Fermi Large Area Telescope Observations of Markarian 421: The Missing Piece of its Spectral Energy Distribution

TLDR: In this paper, the gamma-ray activity of the high-synchrotron-peaked BL Lacertae object Markarian 421 (Mrk 421) during the first 1.5 years of Fermi operation was reported.

Abstract: We report on the gamma-ray activity of the high-synchrotron-peaked BL Lacertae object Markarian 421 (Mrk 421) during the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT) gamma-ray spectrum above 0.3 GeV can be well described by a power-law function with photon index Gamma = 1.78 +/- 0.02 and average photon flux F(>0.3 GeV) = (7.23 +/- 0.16) x 10(-8) ph cm(-2) s(-1). Over this time period, the Fermi-LAT spectrum above 0.3 GeV was evaluated on seven-day-long time intervals, showing significant variations in the photon flux (up to a factor similar to 3 from the minimum to the maximum flux) but mild spectral variations. The variability amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in gamma-rays measured by Fermi-LAT, and these two energy ranges are not significantly correlated. We also present the first results from the 4.5 month long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments that provided excellent temporal and energy coverage of the source throughout the entire campaign (2009 January 19 to 2009 June 1). During this campaign, Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broadband SED was reproduced with a leptonic (one-zone synchrotron self-Compton) and a hadronic model (synchrotron proton blazar). Both frameworks are able to describe the average SED reasonably well, implying comparable jet powers but very different characteristics for the blazar emission site.

Figures

Figure 2. Multifrequency light curves of Mrk 421 with seven-day-long time bins obtained with three all-sky-monitoring instruments: RXTE/ASM (2–10 keV, top), Swift/BAT (15–50 keV, second from top), and Fermi-LAT for two different energy ranges (0.2–2 GeV, third from top, and >2 GeV, bottom). The light curves cover the period from 2008 August 5 (MJD 54683) to 2009 March 12 (MJD 55248). Vertical bars denote 1σ uncertainties and horizontal error bars show the width of the time interval. The black dashed lines and legends show the results from constant fits to the entire data set. The vertical dashed lines denote the time intervals with the extensive multifrequency campaigns during the 2009 and 2010 seasons.

Figure 10. Expanded view of the high energy bump of the SED data and model presented in Figure 9.

Table 3 Parameter Values from the SPB Model Fit to the SED from Mrk 421 Shown in Figure 9

Figure 3. Multifrequency light curves of Mrk 421 with three-day-long time bins obtained with three all-sky-monitoring instruments: RXTE/ASM (2–10 keV, top), Swift/BAT (15–50 keV, second from top), and Fermi-LAT for two different energy ranges (0.2–2 GeV, third from top, and >2 GeV, bottom). The light curves cover the period from 2009 October 4 to 2010 March 12. Vertical bars denote 1σ uncertainties and horizontal error bars show the width of the time interval. The black dashed lines and legends show the results from constant fits to the entire data set. The vertical dashed lines denote the beginning of the extensive multifrequency campaign on Mrk 421 during the 2010 season.

Table 2 List of Instruments Participating in the Multifrequency Campaign and Used in the Compilation of the SED Shown in Figure 8

Figure 6. Time and energy coverage during the multifrequency campaign. For the sake of clarity, the minimum observing time displayed in the plot was set to half a day.

Full text

The Astrophysical Journal, 736:131 (22pp), 2011 August 1 doi:10.1088/0004-637X/736/2/131
C

