Issue |
A&A
Volume 519, September 2010
|
|
---|---|---|
Article Number | A32 | |
Number of page(s) | 10 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200913945 | |
Published online | 08 September 2010 |
MAGIC TeV gamma-ray observations of Markarian 421 during multiwavelength campaigns in 2006
J. Aleksic1 - H. Anderhub2 - L. A. Antonelli3 - P. Antoranz4 - M. Backes5 - C. Baixeras6 - S. Balestra4 - J. A. Barrio4 - D. Bastieri7 - J. Becerra González8 - J. K. Becker5 - W. Bednarek9 - A. Berdyugin10 - K. Berger9 - E. Bernardini11 - A. Biland2 - R. K. Bock12,7 - G. Bonnoli13 - P. Bordas14 - D. Borla Tridon12 - V. Bosch-Ramon14 - D. Bose4 - I. Braun2 - T. Bretz15 - D. Britzger12 - M. Camara4 - E. Carmona12 - A. Carosi3 - P. Colin12 - S. Commichau2 - J. L. Contreras4 - J. Cortina1 - M. T. Costado8,16 - S. Covino3 - F. Dazzi17,26 - A. De Angelis17 - E. de Cea del Pozo18 - R. De los Reyes4,28 - B. De Lotto17 - M. De Maria17 - F. De Sabata17 - C. Delgado Mendez8,27 - M. Doert5 - A. Domínguez19 - D. Dominis Prester20 - D. Dorner2 - M. Doro7 - D. Elsaesser15 - M. Errando1 - D. Ferenc21 - M. V. Fonseca4 - L. Font6 - R. J. García López8,16 - M. Garczarczyk8 - M. Gaug8 - N. Godinovic20 - D. Hadasch18 - A. Herrero8,16 - D. Hildebrand2 - D. Höhne-Mönch15 - J. Hose12 - D. Hrupec20 - C. C. Hsu12 - T. Jogler12 - S. Klepser1 - T. Krähenbühl2 - D. Kranich2 - A. La Barbera3 - A. Laille21 - E. Leonardo13 - E. Lindfors10 - S. Lombardi7 - F. Longo17 - M. López7 - E. Lorenz2,12 - P. Majumdar11 - G. Maneva22 - N. Mankuzhiyil17 - K. Mannheim15 - L. Maraschi3 - M. Mariotti7 - M. Martínez1 - D. Mazin1 - M. Meucci13 - J. M. Miranda4 - R. Mirzoyan12 - H. Miyamoto12 - J. Moldón14 - M. Moles19 - A. Moralejo1 - D. Nieto4 - K. Nilsson10 - J. Ninkovic12 - R. Orito12 - I. Oya4 - R. Paoletti13 - J. M. Paredes14 - S. Partini 13 - M. Pasanen10 - D. Pascoli7 - F. Pauss2 - R. G. Pegna13 - M. A. Perez-Torres19 - M. Persic17,23 - L. Peruzzo7 - F. Prada19 - E. Prandini7 - N. Puchades1 - I. Puljak20 - I. Reichardt1 - W. Rhode5 - M. Ribó14 - J. Rico24,1 - M. Rissi2 - S. Rügamer15 - A. Saggion7 - T. Y. Saito12 - M. Salvati3 - M. Sánchez-Conde19 - K. Satalecka11 - V. Scalzotto7 - V. Scapin17 - T. Schweizer12 - M. Shayduk12 - S. N. Shore25 - A. Sierpowska-Bartosik9 - A. Sillanpää10 - J. Sitarek12,9 - D. Sobczynska9 - F. Spanier15 - S. Spiro3 - A. Stamerra13 - B. Steinke12 - N. Strah5 - J. C. Struebig15 - T. Suric20 - L. Takalo10 - F. Tavecchio3 - P. Temnikov22 - D. Tescaro1 - M. Teshima12 - D. F. Torres24,18 - H. Vankov22 - R. M. Wagner12 - V. Zabalza14 - F. Zandanel19 - R. Zanin1
1 - IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
2 - ETH Zurich, 8093, Switzerland
3 - INAF National Institute for Astrophysics, 00136 Rome, Italy
4 - Universidad Complutense, 28040 Madrid, Spain
5 - Technische Universität Dortmund, 44221 Dortmund, Germany
6 - Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
7 - Università di Padova and INFN, 35131 Padova, Italy
8 - Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
9 - University of ódz,
90236
ódz, Poland
10 - Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
11 - Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
12 - Max-Planck-Institut für Physik, 80805 München, Germany
13 - Università di Siena, and INFN Pisa, 53100 Siena, Italy
14 - Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
15 - Universität Würzburg, 97074 Würzburg, Germany
16 - Depto. de Astrofisica, Universidad, 38206 La Laguna, Tenerife,
Spain
17 - Università di Udine, and INFN Trieste, 33100 Udine, Italy
18 - Institut de Ciències de l'Espai (IEEC-CSIC), 08193 Bellaterra,
Spain
19 - Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
20 - Croatian MAGIC Consortium, Institute R. Boskovic, University of
Rijeka and University of Split, 10000 Zagreb, Croatia
21 - University of California, Davis, 95616-8677, USA
22 - Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
23 - INAF/Osservatorio Astronomico and INFN, 34143 Trieste, Italy
24 - ICREA, 08010 Barcelona, Spain
25 - Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
26 - Supported by INFN Padova
27 - Now at: Centro de Investigaciones Energéticas,
Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
28 - Now at: Max-Planck-Institut für Kernphysik,
69029 Heidelberg, Germany
Received 22 December 2009 / Accepted 19 May 2010
Abstract
Context. Wide-range spectral coverage of blazar-type
active galactic nuclei is of paramount importance for understanding the
particle acceleration mechanisms assumed to take place in their jets.
