10TeV−1cm−2s−1,
ity. a = −2.20±0.05 and b = −0.11±0.03. The ϑ2
distribution and the energy spectrum have been
4. RESULTS OF THE MAGIC OBSER- calculated in the very same way as for the blazar
VATIONS samplebystackingthethreeindividualCrabNeb-
ula samples. The integral flux above 150GeV is
During the observation campaign no signifi- determined to
cant detection of any individual object could be F =(2.81±0.05)·10−10cm−2s−1. It will
>150GeV
achieved. The results can be found in Table 3. be used for comparison with the integral upper
None of the objects showed flaring activity in limits derived from the blazars. Figure 1 displays
the VHE band on a significant level on diurnal the energy spectrum of the stacked excess of the
timescales within the observation time windows. CrabNebulaincomparisontothepublishedspec-
Flaring activity is defined here as an offset of 3 trum. Theintegralfluxabove150GeVamountsto
standard deviations from the mean measured γ- 91% of that determined in Albert et al. (2008c).
rateforeachobject. However,fluxvariationsbya A comparison to previous measurements of ex-
factor three would still prevent an individual ob- periments like HEGRA, H.E.S.S or Whipple is
ject ofthe sample ofbeing detectedwith highsig- difficult because of the higher energy threshold
nificance. In this Section we present the upper of these measurements (above 400GeV). Due to
limits obtained for all 21 objects. the hardening of the Crab spectrum towards the
Three of the objects were partially observed peak below 100GeV a simple extrapolationof the
during anopticalhighstate within a targetof op- power-law spectra found there, overestimates the
portunity campaign. The trigger criterion was an flux at 150GeV leading to integral flux ratios of
increase in the optical flux of the core of more ∼70−−80%above150GeV(cf.Aharonian et al.
than 50%. The objects are 1ES 0033+595, RGB 2000, 2006; Grube 2007).
J1117+202,and B2 1215+30. Significant activity The standard MAGIC integral sensitivity is
or variability in the VHE γ-ray regime could not ∼1.6%oftheCrabNebulafluxabove280GeVfor
be detected. detectingasignalwith5σin50hours(Aliu et al.
2009). Includinglowerenergiesintheintegralsen-
4.1. Crab Nebula Observations sitivity determination, the value increases. The
analysispresentedinthisworkhasanintegralsen-
ForacomparativeanalysisasampleoftheCrab
sitivityabove150GeVof3.8%oftheCrabNebula
Nebula data has been usedspanning a time range
flux. This is mainly due to the long-term charac-
from Oct 2005 to Jan 2008. Three data sets
teristics of the observations, because the analysis
have been chosen to account for the three differ-
isaimedatarobustandconservativetreatmentof
enthardwareconditionsduringtheblazarobserva-
the data; in addition, also data before the instal-
tions: 300MHz readout system without and with
lation of the 2GHz system are considered, where
optical splitters and 2GHz readout system, later
thestandardMAGICsensitivityabove280GeVis
onreferredto as 300MHz, 300MHz and2GHz
OS
also less with ∼1.9% of the Crab Nebula flux.
systems, respectively. The θ distribution of the
subsamples have been matched to the one of the
blazar sample; the overall observation time after
6
Table 2
Observations of the Crab Nebula
Season FADC t θ Excess Backgr. Sign. E
exp thr
system [h] [◦] events events σ [GeV]
10/2005- 03/2006 300MHz 3.8 6 – 37 967 209 36.0 165
09/2006- 01/2007 300MHz 8.1 7 – 43 2086 523 51.0 165
OS
02/2007- 01/2008 2GHz 7.3 8 – 30 2133 455 53.5 165
Combined – 19.1 6 – 43 5188 1188 82.2 165
Note.—Observations of the Crab Nebula used for a performance test of the stacking
method and comparison to the flux upper limits of the blazars. The final spectrum (cf.