2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
FERMI LARGE AREA TELESCOPE OBSERVATIONS OF MARKARIAN 421:
THE MISSING PIECE OF ITS SPECTRAL ENERGY DISTRIBUTION
A. A. Abdo
1,110
, M. Ackermann
2
, M. Ajello
2
, L. Baldini
3
, J . Ballet
4
, G. Barbiellini
5,6
, D. Bastieri
7,8
, K. Bechtol
2
,
R. Bellazzini
3
, B. Berenji
2
, R. D. Blandford
2
, E. D. Bloom
2
, E. Bonamente
9,10
, A. W. Borgland
2
, A. Bouvier
11
,
J. Bregeon
3
, A. Brez
3
, M. Brigida
12,13
,P.Bruel
14
, R. Buehler
2
, S. Buson
7,8
, G. A. Caliandro
15
, R. A. Cameron
2
,
A. Cannon
16,17
, P. A. Caraveo
18
, S. Carrigan
8
, J. M. Casandjian
4
, E. Cavazzuti
19
, C. Cecchi
9,10
,
¨
O. ¸Celik
16,20,21
,
E. Charles
2
, A. Chekhtman
22,110
, J. Chiang
2
,S.Ciprini
10
, R. Claus
2
, J . Cohen-Tanugi
23
, J . Conrad
24,25,111
, S. Cutini
19
,
A. de Angelis
26
, F. de Palma
12,13
, C. D. Dermer
27
, E. do Couto e Silva
2
, P. S. Drell
2
, R. Dubois
2
, D. Dumora
28
,
L. Escande
28,29
, C. Favuzzi
12,13
, S. J. Fegan
14
,J.Finke
27
, W. B. Focke
2
, P. Fortin
14
, M. Frailis
26,30
, L. Fuhrmann
31
,
Y. Fukazawa
32
,T.Fukuyama
33
,S.Funk
2
,P.Fusco
12,13
, F. Gargano
13
, D. Gasparrini
19
, N. Gehrels
16
,
M. Georganopoulos
21
, S. Germani
9,10
, B. Giebels
14
, N. Giglietto
12,13
,P.Giommi
19
, F. Giordano
12,13
, M. Giroletti
34
,
T. Glanzman
2
, G. Godfrey
2
,I.A.Grenier
4
, S. Guiriec
35
, D. Hadasch
15
, M. Hayashida
2
,E.Hays
16
, D. Horan
14
,
R. E. Hughes
36
,G.J
´
ohannesson
37
,A.S.Johnson
2
,W.N.Johnson
27
, M. Kadler
20,38,39,40
, T. Kamae
2
, H. Katagiri
32
,
J. Kataoka
41
,J.Kn
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odlseder
42
,M.Kuss
3
, J . Lande
2
, L. Latronico
3
,S.-H.Lee
2
,F.Longo
5,6
, F. Loparco
12,13
,B.Lott
28
,
M. N. Lovellette
27
, P. Lubrano
9,10
, G. M. Madejski
2
, A. Makeev
22,110
, W. Max-Moerbeck
43
, M. N. Mazziotta
13
,
J. E. McEnery
16,44
, J. Mehault
23
,P.F.Michelson
2
, W. Mitthumsiri
2
, T. Mizuno
32
, C. Monte
12,13
, M. E. Monzani
2
,
A. Morselli
45
, I. V. Moskalenko
2
, S. Murgia
2
, T. Nakamori
41
, M. Naumann-Godo
4
, S. Nishino
32
, P. L. Nolan
2
,
J. P. Norris
46
,E.Nuss
23
,T.Ohsugi
47
, A. Okumura
33
, N. Omodei
2
, E. Orlando
2,48
,J.F.Ormes
46
, M. Ozaki
33
,
D. Paneque
2,49
, J. H. Panetta
2
, D. Parent
22,110
, V. Pavlidou
43
, T. J. Pearson
43
, V. Pelassa
23
, M. Pepe
9,10
,
M. Pesce-Rollins
3
, M. Pierbattista
4
,F.Piron
23
, T. A. Porter
2
,S.Rain
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o
12,13
, R. Rando
7,8
, M. Razzano
3
, A. Readhead
43
,
A. Reimer
2,50
,O.Reimer
2,50
, L. C. Reyes
51
, J . L. Richards
43
,S.Ritz
11
, M. Roth
52
, H. F.-W. Sadrozinski
11
, D. Sanchez
14
,
A. Sander
36
,C.Sgr
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o
3
,E.J.Siskind
53
,P.D.Smith
36
, G. Spandre
3
, P. Spinelli
12,13
, Ł. Stawarz
33,54
, M. Stevenson
43
,
M. S. Strickman
27
,D.J.Suson
55
, H. Takahashi
47
, T. Takahashi
33
, T. Tanaka
2
, J. G. Thayer
2
, J. B. Thayer
2
,
D. J. Thompson
16
, L. Tibaldo
4,7,8,112
, D. F. Torres
15,56
, G. Tosti
9,10
, A. Tramacere
2,57,58
, E. Troja
16,113
,T.L.Usher
2
,
J. Vandenbroucke
2
, V. Vasileiou
20,21
, G. Vianello
2,57
, N. Vilchez
42
, V. Vitale
45,59
,A.P.Waite
2
,P.Wang
2
,
A. E. Wehrle
60
,B.L.Winer
36
,K.S.Wood
27
,Z.Yang
24,25
,Y.Yatsu
61
,
T. Ylinen
25,62,63
,J.A.Zensus
31
, M. Ziegler
11
(The Fermi-LAT Collaboration)
J. Aleksi
´
c
64
, L. A. Antonelli
65
, P. Antoranz
66
, M. Backes
67
, J . A. Barrio
68
, J. Becerra Gonz
´
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69,70
, W. Bednarek
71
,
A. Berdyugin
72
, K. Berger
70
, E. Bernardini
73
, A. Biland
74
, O. Blanch
64
,R.K.Bock
49
, A. Boller
74
, G. Bonnoli
65
,
P. Bordas
75
, D. Borla Tridon
49
, V. Bosch-Ramon
75
,D.Bose
68
, I . Braun
74
, T. Bretz
76
, M. Camara
68
, E. Carmona
49
,
A. Carosi
65
, P. Colin
49
, E. Colombo
69
, J. L. Contreras
68
, J . Cortina
64
, S. Covino
65
, F. Dazzi
26,114
, A. de Angelis
26
,
E. De Cea del Pozo
15
, C. Delgado Mendez
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, B. De Lotto
78
, M. De Maria
78
, F. De Sabata
78
, A. Diago Ortega
69,70
,
M. Doert
67
,A.Dom
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79
, D. Dominis Prester
80
, D. Dorner
74
, M. Doro
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, D. Elsaesser
76
, D. Ferenc
80
,
M. V. Fonseca
68
, L. Font
81
,R.J.Garc
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opez
69,70
, M. Garczarczyk
69
,M.Gaug
69
, G. Giavitto
64
, N. Godinovi
80
,
D. Hadasch
15
, A. Herrero
69,70
, D. Hildebrand
74
,D.H
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,J.Hose
49
, D. Hrupec
80
, T. Jogler
49
, S. Klepser
64
,
T. Kr
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uhl
74
, D. Kranich
74
,J.Krause
49
, A. La Barbera
65
, E. Leonardo
66
, E. Lindfors
72
, S. Lombardi
7,8
,M.L
´
opez
7,8
,
E. Lorenz
49,74
, P. Majumdar
73
, E. Makariev
82
, G. Maneva
82
, N. Mankuzhiyil
26
, K. Mannheim
76
, L. Maraschi
83
,
M. Mariotti
7,8
, M. Mart
´
ınez
64
, D. Mazin
64
, M. Meucci
66
, J . M. Miranda
66
, R. Mirzoyan
49
, H. Miyamoto
49
,J.Mold
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on
75
,
A. Moralejo
64
, D. Nieto
68
,K.Nilsson
84
,R.Orito
49
,I.Oya
68
, R. Paoletti
66
, J. M. Paredes
75
, S. Partini
66
, M. Pasanen
72
,
F. Pauss
74
, R. G. Pegna
66
, M. A. Perez-Torres
79
, M. Persic
26,85
, J. Peruzzo
7,8
, J. Pochon
69
, F. Prada
79
,
P. G. Prada Moroni
66
, E. Prandini
7,8
, N. Puchades
64
,I.Puljak
80
, T. Reichardt
64
, W. Rhode
67
,M.Rib
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o
75
,J.Rico
56,64
,
M. Rissi
74
,S.R
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76
, A. Saggion
7,8
, K. Saito
49
, T. Y. Saito
49
, M. Salvati
65
,M.S
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anchez-Conde
69,70
, K. Satalecka
73
,
V. Scalzotto
7,8
, V. Scapin
26
, C. Schultz
7,8
, T. Schweizer
49
, M. Shayduk
49
,S.N.Shore
3,86
, A. Sierpowska-Bartosik
71
,
A. Sillanp
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a
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a
72
, J. Sitarek
49,71
, D. Sobczynska
71
, F. Spanier
76
, S. Spiro
65
, A. Stamerra
66
, B. Steinke
49
, J. Storz
76
,
N. Strah
67
, J. C. Struebig
76
,T.Suric
80
, L. O. Takalo
72
, F. Tavecchio
83
,P.Temnikov
82
, T. Terzi
´
c
80
, D. Tescaro
64
,
M. Teshima
49
, H. Vankov
82
, R. M. Wagner
49
, Q. Weitzel
74
, V. Zabalza
75
, F. Zandanel
79
,R.Zanin
64
(The MAGIC Collaboration)
and
M. Villata
87
, C. Raiteri
87
, H. D. Aller
88
, M. F. Aller
88
,W.P.Chen
89
, B. Jordan
90
, E. Koptelova
89
,
O. M. Kurtanidze
91
,A.L
¨
ahteenm
¨
aki
92
, B. McBreen
17
, V. M. Larionov
93,94,95
,C.S.Lin
89
, M. G. Nikolashvili
91
,
R. Reinthal
72
, E. Angelakis
31
, M. Capalbi
19
, A. Carrami
˜
nana
96
, L. Carrasco
96
, P. Cassaro
97
, A. Cesarini
98
,
A. Falcone
99
,M.A.Gurwell
100
, T. Hovatta
92
, Yu. A. Kovalev
101
, Y. Y. Kovalev
31,101
, T. P. Krichbaum
31
,
H. A. Krimm
20,40
,M.L.Lister
102
, J . W. Moody
103
, G. Maccaferri
104
, Y. Mori
61
, I . Nestoras
31
, A. Orlati
104
,
C. Pace
103
, C. Pagani
105
, R. Pearson
103
, M. Perri
19
,B.G.Piner
106
,E.Ros
31,107
, A. C. Sadun
108
,
T. Sakamoto
16
, J . Tammi
92
, and A. Zook
109
1