The Major Atmospheric Gamma Imaging Cerenkov (MAGIC) telescope
participated in three multiwavelength (MWL) campaigns, observing the
blazar Markarian (Mkn) 421 during the nights of April 28 and
29, 2006, and June 14, 2006.
Aims. We analyzed the corresponding MAGIC very-high
energy observations during 9 nights from April 22 to 30, 2006 and on
June 14, 2006. We inferred light curves with sub-day resolution and
night-by-night energy spectra.
Methods. MAGIC detects -rays by observing extended
air showers in the atmosphere. The obtained air-shower images were
analyzed using the standard MAGIC analysis chain.
Results. A strong -ray signal was detected from
Mkn 421 on all observation nights. The flux (E>250 GeV)
varied on night-by-night basis between
(0.57 Crab units) and
(2.0 Crab units) in April 2006. There is a clear indication for
intra-night variability with a doubling time of
min
on the night of April 29, 2006, establishing once more rapid flux
variability for this object. For all individual nights
-ray spectra
could be inferred, with power-law indices ranging from 1.66 to 2.47. We
did not find statistically significant correlations between the
spectral index and the flux state for individual nights. During the
June 2006 campaign, a flux substantially lower than the one measured by
the Whipple 10-m telescope four days later was found. Using a
log-parabolic power law fit we deduced for some data sets the location
of the spectral peak in the very-high energy regime. Our results
confirm the indications of rising peak energy with increasing flux, as
expected in leptonic acceleration models.
Key words: radiation mechanisms: non-thermal - BL Lacertae objects: individual: Mkn 421 - gamma rays: galaxies
1 Introduction
The active galactic nucleus (AGN) Markarian
(Mkn) 421 was the first extragalactic source detected in
the TeV energy range, using imaging atmospheric Cerenkov telescopes
(IACTs, Petry
et al. 1996; Punch et al. 1992). With a
redshift
of z=0.030 it is the closest known and, along with
Mkn 501, the best-studied TeV -ray emitting blazar
. So far, flux variations
by more than one order of magnitude (e.g., Fossati
et al. 2008), and occasional flux doubling times as
short as 15 min (Gaidos
et al. 1996; Aharonian et al. 2002; Schweizer
et al. 2008) have been observed. Variations in the
hardness of the TeV
-ray
spectrum during flares were reported by several groups (e.g. Fossati
et al. 2008; Krennrich et al. 2002;
Aharonian
et al. 2005). Simultaneous observations in the X-ray
and very-high energy (VHE;
)
bands show strong evidence for correlated flux variability
(Bazejowski
et al. 2005; Fossati et al. 2008; Krawczynski
et al. 2001). With a long history of observations,
Mkn 421 is an ideal candidate for long-term and
statistical studies
of its emission (Tluczykont
et al. 2007; Goebel et al. 2008a; Hsu et al. 2009).
Mkn 421 has been detected and studied at
basically all wavelengths of
the electromagnetic spectrum from radio waves up to VHE -rays. Its
wide-range spectral energy distribution (SED) shows the typical
double-peak
structure of AGN. Mkn 421 is a so-called blazar.
These constitute a
rare subclass of AGNs with beamed emission closely aligned to our line
of
sight. In blazars, the low-energy peak at keV energies is thought to
arise
dominantly from synchrotron emission of electrons, while the origin of
the
high-energy (GeV-TeV) bump is still debated. The SED is commonly
interpreted
as being due to the beamed, non-thermal emission of synchrotron and
inverse-Compton radiation from ultrarelativistic electrons. These are
assumed
to be accelerated by shocks moving along the jets at relativistic bulk
speed.
For most of the observations, the SED can be reasonably well described
by
homogeneous one-zone synchrotron-self-Compton (SSC) models
(e.g. Maraschi
et al. 1992; Costamante & Ghisellini 2002;
Marscher
& Gear 1985). Hadronic models (Mücke et al. 2003; Mannheim
et al. 1996),
however, can also explain the observed features. A way to distinguish
between
the different emission models is to determine the positions, evolution
and
possible correlations (see, e.g., Wagner
2008b, for a review) of both peaks
in the SED, using simultaneous, time-resolved data covering a broad
energy
range, e.g., as obtained in multiwavelength (MWL) observational
campaigns.
In this Paper we present results from Major Atmospheric
Gamma-ray Imaging Cerenkov
(MAGIC) telescope VHE -ray
observations of Mkn 421 during
eight nights from April 22 to 30, 2006, and on June 14, 2006. For most
of the
days, optical R-band observations were conducted
with the KVA telescope.
Simultaneous observations were performed by Suzaku (Mitsuda et al. 2007) and
HESS, as well as by XMM-Newton (Jansen
et al. 2001) on April 28 and 29, 2006,
respectively. During both nights, we carried out particularly long,
uninterrupted observations in the VHE energy band of
3 h
duration
each. An onset of activity in the X-ray band triggered an INTEGRAL-led
target-of-opportunity (ToO) campaign, which took place from June 14-25,
2006
for a total of 829 ks (Lichti
et al. 2008). Within this campaign, MAGIC observed
Mkn 421 at rather high zenith angles from 43 to 52
degrees in
parallel with INTEGRAL on June 14, 2006.
In the following sections, we describe the data sets and the
analysis applied
to the VHE -ray
data, the determination of spectra for all observation
nights, and put the results into perspective with other VHE
-ray
observations of Mkn 421. The interpretation of these
data in a MWL
context is presented in Acciari
et al. (2009) and subsequent papers.