Eq. 2) is obtained as combination of all the subsamples.
are also comparedto the integral flux of the Crab
-1-2] sm 10-4 Nebula above the individual thresholds.
-1 V Theenergyestimationforeachsourcewasdone
E [Te 10-5 based on Monte Carlo simulated γ events follow-
N/d ing a power-law distribution with Γ = −3.0 for
d
a power law dN/dE ∝ EΓ. This was done in
10-6
order to fit better the average spectral slope for
the blazars in the VHE regime. For the integral
10-7
upper limit calculation the same input spectrum
(Γ = −3.0) was used. The resulting upper lim-
10-8 its vary between 1.6% and 13.6% of the Crab
102 103
E [GeV]
Nebula flux above the individual energy thresh-
old. The energy thresholds lie between 120GeV
Fig. 1.— Observations of three data sets of the Crab and 230GeV, due to differences in the θ distribu-
NebulabetweenOctober2005andJanuary2008. Thered, tions of the individual data samples. The Monte
blue and magenta colored data represent the data sets of Carlo simulations have been chosen to match ex-
the 300MHz, 300MHzOS and the 2GHz systems, respec-
actly the θ distribution of each data sample. The
tively. The black curve shows the combined energy spec-
trum obtained with the stacking method. For comparison resultsofthespectralanalysiscanbefoundinTa-
the published spectrum from Albertetal. (2008c) is plot- ble 3, too.
tedas dashedgreenline. Notethat thevertical errorbars
DiscoveryofVHEγ-raysfromRXJ0319.8+1845
arehiddenbythemarks.
and 1ES 0806+524 has recently been reported
by the VERITAS collaboration (Acciari et al.
4.2. Upper Limits 2009; Ong & Fortin 2009), as well as from PKS
1424+240 which was confirmed by the MAGIC
The upper limits (U.L.) onthe excess rates are
collaboration in a campaign independent of the
calculatedonaconfidencelevelof3σ (99.7%)us-
observations presented here (Acciari et al. 2010;
ing the method from Rolke et al. (2005). Integral
Teshima 2009). The measured VHE flux for
flux upper limits above a given energy are then
the latter source was significantly higher than
calculated from them. The integral flux for each
in previous observations with the MAGIC tele-
source is given above the energy threshold of the
scope. 1ES 0229+200 and 1ES 0414+009 have
analysis, which is defined as the maximum of the
differentialdistribution dN/dE vs E ofsimulated been detected by the H.E.S.S. telescope ar-
ray in 2006 (Aharonian et al. 2007b) and 2009
γ-showers surviving all cuts. The integral fluxes
7
Table 3
Results of the analysis
Object t θ Excess Backgr. Scale Sign. E a U.L. U.L.
exp thr
[h] [◦] events events σ [GeV] c.u.b f.u.c
1ES 0033+595 5.2 31 – 41 60.0 331.0 0.33 2.8 170 9.7 2.4
1ES 0120+340 10.7 6 – 18 20.7 437.3 0.33 0.9 120 8.2 3.1
1ES 0229+200 8.0 8 – 37 55.0 572.0 0.33 2.0 120 13.6 5.1
RX J0319.8+1845 11.2 10 – 31 –23.4 631.4 0.59 –0.7 120 1.6 0.6
1ES 0323+022 11.4 26 – 46 –45.3 751.3 0.33 –1.5 170 6.9 1.7
1ES 0414+009 18.2 28 – 36 71.3 1020.7 0.33 1.9 170 7.7 1.9
1RXS J044127.8+150455 26.9 13 – 36 18.3 1825.7 0.33 0.4 120 3.2 1.2
1ES 0647+250 29.2 3 – 32 64.3 1797.7 0.33 1.3 120 4.3 1.6
1ES 0806+524 17.5 24 – 36 17.0 752.0 0.33 0.5 140 7.2 2.2
1ES 0927+500 16.7 21 – 26 28.3 702.7 0.33 0.9 140 5.6 1.7
1ES 1011+496 14.5 21 – 29 89.0 590.0 0.33 3.1 140 6.9 2.1
1ES 1028+511 37.1 22 – 36 65.7 2312.3 0.33 1.2 140 3.3 1.0