The Astrophysical Journal, 736:131 (22pp), 2011 August 1 Abdo et al.
1
National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USA
2
W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator
Laboratory, Stanford University, Stanford, CA 94305, USA; dpaneque@mppmu.mpg.de, anita.reimer@uibk.ac.at
3
Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy
4
Laboratoire AIM, CEA-IRFU/CNRS/Universit
´
e Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif sur Yvette, France
5
Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy
6
Dipartimento di Fisica, Universit
`
a di Trieste, I-34127 Trieste, Italy
7
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy
8
Dipartimento di Fisica “G. Galilei,” Universit
`
a di Padova, I-35131 Padova, Italy
9
Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, I-06123 Perugia, Italy
10
Dipartimento di Fisica, Universit
`
a degli Studi di Perugia, I-06123 Perugia, Italy
11
Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz,
Santa Cruz, CA 95064, USA
12
Dipartimento di Fisica “M. Merlin” dell’Universit
`
a e del Politecnico di Bari, I-70126 Bari, Italy
13
Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
14
Laboratoire Leprince-Ringuet,
´
Ecole polytechnique, CNRS/IN2P3, Palaiseau, France
15
Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain
16
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
17
University College Dublin, Belfield, Dublin 4, Ireland
18
INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, Italy
19
Agenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), Italy
20
Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
21
Department of Physics and Center for Space Sciences and Technology, University of Maryland Baltimore County, Baltimore,
MD 21250, USA; georgano@umbc.edu
22
College of Science, George Mason University, Fairfax, VA 22030, USA
23
Laboratoire de Physique Th
´
eorique et Astroparticules, Universit
´
e Montpellier 2, CNRS/IN2P3, Montpellier, France
24
Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden
25
The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, SE-106 91 Stockholm, Sweden
26
Dipartimento di Fisica, Universit
`
a di Udine and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Gruppo Collegato di Udine, I-33100 Udine, Italy
27
Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA; justin.finke@nrl.navy.mil
28
Universit
´
e Bordeaux 1, CNRS/IN2p3, Centre d’
´
Etudes Nucl
´
eaires de Bordeaux Gradignan, 33175 Gradignan, France
29
CNRS/IN2P3, Centre d’
´
Etudes Nucl
´
eaires Bordeaux Gradignan, UMR 5797, Gradignan, 33175, France
30
Osservatorio Astronomico di Trieste, Istituto Nazionale di Astrofisica, I-34143 Trieste, Italy
31
Max-Planck-Institut f
¨
ur Radioastronomie, Auf dem H
¨
ugel 69, 53121 Bonn, Germany
32
Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
33
Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
34
INAF Istituto di Radioastronomia, 40129 Bologna, Italy
35
Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899, USA
36
Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA
37
Science Institute, University of Iceland, IS-107 Reykjavik, Iceland
38
Dr. Remeis-Sternwarte Bamberg, Sternwartstrasse 7, D-96049 Bamberg, Germany
39
Erlangen Centre for Astroparticle Physics, D-91058 Erlangen, Germany
40
Universities Space Research Association (USRA), Columbia, MD 21044, USA
41
Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo, 169-8555 Japan
42
Centre d’
´
Etude Spatiale des Rayonnements, CNRS/UPS, BP 44346, F-30128 Toulouse Cedex 4, France
43
Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
44
Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
45
Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata,” I-00133 Roma, Italy
46
Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
47
Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
48
Max-Planck Institut f
¨
ur extraterrestrische Physik, 85748 Garching, Germany
49
Max-Planck-Institut f
¨
ur Physik, D-80805 M
¨
unchen, Germany
50
Institut f
¨
ur Astro- und Teilchenphysik and Institut f
¨
ur Theoretische Physik, Leopold-Franzens-Universit
¨
at Innsbruck, A-6020 Innsbruck, Austria
51
Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
52
Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
53
NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA
54
Astronomical Observatory, Jagiellonian University, 30-244 Krak
´
ow, Poland
55
Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA
56
Instituci
´
o Catalana de Recerca i Estudis Avan¸cats (ICREA), Barcelona, Spain
57
Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy
58
INTEGRAL Science Data Centre, CH-1290 Versoix, Switzerland
59
Dipartimento di Fisica, Universit
`
a di Roma “Tor Vergata,” I-00133 Roma, Italy
60
Space Science Institute, Boulder, CO 80301, USA
61
Department of Physics, Tokyo Institute of Technology, Meguro City, Tokyo 152-8551, Japan
62
Department of Physics, Royal Institute of Technology (KTH), AlbaNova, SE-106 91 Stockholm, Sweden
63
School of Pure and Applied Natural Sciences, University of Kalmar, SE-391 82 Kalmar, Sweden
64
Institut de F
´
ısica d’Altes Energies (IFAE), Edifici Cn, Universitat Aut
`
onoma de Barcelona (UAB), E-08193 Bellaterra (Barcelona), Spain; diegot@ifae.es
65
INAF National Institute for Astrophysics, I-00136 Roma, Italy
66
Universit
`
a di Siena and INFN Pisa, I-53100 Siena, Italy
67
Technische Universit
¨
at Dortmund, D-44221 Dortmund, Germany
68
Universidad Complutense, E-28040 Madrid, Spain
69
Instituto de Astrof
´
ısica de Canarias, E38205 - La Laguna (Tenerife), Spain
70
Departamento de Astrofisica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
2