VHE -ray
observations in April and June 2006 have also been carried out
by the Whipple telescope (Horan
et al. 2009), by the VERITAS (Fegan
2008), and
TACTIC (Yadav et al. 2007)
collaborations, although not simultaneously with our
observations.
2 The MAGIC telescope
The VHE -ray
observations were conducted with the MAGIC telescope
located on the Canary island La Palma (2200 m above sea level,
28
45'N, 17
54'W). At the
time of our observations in 2006,
MAGIC was a single-dish 17-m Ø instrument
for the detection of
atmospheric air showers induced by
-rays. Its hexagonally-shaped
camera
with a field of view (FOV) of
mean diameter comprises 576
high-sensitivity photomultiplier tubes (PMTs): 180 pixels of
Ø
surround the inner section of the camera of 394 pixels of
Ø
(=
Ø
FOV). The trigger is formed by a coincidence of
4 neighboring pixels.
Presently the accessible trigger energy range (using the MAGIC standard
trigger, Meucci et al. 2007)
spans from 50-60 GeV (at small zenith angles) up to tens of
TeV.
Further details, telescope parameters, and performance information can
be found
in Cortina
et al. (2005); Albert et al. (2008a); Baixeras
et al. (2004).
3 Observations and data analysis
The observations were carried out during dark nights, employing the
so-called
wobble mode (Daum et al. 1997),
in which two opposite sky directions, each
0.4
off the source, are tracked alternatingly for 20 min each. The
on-source data are defined by calculating image parameters with respect
to the source
position, whereas background control (``off'') data are obtained from
the same data set, but with image parameters calculated with respect to
three positions, arranged symmetrically to the on-source region with
respect to the camera center.
The simultaneous measurement of signal and background makes additional
background control data unnecessary. In order to avoid an unwanted
contribution
from source
-events
in the off sample, and to guarantee the statistical
independence between the on and the off samples in the signal region,
events
included in the signal region of the on sample were excluded from the
off
sample and vice versa.
The data were analyzed following the standard MAGIC analysis procedure (Bretz & Dorner 2008; Bretz & Wagner 2003). After calibration (Albert et al. 2008c) and extracting the signal at the pulse maximum using a spline method, the air-shower images were cleaned of noise from night-sky background light by applying a three-stage image cleaning. The first stage requires a minimum number of 6 photoelectrons in the core pixels and 3 photoelectrons in the boundary pixels of the images (see, e.g. Fegan 1997). These tail cuts are scaled according to the larger size of the outer pixels of the MAGIC camera. Only pixels with at least two adjacent pixels with a signal arrival time difference lower than 1.75 ns survive the second cleaning stage. The third stage repeats the cleaning of the second stage, but requires only one adjacent pixel within the 1.75 ns time window.
The data were filtered by rejecting trivial background events, such as accidental noise triggers, triggers from nearby muons, or data taken during adverse atmospheric conditions (e.g., low atmospheric transmission). 12.7 h out of the total 15.0 hours' worth of data survived the latter quality selection and were used for further analysis.
We calculated image parameters (Hillas
1985) such as WIDTH, LENGTH, SIZE, CONC,
M3LONG (the third moment of the light distribution along the major
image axis),
and LEAKAGE (the fraction of light contained in the outermost ring of
camera
pixels) for the surviving events. For the /hadron separation, a
SIZE-dependent parabolic cut in AREA
WIDTH
LENGTH
was used (Riegel et al. 2005). The
cut parameters for the assessment of the detection significance were
optimized on Mkn 421 data from close-by days. For
the data of June 14, 2006 at rather large zenith angles, data of
Mkn 501 from
October 2006 were used to determine the optimal cuts. Any significance
in this
work was calculated using Eq. (17) of Li
& Ma (1983) with
.
The primary -ray
energies were reconstructed from the image parameters
using a Random Forest regression method (Albert
et al. 2008b, and references
therein) trained with Monte-Carlo simulated events
(MCs, Majumdar
et al. 2005; Knapp & Heck 2004). The MC
sample is characterized by a power-law
spectrum between 10 GeV and 30 TeV with a
differential spectral photon index of
,
and a point-spread function resembling the experimental one. The
events were selected to cover the same zenith distance range as the
data. For
the spectrum calculation, the area cut parameters were optimized to
yield a
constant MC cut efficiency of 90% over the whole energy range,
increasing the
-ray event
statistics at the threshold.
The Mkn 421 observations presented here are
among the first data
taken by MAGIC after major hardware updates in April 2006 (Goebel et al. 2008b), which
required us to thoroughly examine the data. Despite the hardware
changes, the
MAGIC subsystems performed as expected with the exception of an
unstable
trigger behavior for some PMTs, leading to a significant loss of events
in one
of the six sectors of the camera. In order to proceed with the data
analysis
with serenity and to estimate the effect caused by this inhomogeneity,
a simple
procedure was applied to the data: the expected number of events, as a
function
of energy, for the affected sector was estimated as the mean of the
number of
events in the other five sectors of the camera. (A homogeneous
distribution of
events through the six sectors is expected for normal conditions.) The
difference between the expected and actually measured events was
computed
using the whole data sample in order to have sufficient statistics. We
found a decrease of the differential photon flux of 5.7% between 250
and
400 GeV, 4.6% between 400 and 650 GeV, 2.2% between
650 and 1050 GeV and <
for higher energies for the April 2006 data. Due to the higher
zenith
distance and energy threshold, the method was adapted for June 14, 2006
and
yielded a decrease of 5.2% between 450 and 670 GeV and 2.6%
for higher
energies. However the above mentioned effect is just an average one,
with
estimated flux errors of up to 6.6% showing up for individual nights.