RGB J1117+202 14.9 8 – 38 25.7 804.3 0.33 0.8 120 5.3 2.0
RX J1136.5+6737 14.8 39 – 46 22.7 954.3 0.33 0.6 230 5.7 0.9
B2 1215+30 16.1 1 – 41 119.0 995.0 0.33 3.2 120 9.3 3.5
2E 1415.6+2557 57.4 3 – 36 7.6 3805.4 0.54 0.1 120 3.5 1.3
PKS 1424+240 20.0 5 – 36 51.7 1210.3 0.33 1.3 120 8.2 3.1
RX J1725.0+1152 32.0 17 – 35 70.0 1859.0 0.38 1.4 140 4.2 1.3
1ES 1727+502 6.1 21 – 36 31.0 302.0 0.33 1.5 140 11.8 3.6
1ES 1741+196 11.8 9 – 40 98.7 731.3 0.33 3.1 120 9.6 3.6
B3 2247+381 8.3 10 – 36 21.7 490.3 0.33 0.8 140 5.2 1.6
aPeak response energy for a power law spectrum EΓ with Γ=−3.0
bIntegral flux above E given in units of the flux of the Crab Nebula (crab units, c.u.)
thr
cIntegral flux above E given in flux units f.u. = 10−11cm−2s−1
thr
Note.—Results of the analysis. The upper limits span a range of 1.6 - 13.6% of the Crab Nebula
flux above the corresponding energy threshold.
8
(Hofmann & Fegan 2009), respectively. However, Gaussiandistributionsthetestreturns3.42%and
since the observations presented here were each 77.03%forthecompatibilityoftheblazarandthe
performedin advanceofthe detections mentioned cross-check sample with the standard Gaussian,
above, the inclusion of these sources in the stack- respectively. The cross-check sample is ∼7 times
ing method is justified. The later detections show smallerthantheblazarsample,thussystematicef-
that the X-ray selection of possible targets is a fects in the analysis can only largely be ruled out
reasonable approach. as possible explanation for the shift in the blazar
In order to compare the measured integral distribution.
fluxes with the upper limits presented here they
4.4. Stacking Analysis
are extrapolated to the individual energy thresh-
olds asreportedinTable 3. Inallcasesexcept for
Even if none of the sources was detected in
PKS 1424+240 the upper limits are compatible
a single observation, a cumulative signal search
with the extrapolated reported integral fluxes. seems promising. For this reason the ϑ2-plots of
the individualanalyseshavebeenstackedproduc-
4.3. Significance Distribution
ing one plot for the whole set containing 394.1
Taking a look at the calculated significances of hours of data (cf. Section 3). Figure 3 shows the
theblazarsampleitisevidentthatmostofthein- result, a significance of 4.9 standard deviations
dividualobjectsshowpositivevalues. Plottingthe with 870 excess and 22876 background events.
distributionofthe significances,the meanvalueis About 30% of the stacked excess comes from
notlocatedat0 asexpectedfor skyregionswhere blazarsnow knownasVHE γ-rayemitters. With-
no γ-rays are expected to originate. out these sources the stacked excess amounts to
608 excess events with a significance of 3.8σ in-
In Figure 2 the significance distribution for the
dicating that there are other emitters contained
blazarsample is showntogetherwith the resultof
in the sample. Figure 4 underlines this finding.
a cross-check as described below. As the num-
As expected, the stacked ϑ2-plot of the cross-
ber of individual samples is different for both
check analysis containing no γ-signal gives a sig-
distributions they have been normalized to one.
nificance of −0.1 with −6 excess and 3009 back-
The blazar sample distribution has a mean value
ground events, the result is shown in Figure 3 as
of 1.23±1.17 while the cross-check sample has
well.