The Astrophysical Journal, 736:131 (22pp), 2011 August 1 Abdo et al.
71
University of Ł
´
od
´
z, PL-90236 Ł
´
od
´
z, Poland
72
Tuorla Observatory, University of Turku, FI-21500 Piikki
¨
o, Finland
73
Deutsches Elektronen Synchrotron DESY, D-15738 Zeuthen, Germany
74
ETH Zurich, CH-8093 Zurich, Switzerland
75
Universitat de Barcelona (ICC/IEEC), E-08028 Barcelona, Spain
76
Institut f
¨
ur Theoretische Physik and Astrophysik, Universit
¨
at W
¨
urzburg, D-97074 W
¨
urzburg, Germany
77
Centro de Investigaciones Energ
´
eticas, Medioambientales y Tecnol
´
ogicas (CIEMAT), Madrid, Spain
78
Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, and Universit
`
a di Trieste, I-34127 Trieste, Italy
79
Instituto de Astrof
´
ısica de Andaluc
´
ıa, CSIC, E-18080 Granada, Spain
80
Croatian MAGIC Consortium, Institute R. Bo
ˇ
skovi
´
c, University of Rijeka and University of Split, HR-10000 Zagreb, Croatia
81
Universitat Aut
´
onoma de Barcelona, E-08193 Bellaterra, Spain
82
Institute for Nuclear Research and Nuclear Energy, BG-1784 Sofia, Bulgaria
83
INAF Osservatorio Astronomico di Brera, I-23807 Merate, Italy
84
Finnish Centre for Astronomy with ESO (FINCA), University of Turku, FI-21500 Piikii
¨
o, Finland
85
INAF Osservatorio Astronomico di Trieste, I-34143 Trieste, Italy
86
Dipartimento di Fisica “Enrico Fermi,” Universit
`
a di Pisa, Pisa I-56127, Italy
87
INAF, Osservatorio Astronomico di Torino, I-10025 Pino Torinese (TO), Italy
88
Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042, USA
89
Graduate Institute of Astronomy, National Central University, Jhongli 32054, Taiwan
90
School of Cosmic Physics, Dublin Institute for Advanced Studies, Dublin, 2, Ireland
91
Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
92
Aalto University Mets
¨
ahovi Radio Observatory, FIN-02540 Kylmala, Finland
93
Isaac Newton Institute of Chile, St. Petersburg Branch, St. Petersburg, Russia
94
Pulkovo Observatory, 196140 St. Petersburg, Russia
95
Astronomical Institute, St. Petersburg State University, St. Petersburg, Russia
96
Instituto Nacional de Astrof
´
ısica,
´
Optica y Electr
´
onica, Tonantzintla, Puebla 72840, Mexico
97
INAF Istituto di Radioastronomia, Sezione di Noto,Contrada Renna Bassa, 96017 Noto (SR), Italy
98
Physics Department, National University of Ireland Galway, Ireland
99
Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
100
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
101
Astro Space Center of the Lebedev Physical Institute, 117997 Moscow, Russia
102
Department of Physics, Purdue University, West Lafayette, IN 47907, USA
103
Department of Physics and Astronomy, Brigham Young University, Provo, Utah 84602, USA
104
INAF Istituto di Radioastronomia, Stazione Radioastronomica di Medicina, I-40059 Medicina (Bologna), Italy
105
Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
106
Department of Physics and Astronomy, Whittier College, Whittier, CA, USA
107
Universitat de Val
`
encia, 46010 Val
`
encia, Spain
108
Department of Physics, University of Colorado, Denver, CO 80220, USA
109
Department of Physics and Astronomy, Pomona College, Claremont CA 91711-6312, USA
Received 2010 November 25; accepted 2011 May 19; published 2011 July 15
ABSTRACT
We report on the γ -ray activity of the high-synchrotron-peaked BL Lacertae object Markarian 421 (Mrk 421) during
the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope
(LAT) γ -ray spectrum above 0.3 GeV can be well described by a power-law function with photon index Γ = 1.78±
0.02 and average photon flux F (>0.3GeV)= (7.23 ±0.16) ×10
−8
ph cm
−2
s
−1
. Over this time period, the Fermi-
LAT spectrum above 0.3 GeV was evaluated on seven-day-long time intervals, showing significant variations in the
photon flux (up to a factor ∼3 from the minimum to the maximum flux) but mild spectral variations. The variability
amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in γ -rays
measured by Fermi-LAT, and these two energy ranges are not significantly correlated. We also present the first results
from the 4.5 month long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE,MAGIC,the
F-GAMMA, GASP-WEBT, and other collaborations and instruments that provided excellent temporal and energy
coverage of the source throughout the entire campaign (2009 January 19 to 2009 June 1). During this campaign,
Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides
an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broadband
SED was reproduced with a leptonic (one-zone synchrotron self-Compton) and a hadronic model (synchrotron
proton blazar). Both frameworks are able to describe the average SED reasonably well, implying comparable jet
powers but very different characteristics for the blazar emission site.
Key words: acceleration of particles – BL Lacertae objects: general – BL Lacertae objects: individual (Mrk 421) –
galaxies: active – gamma rays: general – radiation mechanisms: non-thermal
Online-only material: color figures
110
Resident at Naval Research Laboratory, Washington, DC 20375, USA.
111
Royal Swedish Academy of Sciences Research Fellow, funded by a grant
from the K. A. Wallenberg Foundation.
112
Partially supported by the International Doctorate on Astroparticle Physics
(IDAPP) program.
113
NASA Postdoctoral Program Fellow, USA.
1. INTRODUCTION
Blazars are active galaxies believed to have pairs of relativistic
jets flowing in opposite directions closely aligned to our line of
114
Supported by INFN Padova.
3