To mitigate the effect of the inhomogeneity, instead of an (already increased) energy threshold of 250 GeV, higher thresholds of 350 or 450 GeV were applied for some observation nights. In this way we made sure that the estimated systematic error remains within reasonable limits.
For the calculation of the individual light curves as well as
for the overall
April 2006 lightcurve, the flux between 250 GeV and
350 GeV was extrapolated
for the nights with higher threshold. We assumed a power-law behavior
in this
energy range, with the spectral index determined for the first three
energy bins of
the whole April dataset (i.e., ). The flux
normalization for
each night has been determined at 500 GeV by a fit to the
first three
differential spectral points, an energy range which is reliable for all
affected nights.
Table 1
summarizes the analyzed data sets. The statistical
significance of any detection is assessed by applying a cut in ,
where
is the angular distance between the expected source position and
the reconstructed
-ray
arrival direction. The arrival directions of the
showers in equatorial coordinates were calculated using the DISP method
(Fomin
et al. 1994; Lessard et al. 2001). We
replaced the constant coefficient
in the
parameterization of DISP in the original approach by a term which is
dependent
on LEAKAGE, SIZE, and SLOPE,
![]() |
(1) |
k = 0 for



Table 1: Some characteristic parameters of the different data sets of the campaign.
All stated errors are statistical errors only; we estimate our systematic errors to be 16% for the energy scale, 11% for absolute fluxes and flux normalizations, and 0.2 for the spectral slopes (Albert et al. 2008a), not including the additional systematic flux errors mentioned above.
A second, independent analysis of the data yielded compatible results to those presented here.
4 Results
4.1 Results for April 22-30, 2006
MAGIC observed Mkn 421 from MJD 53 847 to MJD 53 855. During the observations, two MWL campaigns were carried out simultaneously with Suzaku and with XMM-Newton on MJD 53 854 and MJD 53 855, respectively. Mkn 421 was also observed as part of the monitoring program of the Whipple 10-m telescope (see Horan et al. 2009), albeit about 3.5 h after the MAGIC observations stopped, due to the different longitudes of the two instruments.
A strong -ray
signal from the source was detected in all eight
observation nights. In total, 3165 excess events were recorded over a
background of 693 events for energies >250 GeV,
yielding an overall significance of
.
Mkn 421 exhibited an average flux of
.
When compared to earlier observations (see,
e.g. Tluczykont
et al. 2007; Albert et al. 2007a; Goebel et al.
2008a; Steele
et al. 2008), our observations indicate an elevated
flux
state of Mkn 421. We found high flux states in the
nights of
MJD 53 850,
,
MJD 53853,
,
and MJD 53 856,
(Fig. 1).
In the remaining nights (we assumed nights with fluxes below
as non-flare nights), Mkn 421 exhibited a
low-flux average of
.
The analysis results on a night-by-night
basis are summarized in Table 2, and
include the nightly
numbers for excess and background events, significances, and average
integral fluxes
above 250 GeV (where the nights with an energy cut of
350 GeV where
extrapolated down to 250 GeV, see Sect. 3 for
details).
The results of a spectral fit based on a simple power law (PL) of the
form
are also shown.
![]() |
Figure 1:
VHE (E>250 GeV) light curve for
Mkn 421 observations in April 2006. The data points
represent average nightly fluxes. The observation windows of the
Suzaku (MJD 53 853.28-53 854.27) and
XMM-Newton (MJD 53 854.87-53 855.35) MWL
campaigns
are marked by the gray-shaded areas. A ``mean low flux'' (solid line)
was
averaged over all data points below |
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![]() |
Figure 2:
VHE (
|
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Table 2: Analysis results.
The energy thresholds of the individual observations are also given in Table 2. As the analysis threshold is always lower than the applied energy cut, the latter one defines the energy threshold value.
The strong -ray
signal allowed to infer light curves with a resolution
below one hour for all of the observation nights, which are shown in
Fig. 2
(see Table 4
for the
light curve data). Most light curves are compatible with a constant
flux during
the nightly observation time (see Table 2 for all
constant-fit
values), while on MJD 53 855 a clear
intra-night variability is apparent. A fit with a constant function
yields an
unacceptable
(
)
for this
night, and the data suggest a flux halving time of
min.
Note that
this interesting observation window has also been covered by XMM-Newton
observations in the X-ray band (Acciari
et al. 2009).
4.2 Results for June 14, 2006
An onset of activity to 2 times
the average quiescent-flux level of
Mkn 421 was measured in April 2006 by the RXTE
all-sky monitor
(ASM) instrument. It triggered an INTEGRAL ToO
campaign from June 14, 2006
to 25 for a total of 829 ks (Lichti
et al. 2008). This >30 mCrab flux
remained
until September 2006. During the 9-day campaign,
Mkn 421 was
targeted by various instruments in the radio, optical, X-ray and VHE
wavebands.
Results are reported in Lichti
et al. (2008). On June 14, 2006, MAGIC
observed
Mkn 421 at rather high zenith angles in parallel
with the OMC, JEM-X,
and IBIS measurements aboard INTEGRAL. Further
VHE coverage was provided
by the Whipple 10-m telescope on June 18/19/21, 2006
(Lichti et al. 2008).
The MAGIC observations on June 14, 2006 lasted for 50 min.
The
high zenith angles of 43 to 52 degrees of this observations
and the previously
mentioned inhomogeneities result in an energy threshold of
GeV. In spite of the
overall rather difficult
observational circumstances caused by the high zenith angle
observations
(Tonello 2006;
Albert
et al. 2006), a firm detection on the 7.5-
significance
level was achieved.