−0.08±0.85. This result can be expected due to
the fact that our sample is biasedby the selection
4.5. Energy Spectrum
toward potential VHE γ-ray emitters.
In order to test if the positive signal in the From the combined excess a differential energy
blazar sample originates from a systematic effect spectrum can be calculated. The differential en-
oftheobservationsoranalysischain,wealsocross- ergy spectrum dF/dE for one source is calculated
checkedthisresultwithdatasetsobtainedasOFF binwise by dividing the product of the number of
pointings associatedto different ON source obser- excess events Nexc,i and the spill overfactor ai by
vations not treated in this paper. These data sets the product of effective collection area Aeff,i and
weretakenunder similarconditionsasthe blazars exposure time texp. In order to derive an energy
coveringthe wholerangeofθ oftheblazarsample spectrum of the stacked excess, the mean values
and processed with the very same analysis chain. ofai andAeff,i weightedwiththeobservationtime
The OFF observations were analyzed in wobble have to be taken:
mode with respect to two fake source positions
a ·t
in the camera displaced by 0.4◦from the camera ha i = n i,n exp,n (3)
i
P t
center. Table 4 gives a list of these observations n exp,n
and results. Although the fit parametersof Gaus- hA i = nPAeff,i,n·texp,n (4)
sian fits to both distributions do not permit any eff,i P ntexp,n
conclusive statement, a Kolmogorov-Smirnovtest P
with n being the number of objects to be stacked
of the compatibility of the blazar with the cross-
and the energy bin i. The differential quotient
checksamplegivesaprobabilityof1.56%. Forthe
9
]0.35
u.
a.
[
s 0.3
t
n
u
o
C
0.25
0.2
0.15
0.1
0.05
0
-5 -4 -3 -2 -1 0 1 2 3 4 5
Significance
Fig. 2.—Significancedistributionsoftheblazar(red,hatcheduplefttolowright)andthecross-checksample(blue,hatched
low left to up right). The different distributions are normalized to one, so the vertical axis gives the percentage of the whole
blazar or cross-check sample, respectively. The blazar sample distributionhas a mean value of 1.23±1.17 and the cross-check
sample−0.08±0.85. ForcomparisonaGaussianwithmeanvalue0andstandarddeviation1isplottedasblackcurve.
dNi/dE for each bin can then be calculated as In Figure 5 the measured spectrum is shown.
dNi = nNexc,i,n·haii (5) 5. DISCUSSION
dE Pt ·hA i·∆E
n exp,n eff,i i
with the energyPbin width ∆E . The mean en- The positive mean significance distribution
i
ergy spectrum in the observer’s frame for all 21 indicates that the X-ray selected blazars stud-
blazars considered in the stacking analysis can be ied here constitute a fairly representative sam-
well described by a power law, ple of generic VHE emitters, as suggested by
Costamante & Ghisellini (2002). The recent dis-
dN 1 E −3.16±0.51
=(2.5±0.7)·10−11 · coveries of individual blazars from the sample in-
dE TeVcm2s (cid:18)E (cid:19)
0 deedcorroboratethisfinding. Thenextgeneration
(6)
of Cherenkov experiments – MAGIC-II, H.E.S.S.
with E = 200GeV. The differential flux at
0 2, and later on the Cherenkov Telescope Array
200GeV corresponds to 1.9% of the one of the
(CTA, Wagner et al. 2009) – will therefore have
Crab Nebula. The integral flux above 150GeV
good chances to detect an increasing fraction of
is determined as F = 4.3·10−12cm−2s−1 corre-
all known X-ray blazars.
spondingto1.5%oftheintegralCrabNebulaflux
above 150GeV. 5.1. Gamma-ray Background
On average, each blazar contributes with
At 200GeV, the attenuation caused by the
(2.1±0.3)/h excess events to the cumulative ex-
EBL is negligible, according to the model of
cess as is illustrated in Figure 4. The objects are
Kneiske & Dole (2010), so the calculation of
ordered in right ascension.
10