The Astrophysical Journal, 736:131 (22pp), 2011 August 1 Abdo et al.
sight. Their spectral energy distributions (SEDs) are dominated
by beamed jet emission and take the form of two broad non-
thermal components, one at low energies, peaking in the radio
through optical, and one at high energies, peaking in the γ -rays.
Some blazars have been well monitored for decades and along
a wide range of wavelengths. Although there is ample evidence
for the electron synchrotron origin of the low-energy bump, the
existing data do not allow an unambiguous identification of the
radiation mechanism responsible for the high-energy bump. One
reason for this is that the high-energy bump is poorly constrained
due to the lack of observations at energies between ∼0.1 MeV
and 0.3 TeV. This gap was filled to some extent by EGRET on
board the Compton Gamma-Ray Observatory (Hartman et al.
1999). However, its moderate sensitivity and limited observing
time precluded detailed cross-correlation studies between γ -ray
and lower-energy wavebands. On the other hand, the current
generation of TeV imaging atmospheric Cherenkov telescopes
(IACTs)—the High Energy Stereoscopic System, the Major
Atmospheric Gamma Imaging Cherenkov telescope (MAGIC),
and the Very Energetic Radiation Imaging Telescope Array
System, which have good sensitivity at energies as low as
0.1 TeV—did not start scientific operation until 2004, that is,
well after EGRET had stopped operating.
This has changed with the launch of the Fermi Gamma-ray
Space Telescope in 2008 June. In science operation since 2008
August, its Large Area Telescope (LAT) instrument (Atwood
et al. 2009) views the entire sky in the 20 MeV to greater
than 300 GeV range every three hours. The one-year First
LAT Active Galactic Nuclei Catalog (1LAC; Abdo et al.
2010b) contains around 600 blazars, a factor of ∼10 greater
than EGRET detected during its entire operational lifetime.
For the first time, simultaneous observations of Fermi with
the latest generation of IACTs can cover the entire high-
energy bump. Combining this with simultaneous low-energy
observations gives an unprecedented multiwavelength view of
these enigmatic objects.
Blazars found in low states are particularly poorly studied.
This is due in part to the lower sensitivity of previous instru-
ments, and in part to the fact that multiwavelength monitoring
programs, including space-based instruments, are mostly trig-
gered when an object enters a particularly bright state, as ob-
served by ground-based optical telescopes and all-sky monitors
such as the RXTE (Bradt et al. 1993) All Sky Monitor (ASM)
or the Swift (Gehrels et al. 2004) Burst Alert Telescope (BAT).
Having a well-measured low-state SED will be useful for con-
straining models and as a baseline to which other, flaring states
can be compared. This will be crucial for answering many of
the questions regarding these objects.
Markarian 421 (Mrk 421; R.A. = 11
h
4
m
27.
s
31, decl. = 38
◦
12