The corresponding differential energy spectrum is shown in
Fig. 3.
Between 450 GeV and 2.2 TeV, it can be described by a
simple power-law of the form
![]() |
(3) |
For comparison we also show the spectral points reported by the Whipple 10-m telescope averaged over the nights of June 18/19/21, 2006. Generally, there might be systematic differences between the Whipple and MAGIC measurements. It could, however, be shown that such inter-instrument systematic effects are rather small and under control, e.g. those between MAGIC and HESS (Mazin et al. 2005). Particularly the Crab nebula spectra measured by Whipple and MAGIC agree quite well (Albert et al. 2008a). The Mkn 421 flux measured by the Whipple 10-m telescope four days after the MAGIC observation is substantially higher than our measurements (Fig. 3), pointing to a clear evolution of the source emission level within the INTEGRAL campaign.
5 Discussion
In leptonic acceleration models, e.g., SSC models, a shift of the
high-energy
peak (attributed to Inverse Compton radiation) in the spectral energy
distribution towards higher energies with an increasing flux level is
expected.
In the VHE domain, such a shift can be traced by spectral hardening.
Variations
in the hardness of the TeV -ray
spectrum during flares were reported by
several groups (e.g., Fossati
et al. 2008; Krennrich et al. 2002;
Aharonian
et al. 2005). We tested for a
correlation of the spectral hardness with the flux level of the
de-absorbed
spectrum (i.e. after removing any attenuation effects caused by the
Extragalactic Background Light [EBL], cf. Nikishov 1962; Hauser &
Dwek 2001; Gould
& Schréder 1966)
in our data (Fig. 4),
but found that the correlation neither
can be described by a constant fit (
,
)
nor by a linear dependence of spectral hardness and flux level
(
,
),
giving no clear preference for
either. Although clear flux variations are present in the data set, the
overall
dynamical range of 3.9 in flux might be too small to see a significant
spectral hardening with increasing flux.
![]() |
Figure 3:
Differential photon spectrum for Mkn 421 for the
observation night of June 14, 2006 (black data points). A
power-law fit to the spectrum
results in a spectral slope of |
Open with DEXTER |
![]() |
Figure 4:
Spectral index vs. flux at 0.5 TeV deduced from a simple
power-law
fit after EBL de-absorption for Mkn 421.
The |
Open with DEXTER |
The individual night-by-night spectra during the campaign in April 2006
are
shown in Fig. 5.
All spectral data points are summarized in
Table 6.
For the nights of April 22, 26, and 29, 2006, there seems to be
evidence for a resolved peak, but a likelihood ratio test (e.g., Mazin & Goebel 2007) yields
significant curvature only for April 27, 2006.
We used a logarithmic curvature term, corresponding to a parabolic
power-law (log-P) in a
vs.
representation (Massaro et al.
2004), and a power-law with exponential cutoff (PL+C) of the
form
and
respectively. The likelihood ratio test results in a clear preference towards a log-P or a PL+C compared to a simple power-law with a probability of



![]() |
Figure 5: Differential energy spectra for Mkn 421 for April 2006 before (gray points) and after (black points) correcting for EBL absorption. For the apparently hard spectra on April 22, 26, 27, and 29, 2006, log-P (Eq. (4)) and PL+C (Eq. (5)) fits were performed (red solid and blue dashed curves, respectively). |
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Table 3: Special fit results.
Table 4: Light curve data.
Table 5:
values for the PL, log-P, and PL+C fits performed in Fig. 5.
Table 6: Energy spectra for all observation nights under study after EBL de-absorption.
![]() |
Figure 6: Derived peak position using the log-P (Eq. (6)) versus flux at 1 TeV for the data sets presented in Table 3. Historical data, taken from Albert et al. (2007a), are shown in gray. Our data confirm the indication of a correlation between the two parameters. |
Open with DEXTER |
The curved power laws enable to locate a peak in the de-absorbed
spectrum at
for the log-P and at
if
for the PL+C fit.
For simplicity we
determined
of the log-P by using the apex form of the parabola
in a
logarithmic representation:
which naturally yields both



The observation of a relation between flux (and thus, fluence)
and the position
of the VHE peak in the SED could be signalling a relation similar to
the one
suggested by Amati et al. (2002)
and observed by Sakamoto
et al. (2006) for gamma-ray
bursts. Since the TeV -ray
production is assumed to take place in a
relativistic jet, and many of the same radiative processes are involved
(on a
larger scale, of course) it might be a similar (or related) mechanism
at work
on a different scale. A trend towards a relation between flux and
spectral
index in the TeV energy range has also been noted by Wagner (2008a),
studying 17 known TeV blazars, and by Tramacere
(2010) in the X-ray band,
after a deep spectral analysis of all Swift
observations of Mrk 421
between April and July 2006.
Although the peak energy measured on April 27, 2006 exceeds that of the All April Data and Low-State data set, it is, despite having a higher flux, comparable with that derived for the High-State data set. This discrepancy in terms of the expected behaviour in SSC models can be explained with the different nature of the data sets: the April 27, 2006 data represent a rather particular, 1.4 h long episode of an individual flare event, whereas the High-State data set is an average of three individual flares. Due to the sparse sampling, most probably each of these observations caught different epochs of the individual flare evolutions, during which the spectral shape can change considerably in terms of spectral index and curvature (see, e.g., Katarzynski et al. 2006). Hence the two data sets are not necessarily directly comparable.
The values of the derived cutoff energies are also suggesting this behavior, showing, with the exception of April 27, 2006, an increase with rising flux, thus indicating a source-intrinsic rather than a cosmological reason for the cutoff feature. This is in accordance with the Kneiske & Dole (2008) lower-limit model, predicting an EBL cutoff for Mkn 421 at around 13 TeV.
![]() |
Figure 7: EBL de-absorbed historical spectra of Mkn 421 (see Albert et al. 2007a, for references) along with selected spectra from the April 2006 campaign and the flare spectrum of Donnarumma et al. (2009). The solid line is the result of a fit using Eq. (4). Note that the historical data were deabsorbed using the model of Primack et al. (2005), our data and those from Donnarumma et al. (2009) with the model of Kneiske & Dole (2008). |
Open with DEXTER |
In Fig. 7, we compare ``historical'' spectra measured between 1998 and 2005 with the low-state and high-state spectra derived from the observations reported here. It is obvious that our low-state spectrum represents one of the lowest flux states ever measured in VHE for Mkn 421, whereas the high state spectrum shows no exceptionally high flux level of this source. Both spectra are harder than historical spectra with comparable flux levels, in particular harder than the VERITAS spectrum (Donnarumma et al. 2009), enabling one of the best measurements of the turnover of the SED in a low flux state. While a previous observation yielded a rather flat spectrum in the VHE regime (Aharonian et al. 2002), we conclude that we measured a rather clear peak (flat structure in the SED). The low-state spectrum has a shape similar to the one measured by HEGRA CT1, although at an approximately three times lower flux level. The high-state spectral shape resembles the high-state Whipple spectrum, which in turn has an about three times higher flux. This tendency can also be seen in Fig. 6, which shows that the fluxes we derive are systematically lower than historical measurements for comparable peak energies. Within the SSC framework this difference in flux for comparable spectral shapes can be caused by, e.g., a lower number of electrons with the same energy distribution as in the high-flux case.
In summary, we followed the evolution of a sequence of mild
flares of the
blazar Mkn 421 during one week from April 22 to 30,
2006, peaking at
(
2.0 Crab
units). The nocturnal observations lasted at least for about one hour
and allowed
for the reconstruction of night-by-night spectra. During three
observation
nights high fluxes were recorded, in which, however, no variability
could be
measured. In two of these nights, rather hard spectral indices were
found, but
this was also the case for the night with the lowest flux. During the
night of
April 29, 2006, with a not particularly high flux of
(
0.65 Crab
units), clear intra-night variability with a flux-doubling time of
min
was observed.
According to a likelihood ratio test, the spectra of some data sets were better described by curved power laws than simple power laws, enabling us to calculate peak and cutoff energies in the VHE regime. The derived peak values are consistent with an evolution of the peak energy with the flux, as suggested by historical data. Indications of an intrinsic cutoff in the spectra of Mkn 421, as found in former observations, are confirmed by our results.
During the INTEGRAL-triggered MWL
campaign in June 2006 we observed
Mkn 421 in one night at high zenith angles. Our
measurements
complement the three-night observations conducted by the Whipple 10-m
telescope
four days later. Taking the MAGIC and Whipple results together, a
variability
of Mkn 421 also during the INTEGRAL
observations is evident.
The energy coverage of the Whipple telescope spectrum (
GeV)
was not sufficient to assess any spectral evolution by comparing it
to the MAGIC spectrum (
TeV).
The determined fluxes and spectra will be further used for studies of the SED taking into account data taken at other photon energies in detailed MWL analyses (publications in preparation).
AcknowledgementsWe thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The support of the German BMBF and MPG, the Italian INFN and Spanish MICINN is gratefully acknowledged. This work was also supported by ETH Research Grant TH 34/043, by the Polish MNiSzW Grant N N203 390834, and by the YIP of the Helmholtz Gemeinschaft.
References
- Acciari, V. A., Aliu, E., Aune, T., et al. (VERITAS and MAGIC Collabs.) 2009, ApJ, 703, 169 [Google Scholar]
- Aharonian, F., Akhperjanian, A., Beilicke, M., et al. (HEGRA Collab.) 2002, A&A, 393, 89 [Google Scholar]
- Aharonian, F., Akhperjanian, A., Beilicke, M., et al. (HEGRA Collab.) 2004, ApJ, 614, 897 [Google Scholar]
- Aharonian, F., Akhperjanian, A. G., Aye, K.-M., et al. (HESS Collab.) 2005, A&A, 437, 95 [Google Scholar]
- Albert, J., Aliu, E., Anderhub, H., et al. (MAGIC Collab.) 2006, ApJ, 638, L101 [Google Scholar]
- Albert, J., Aliu, E., Anderhub, H., et al. (MAGIC Collab.) 2007a, ApJ, 663, 125 [Google Scholar]
- Albert, J., Aliu, E., Anderhub, H., et al. (MAGIC Collab.) 2007b, Nucl. Instrum. Meth., A583, 494 [Google Scholar]