31.

8, J2000, redshift z = 0.031) is a high-synchrotron-
peaked (HSP) BL Lac object (according to the classification
presented in Abdo et al. (2010c)) that is one of the brightest
sources in the extragalactic X-ray/TeV sky. Mrk 421 was
actually the first extragalactic object to be discovered as a TeV
emitter (Punch et al. 1992), and one of the fastest varying
γ -ray sources (Gaidos et al. 1996). During the last decade,
there were a large number of publications on the very high
energy (VHE) γ -ray spectrum of this source, which has been
measured with almost all the existing IACTs (Krennrich et al.
2002; Aharonian et al. 2002, 2003, 2005; Albert et al. 2007a;
Acciari et al. 2009). Among other things, we learned that the
source shows evidence for a spectral hardening with increasing
flux. The SED and the multifrequency correlations of Mrk 421
have also been intensively studied in the past through dedicated
multifrequency observations of the source (Katarzy
´
nski et al.
2003;Bła
˙
zejowski et al. 2005; Revillot et al. 2006; Fossati et al.
2008; Horan et al. 2009), which showed a positive but very
complex relation between X-rays and VHE γ -rays, and that a
simple one-zone synchrotron self-Compton (SSC) model with
an electron distribution parameterized with one or two power
laws seemed to describe the collected SED well during the
observing campaigns. During a strong flare in 2008 June, the
source was also detected with the gamma-ray telescope AGILE
and, for the first time, a hint of correlation between optical
and TeV energies was reported by Donnarumma et al. (2009).
Despite the large number of publications on Mrk 421, the
details of the physical processes underlying the blazar emission
are still unknown. The main reasons for this are the sparse
multifrequency data during long periods of time, and the
moderate sensitivity available in the past to study the γ -ray
emission of this source. In addition, as occurs often with studies
of blazars, many of the previous multifrequency campaigns
were triggered by an enhanced flux level at X-rays and/or
γ -rays, and hence many of the previous studies of this source
are biased toward “high-activity” states, where perhaps distinct
physical processes play a dominant role. Moreover, we have very
little information from the MeV–GeV energy range: nine years
of operation with EGRET resulted in only a few viewing periods
with a signal significance of barely five standard deviations
(σ hereafter; Hartman et al. 1999), which precluded detailed
correlation studies with other energy bands.
We took advantage of the new capabilities provided by Fermi-
LAT and the new IACTs, as well as the existing capabilities
for observing at X-ray and lower frequencies, and organized a
multifrequency (from radio to TeV) campaign to observe Mrk
421 over 4.5 months. The observational goal for this campaign
was to sample Mrk 421 every two days, which was accomplished
at optical, X-ray, and TeV energies whenever the weather and/
or technical operations allowed. Fermi-LAT operated in survey
mode and thus the source was constantly observed at
γ -ray
energies.
In this paper, we report the overall SED averaged over the
duration of the observing campaign. A more in-depth analysis
of the multifrequency data set (variability, correlations, and
implications) will be given in a forthcoming paper.
This work is organized as follows: In Section 2 we introduce
the LAT instrument and report on the data analysis. In Section 3
we report the flux/spectral variability in the γ -ray range ob-
served by Fermi-LAT during the first 1.5 years of operation, and
compare it with the flux variability obtained with RXTE/ASM
and Swift/BAT, which are also all-sky instruments. In Section 4
we report on the spectrum of Mrk 421 measured by Fermi,
and Section 5 reports on the overall SED collected during the
4.5 month long multiwavelength campaign organized in 2009.
Section 6 is devoted to SED modeling of the multifrequency
data with both a hadronic and a leptonic model, and in Section 7
we discuss the implications of the experimental and modeling
results. Finally, we conclude in Section 8.
2. FERMI-LAT DATA SELECTION AND ANALYSIS
The Fermi-LAT is a γ -ray telescope operating from 20 MeV
to >300 GeV. The instrument is an array of 4 × 4 identical
towers, each one consisting of a tracker (where the photons
are pair-converted) and a calorimeter (where the energies of
the pair-converted photons are measured). The entire instru-
ment is covered with an anticoincidence detector to reject the
4

The Astrophysical Journal, 736:131 (22pp), 2011 August 1 Abdo et al.
MJD
54700 54800 54900 55000 55100 55200
]
-1
s
-2
ph cm
-8
Flux [10
5
10
15
/ ndf
2
χ
158.9 / 82
Prob 7.551e-07
p0
0.15± 6.783
/ ndf
2
χ
158.9 / 82
Prob 7.551e-07
p0
0.15± 6.783
MJD
54700 54800 54900 55000 55100 55200
Photon index
1.5
2
2.5
/ ndf
2
χ
86.61 / 82
Prob 0.3426
p0
0.01563± 1.759
/ ndf
2
χ
86.61 / 82
Prob 0.3426
p0
0.01563± 1.759
]
-1
s
-2
cm
-8
F [10
5
Photon index
1.5
2
2.5
15
10
Figure 1. Left: γ -ray flux at photon energies above 0.3 GeV (top) and spectral photon index from a power-law fit (bottom) for Mrk 421 for seven-day-long time
intervals from 2008 August 5 (MJD 54683) to 2009 March 12 (MJD 55248). Vertical bars denote 1σ uncertainties and the horizontal error bars denote the width of
the time interval. The black dashed line and legend show the results from a constant fit to the entire data set. Right: scatter plot of the photon index vs. flux.
(A color version of this figure is available in the online journal.)
charged-particle background. LAT has a large peak effective
area (0.8m
2
for 1 GeV photons), an energy resolution typically
better than 10%, and a field of view of about 2.4 sr with an
angular resolution (68% containment angle) better than 1
◦
for
energies above 1 GeV. Further details on the description of LAT
are given by Atwood et al. (2009).
The LAT data reported in this paper were collected from
2008 August 5 (MJD 54683) to 2010 March 12 (MJD 55248).
During this time, the Fermi-LAT instrument operated almost
entirely in survey mode. The analysis was performed with
the Science Tools software package version v9r15p6. Only
events having the highest probability of being photons, those
in the “diffuse” class, were used. The LAT data were extracted
from a circular region with a 10
◦
radius centered at the
location of Mrk 421. The spectral fits were performed using
photon energies greater than 0.3 GeV, where the effective
area of the instrument is large (>0.5m
2
) and the angular
resolution relatively good (68% containment angle smaller
than 2
◦
). The spectral fits using energies above 0.3 GeV are
less sensitive to possible contamination from non-accounted
(transient) neighboring sources, and have smaller systematic
errors, at the expense of reducing somewhat the number of
photons from the source. In addition, a cut on the zenith
angle (<105
◦
) was also applied to reduce contamination from
the Earth limb γ -rays, which are produced by cosmic rays
interacting with the upper atmosphere.
The background model used to extract the γ -ray signal
includes a Galactic diffuse emission component and an isotropic
component. The model that we adopted for the Galactic com-
ponent is given by the file gll_iem_v02.fit, and the isotropic
component, which is the sum of the extragalactic diffuse emis-
sion and the residual charged particle background, is param-
eterized by the file isotropic_iem_v02.
115
The normalization
of both components in the background model was allowed to
vary freely during the spectral point fitting. The spectral anal-
yses (from which we derived spectral fits and photon fluxes)
were performed with the post-launch instrument response func-
tions P6_V3_DIFFUSE using an unbinned maximum likelihood
method. The systematic uncertainties in the flux were estimated
115
http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html.
as 10% at 0.1 GeV, 5% at 560 MeV and 20% at 10 GeV and
above.
116
3. FLUX AND SPECTRAL VARIABILITY
The sensitivity of Fermi-LAT is sufficient to accurately
monitor the γ -ray flux of Mrk 421 on short timescales (a
few days).
117
The measured γ -ray flux above 0.3 GeV and
the photon index from a power-law (PL) fit are shown in
Figure 1. The data span the time from 2008 August 5 (MJD
54683) to 2009 March 12 (MJD 55248) and they are binned
on time intervals of 7 days. The Test Statistic (TS) values
118
for the 81 time intervals are typically well in excess of 100
(∼10σ ). The number of intervals with TS < 100 is only
nine (11%). The lowest TS value is 30, which occurs for the
time interval MJD 54899–54906. This low signal significance
is due to the fact that the Fermi-LAT instrument did not
operate during the time interval MJD 54901–54905
119
and
hence only three out of the seven days of the interval contain
data. The second lowest TS value is 40, which occurred for
the time interval 54962–54969. During the first 19 months
of Fermi operation, Mrk 421 showed relatively mild γ -ray
flux variations, with the lowest photon flux F (>0.3GeV)=
(2.6 ± 0.9) × 10
−8
cm
−2
s
−1
(MJD 54906–54913; TS = 53)
and the highest F (>0.3GeV)= (13.2 ± 1.9) ×10
−8
cm
−2
s
−1
(MJD 55200–55207; TS = 355). A constant fit to the flux
points from Figure 1 gave a χ
2
= 159 for 82 degrees of
freedom (probability that the flux was constant is 8 × 10
−7
),
hence indicating the existence of statistically significant flux
variability. On the other hand, the photon index measured in
seven-day-long time intervals is statistically compatible with
being constant, as indicated by the results of the constant fit to
all the photon index values, which gave χ
2
=87 for 82 degrees of
freedom (NDF; probability of no variability is 0.34). The scatter
116
See http://fermi.gsfc.nasa.gov/ssc/data/analysis/LAT_caveats.html.
117
The number of photons from Mrk 421 (above 0.3 GeV) detected by LAT in
one day is typically about six.
118
The Test Statistic TS = 2Δ log(likelihood) between models with and
without the source is a measure of the probability of having a point γ -ray
source at the location specified. The TS value is related to the significance of
the signal (Mattox et al. 1996).
119
The LAT did not operate during the time interval MJD 54901–54905 due to
an unscheduled shutdown.
5