- Albert, J., Aliu, E., Anderhub, H., et al. (MAGIC Collab.) 2008a, ApJ, 674, 1037 [Google Scholar]
- Albert, J., Aliu, E., Anderhub, H., et al. (MAGIC Collab.) 2008b, Nucl. Instrum. Meth., A588, 424 [Google Scholar]
- Albert, J., Aliu, E., Anderhub, H., et al. (MAGIC Collab.) 2008c, Nucl. Instrum. Meth., A594, 407 [Google Scholar]
- Aliu, E., Anderhub, H., Antonelli, L. A., et al. (MAGIC Collab.) 2009, Astropart. Phys., 30, 293 [Google Scholar]
- Amati, L., Frontera, F., Tavani, M., et al. 2002, A&A, 390, 81 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Baixeras, C., Bastieri, D., Bigongiari, C., et al. (MAGIC Collab.) 2004, Nucl. Instrum. Meth., A518, 188 [Google Scholar]
- B▯azejowski, H., Blaylock, G., Bond, I. H., et al. 2005, ApJ, 630, 130 [NASA ADS] [CrossRef] [Google Scholar]
- Bretz, T., & Wagner, R. 2003, in Proc. 28th International Cosmic Ray Conference, Tsukuba, Japan, 5, 2947 [Google Scholar]
- Bretz, T., & Dorner, D. (MAGIC Collab.) 2008, AIP Conf. Proc., 1085, 664 [Google Scholar]
- Cortina, J., Armada, A., Biland, A., et al. (MAGIC Collab.) 2005, in Proc. 29th International Cosmic Ray Conference, Pune, India, 5, 359 [Google Scholar]
- Cortina, J., Goebel, F., & Schweizer, T. (MAGIC Collab.) 2009, in Proc. 31st International Cosmic Ray Conference, ▯ódz, Poland, [arXiv:0907.1211] [Google Scholar]
- Costamante, L., & Ghisellini, G. 2002, A&A, 384, 56 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Daum, A., Hermann, G., Hess, M., et al. (HEGRA Collab.) 1997, Astropart. Phys., 8, 1 [Google Scholar]
- Donnarumma, I., Vittorini, V., Vercellone, S., et al. (AGILE Collab., GASP-WEBT Collab., MAGIC Collab., VERITAS Collab.) 2009, ApJ, 691, L13 [Google Scholar]
- Fegan, D. J. 1997, J. Phys. G, 23, 1013 [NASA ADS] [CrossRef] [Google Scholar]
- Fegan, D. J. (VERITAS Collab.) 2008, in Proc. 30th International Cosmic Ray Conference, Merida, Mexico, 3, 901 [Google Scholar]
- Fomin, V. P., Stepanian, A. A., Lamb, R. C., et al. 1994, Astropart. Phys., 2, 137 [NASA ADS] [CrossRef] [Google Scholar]
- Fossati, G., Buckley, J. H., Bond, I. H., et al. 2008, ApJ, 677, 906 [NASA ADS] [CrossRef] [Google Scholar]
- Gaidos, J. A., Akerlof, C. W., Biller, S., et al. 1996, Nature, 383, 319 [NASA ADS] [CrossRef] [Google Scholar]
- Goebel, F., Backes, M., Bretz, T., et al. (MAGIC Collab.) 2008a, in Proc. 30th International Cosmic Ray Conference, Merida, Mexico, 3, 1025 [Google Scholar]
- Goebel F., Bartko, H., Carmona, E., et al. (MAGIC Collab.) 2008b, in Proc. 30th International Cosmic Ray Conference, Merida, Mexico, 3, 1481 [Google Scholar]
- Gould, R. J., & Schréder, G. P. 1966, Phys. Rev. Lett., 16, 252 [Google Scholar]
- Hauser, M. G., & Dwek, E. 2001, ARA&A, 39, 249 [NASA ADS] [CrossRef] [Google Scholar]
- Hillas, A. M. 1985, in Proc. 19th International Cosmic Ray Conference, La Jolla, 3, 445 [Google Scholar]
- Horan, D., Acciari, V. A., Bradbury, S. M., et al. (Whipple Collab.) 2009, ApJ, 695, 596 [Google Scholar]
- Hsu, C.-C., et al. (MAGIC Collab.) 2009, in Proc. 31st International Cosmic Ray Conference, ▯ódz, Poland, [arXiv:0907.0893] [Google Scholar]
- Katarzynski, K., Ghisellini, G., Mastichiadis, A., Tavecchio, F., & Maraschi, L. 2006, A&A, 453, 47 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Knapp, J., & Heck, D. 2004, EAS Simulation with CORSIKA: A Users Manual [Google Scholar]
- Kneiske, T. M., & Dole, H. 2008, AIP Conf. Proc., 1085, 620 [NASA ADS] [CrossRef] [Google Scholar]
- Krennrich, F., Bond, I. H., Bradbury, S. M., et al. 2002, ApJ, 575, L9 [NASA ADS] [CrossRef] [Google Scholar]
- Krawczynski, H., Sambruna, R., Kohnle, A., et al. 2001, ApJ, 559, 187 [NASA ADS] [CrossRef] [Google Scholar]
- Jansen, B., Lumb, D., Altieri, B., et al. 2001, A&A, 365, 1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Lessard, R. W., Buckley, J. H., Connaughton, V., & Le Bohec, S. 2001, Astropart. Phys., 15, 1 [NASA ADS] [CrossRef] [Google Scholar]
- Li, T.-P., & Ma, Y.-Q. 1983, ApJ, 272, 317 [NASA ADS] [CrossRef] [Google Scholar]
- Lichti, G. G., Bottacini, E., Ajello, M., et al. 2008, A&A, 486, 721 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Mannheim, K., Westerhoff, S., Meyer, H., & Fink, H.-H. 1996, A&A, 315, 77 [NASA ADS] [Google Scholar]
- Majumdar, P., Moralejo, A., Bigongiari, C., Blanch, O., & Sobczynska, D. 2005, in Proc. 29th International Cosmic Ray Conference, Pune, India, 5, 203 [Google Scholar]
- Marscher, A. P., & Gear, W. K. 1985, ApJ, 298, 11 [Google Scholar]
- Maraschi, L., Ghisellini, G., & Celotti, A. 1992, ApJ, 397, L5 [NASA ADS] [CrossRef] [Google Scholar]
- Massaro, E., Perri, M., Giommi, P., & Nesci, R. 2004, A&A, 413, 489 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Mazin, D., & Goebel, F. 2007, ApJ, 655, L13 [NASA ADS] [CrossRef] [Google Scholar]