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "C: " ?

The authors report on the γ -ray activity of the high-synchrotron-peaked BL Lacertae object Markarian 421 ( Mrk 421 ) during the first 1. 5 years of Fermi operation, from 2008 August 5 to 2010 March 12. The authors also present the first results from the 4. 5 month long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments that provided excellent temporal and energy coverage of the source throughout the entire campaign ( 2009 January 19 to 2009 June 1 ). The extensive multi-instrument ( radio to TeV ) data set provides an unprecedented, complete look at the quiescent spectral energy distribution ( SED ) for this source. 

Q2. How many electrons are required to be injected in the optical data?

In order to fit the optical data, the lowest energy of the injected electrons is required to be maintained as γe,min ≈ 700 through the steady state. 

Q3. What is the consequence of the dense synchrotron photon energy density?

The consequence is a dense synchrotron photon energy density that facilitates frequent interactions with relativistic protons, resulting in a strong reprocessed/cascade component which leads to a softening of the spectrum occurring mostly below 100 MeV. 

Q4. What is the background model used to extract the ray signal?

The background model used to extract the γ -ray signal includes a Galactic diffuse emission component and an isotropic component. 

Q5. Why did the compiled data provide us with the SED yet of Mrk 421?

Because of the low activity and low variability shown during this campaign, the compiled data provided us with the best SED yet of Mrk 421 in the low/ quiescent state. 

Q6. What are the spectral intervals lacking observations?

The only spectral intervals lacking observations are 1 meV–0.4 eV, and 200 keV–100 MeV, where the sensitivity of the current instruments is insufficient to detect Mrk 421. 

Q7. What other telescopes provided data at near-IR wavelengths?

In addition, the Goddard Robotic Telescope (GRT), the Remote Observatory for Variable Object Research (ROVOR), the New Mexico Skies telescopes, and the Multicolor Imaging Telescopes for Survey and Monstrous Explosions (MITSuME) provided data with various optical filters, while the Guillermo Haro Observatory (OAGH) and the Wyoming Infrared Observatory (WIRO) provided data at near-IR wavelengths. 

Q8. What is the reason for the weaker spectral break observed in the homogenous model?

The question that naturally arises is why, although the EED break postulated by the homogeneous model is at nearly the same energy as the expected cooling break, the spectral break observed is stronger. 

Q9. What are the dominantly radiating particles in the hadronic scenario?

In the hadronic scenario presented in Section 6.1, the dominantly radiating particles are protons, secondary electron/positron pairs, muons, and pions, in addition to the primary electrons. 

Q10. How many breaks are needed to describe the shape of the measured broadband SED?

the authors find that, in order to properly describe the shape of the measured broadband SED during the 4.5 month long campaign, the model requires an electron distribution parameterized with three PL functions (and hence two breaks). 

Q11. Why are the different segments of the EED somewhat connected?

Because of the electrons upscattering the broad energy range of synchrotron photons, the emissionof the different electron segments are somewhat connected, as shown in the bottom plot of Figure 13. 

Q12. What is the effect of the introduction of poorly constrained components on the jet?

The introduction of additional, poorly constrained components would be necessary to account for the subsequent evolution of the jet through the expansion phase where the synchrotron radiation becomes gradually optically thin at centimeter wavelengths. 

Q13. Why is the data collected during this campaign considered an excellent proxy for the low/quiescent state?

because of the low flux, low (multifrequency) variability, and the large density of observations, the collected data during this campaign can be considered an excellent proxy for the low/quiescent state SED of Mrk 421. 

Q14. Why did the authors exclude the 1.4–2.3 keV energy band from the analysis?

These residuals are due to known XRT calibration uncertainties (SWIFT-XRT-CALDB-12125) and hence the authors decided to exclude the 1.4–2.3 keV energy band from the analysis. 

Q15. What can be done to understand the importance of differential beaming?

Studies of the SEDs of sources with different jet orientations (e.g., radio galaxies and blazars) can help to understand the importance of differential beaming, and therefore of relativistic velocity gradients in these flows. 

Q16. What is the jet power of the supermassive black hole in Mrk 421?

In both cases, the computed jet power is a small fraction (∼10−2 to 10−3) of the Eddington luminosity for the supermassive black hole in Mrk 421 (2 × 108 M ), which is LEdd ∼ 1046–1047 erg s−1. 

Q17. What is the contribution of the low energy electrons to the TeV photon flux?

the low energy electrons have also contributed to the TeV photon flux through the emitted synchrotron photons which are being upscattered by the high energy electrons.