- Mazin, D., Goebel, F., Horns, D., et al. 2005, in Proc. 29th International Cosmic Ray Conference, Pune, India, 4, 331 [Google Scholar]
- Meucci, M., Paoletti, R., Cecchi, R., et al. 2007, IEEE Trans. Nucl. Sci., 54, 404 [NASA ADS] [CrossRef] [Google Scholar]
- Mitsuda, K., Bautz, M., Inoue, H., et al. 2007, PASJ, 59, S1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Mücke, A., Protheroe, R. J., Engel, R., Rachen, J. P., & Stanev, T. 2003, Astropart. Phys., 18, 593 [NASA ADS] [CrossRef] [Google Scholar]
- Nikishov, A. I. 1962, Sov. Phys. JETP, 14, 393 [Google Scholar]
- Petry, D., Bradbury, S. M., Konopelko, A., et al. 1996, A&A, 311, L13 [NASA ADS] [Google Scholar]
- Primack, J. R., Bullock, J. S., & Somerville, R. S. 2005, AIP Conf. Proc., 745, 23 [NASA ADS] [CrossRef] [Google Scholar]
- Punch, M., Akerlof, C. W., Cawley, M. F., et al. 1992, Nature, 358, 477 [NASA ADS] [CrossRef] [Google Scholar]
- Riegel, B., Bretz, T., Dorner, D., Berger, K., & Höhne, D. (MAGIC Collab.) 2005, in Proc. 29th International Cosmic Ray Conference, Pune, India, 5, 215 [Google Scholar]
- Sakamoto, T., Barbier, L., Barthelmy, S. D., et al. 2006, ApJ, 636, L73 [NASA ADS] [CrossRef] [Google Scholar]
- Schweizer, T., Wagner, R. M., & Lorenz, E. 2008, AIP Conf. Proc., 1085, 455 [NASA ADS] [CrossRef] [Google Scholar]
- Steele, D., Carini, M. T., Charlot, P., et al. (VERITAS Collab.) 2008, in Proc. 30th International Cosmic Ray Conference, Merida, Mexico, 3, 989 [Google Scholar]
- Tluczykont M., Shayduk, M., Kalekin, O., & Bernardini, E. 2007, J. Phys. Conf. Ser., 60, 318 [NASA ADS] [CrossRef] [Google Scholar]
- Tonello, N. 2006, Ph.D. Thesis, Technische Universität München, MPP-2006-21 [Google Scholar]
- Tramacere, A. 2009, PoS(extremesky2009), 96, 39 [arXiv:1003.5001] [Google Scholar]
- Wagner, R. M. 2008a, MNRAS, 385, 119 [NASA ADS] [CrossRef] [Google Scholar]
- Wagner, R. M. 2008b, PoS(BLAZARS2008), 63, 13 [arXiv:0808.2483] [Google Scholar]
- Yadav, K. K., Chandra, P., Tickoo, A. K., et al. (TACTIC Collab.) 2007, Astropart. Phys., 27, 447 [Google Scholar]
Footnotes
- ... blazar
- See, e.g., http://www.mpp.mpg.de/ rwagner/sources/ for an up-to-date list of VHE
-ray sources.
- ... instrument
- Since 2009, MAGIC is a two-telescope stereoscopic system (Cortina et al. 2009).
- ... 2006
- The respective log-P probabilities are 83%, 48%, 73%, and 96%.
All Tables
Table 1: Some characteristic parameters of the different data sets of the campaign.
Table 2: Analysis results.
Table 3: Special fit results.
Table 4: Light curve data.
Table 5:
values for the PL, log-P, and PL+C fits performed in Fig. 5.
Table 6: Energy spectra for all observation nights under study after EBL de-absorption.
All Figures
![]() |
Figure 1:
VHE (E>250 GeV) light curve for
Mkn 421 observations in April 2006. The data points
represent average nightly fluxes. The observation windows of the
Suzaku (MJD 53 853.28-53 854.27) and
XMM-Newton (MJD 53 854.87-53 855.35) MWL
campaigns
are marked by the gray-shaded areas. A ``mean low flux'' (solid line)
was
averaged over all data points below |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
VHE (
|
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Differential photon spectrum for Mkn 421 for the
observation night of June 14, 2006 (black data points). A
power-law fit to the spectrum
results in a spectral slope of |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Spectral index vs. flux at 0.5 TeV deduced from a simple
power-law
fit after EBL de-absorption for Mkn 421.
The |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Differential energy spectra for Mkn 421 for April 2006 before (gray points) and after (black points) correcting for EBL absorption. For the apparently hard spectra on April 22, 26, 27, and 29, 2006, log-P (Eq. (4)) and PL+C (Eq. (5)) fits were performed (red solid and blue dashed curves, respectively). |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Derived peak position using the log-P (Eq. (6)) versus flux at 1 TeV for the data sets presented in Table 3. Historical data, taken from Albert et al. (2007a), are shown in gray. Our data confirm the indication of a correlation between the two parameters. |
Open with DEXTER | |
In the text |
![]() |
Figure 7: EBL de-absorbed historical spectra of Mkn 421 (see Albert et al. 2007a, for references) along with selected spectra from the April 2006 campaign and the flare spectrum of Donnarumma et al. (2009). The solid line is the result of a fit using Eq. (4). Note that the historical data were deabsorbed using the model of Primack et al. (2005), our data and those from Donnarumma et al. (2009) with the model of Kneiske & Dole (2008). |
Open with DEXTER | |
In the text |
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