Observations of Mrk 421 with INTEGRAL
G. G. Lichti, V. Beckmann, C. Boisson, J. Buckley,
P. Charlot, W. Collmar, B. Degrange, A. Djannati-Atai,
J. Finley, G. Fossati, G. Henri, K. Katarzynski, D. Kieda,
K. Mannheim, A. Marcowith, M. Punch, A. Saggione,
L. Saugé, V. Schönfelder, A. Sillanpaa, D. Smith,
H. Sol, F. Tavecchio, L. Takalo, M. Tornikoski,
A. von Kienlin, T. Weekes
An accepted ToO Proposal for multiwavelength observations
(829 ks)
Aim of the Proposal
• To perform simultaneous or quasi-simultaneous observations of Mrk 421 across the
electromagnetic spectrum with the aim to
measure
– time variability
– spectral characteristics/variability
– intensity-spectrum correlations
• to study the relative properties in different
X-ray bands and between X/γ-rays and TeV
quanta
Mrk 421 is a TeV blazar
• TeV blazars are AGN of the BL Lac type
– radio-loud sources
– high polarization at radio & optical wavelengths 
synchrotron radiation
– strong variability at all wavelengths
– 6 TeV blazars so far firmly detected
• spectral characteristics
– non-thermal emission processes
– 2 smooth broadband-emission components
• emission from a narrow relativistic jet observed under a
small angle (energy flux in the jet: 1044 – 1047 erg/s)
Parameters of Mrk 421
(z = 0.031 dl  130 Mpc)
Schwarzschildradius:
Rs = (3.9 - 15.8) AU
radius of last stable orbit:
r = 3Rs = (11.8 – 47.4) AU
M = (2-8) · 108 M
Keplervelocity at r:
v = 0.41 · c
trot = 25 h – 4.2 d
Structure of a TeV Blazar
relativistic-moving blobs
of Leptons or Hadrons
TeV blazars
Spectrum of TeV Blazars
synchrotron
emission
IC emission
4 keV
414
MeV
4 TeV
X-ray and TeV emission time variability correlates  same e- population
Emission Processes
e- + magnetic fields
 synchrotron radiation
e- + photons
 IC emission
Origin of photons for
IC scattering:
- synchrotron photons
- thermal photons from
disk
- scattered photons
from clouds
However:
Lack of strong emission
lines in BL Lac favour
SSC models
The transition region when Mrk 421 is active
IBIS
JEM-X
SPI
SPI sensitivity
for 3σ detection
JEM-X will detect Mrk 421
with 10σ in ~5000 s!
SPI detects the active Mrk 421 in the
40-100 keV band with 10σ in <104 s
Lightcurves of Mrk 421 from the ASM of RXTE
30 mCrab
Points in red are >3σ detections!
Preliminary ISGRI Maps
20 – 50 keV
50 – 100 keV
100 – 150 keV
39.7 σ
8.7 σ
NGC 4151
160 σ
The Lightcurves of ISGRI (20-80 keV)
steep rise
(2.9 h)
June 14
June 25
The Lightcurves of JEM-X (3-20 keV)
June 14
June 25
Optical Images of the OMC
N
Mrk 421
2`
nearby
star
(V=6)
~15 min
Although a bright star is close
by the photometry of Mrk 421
can be performed with the
OSA analysis tools.
Lightcurves with a high resolution
are available from the OMC!
Preliminary Emission-Region Constraints
from INTEGRAL Observations
Shortest variability time scale: 2.9 hours  size of emission region
l
c  t flare  
1 z
δ = Doppler factor ( 10)
l < 3 · 1015 cm = 203 AU
z = 0.031 (redshift)
c = speed of light
1  2


  (1    cos ) 1   cos 
1
for Θ  0°  β > 0.98
Time Variability of Mrk 421 at TeV Energies
CAT lightcurve at TeV energies (1999-2000)
Mrk 421 shows a very errratic timing
behaviour and a strong flaring activity
emission region < 3.6 AU
Smallest variability time
scale ~30 minutes
Lightcurves at X-Ray & TeV Energies
Whipple observations
X-ray & TeV flares occurred simultaneously to within 1.5 hours
BeppoSAX observations
Flare at TeV and X-ray energies
nearly coincident 
same electron population responsible for X-ray and γ-ray radiation?
from Maraschi et al., Ap. J. 526, L81, 1999
Correlation between X-ray and TeV γ-ray lightcurves
(after Blazejowski et al., Ap. J. 630, 130, 2005)
of
RXTE: 2-60 keV
Δt  5 days
X-ray and TeV γ-ray intensity
correlate, yet only loosely!
However:
The X-rays lag behind the TeV γ-rays in contradiction to the SSC model!
Time Lags
• time lags between X- and γ-rays can help to
distinguish between SSC & EC models
– SSC: Δt  R · c-1 · δ-1 ( 2.8 hours)
• synchrotron photons immediately emitted
• IC photons only after these photons were distributed over
emission volume
– EC: Δt  0 s
• observations so far inconsistent!
• however high-energy X-rays
lag the softer ones (in agreement with pumping e- to higher
energies)! (Fossati et al., Ap. J.
541, 153, 2000)
Energy Dependence of Time Lags at X-Rays
ASCA data show:
soft X-rays (0.5-2 keV)
lag the X-rays from
2-7.5 keV
Interpretation:
radiative cooling of e-!
However reality of these
time lags questioned!
from Takahashi et al., Ap. J. 470, L89, 1996
Evolution of the X-Ray Photon Indices
Higher fluxes have
flatter (harder) spectra!
rising phase
The evolution of the spectrum
is dictated by the interplay of
acceleration, cooling and confinement times. The clockwise
evolution is consistent with
stochastic Fermi acceleration
and synchrotron cooling.
decay phase
Preliminary JEM-X & ISGRI Spectrum
F = A·E-n·exp(-E/Ecut)
A = 0.28 ± 0.02
n = 1.91 ± 0.03
Ecut = (98 ± 16) keV
Emission Maximum at X-ray Energies
For a spectrum with exponential cut off the
emission-maximum energy Ep is given by:
Ep = (2 – n) • Ecut
Inserting the values from above one obtains: Ep = (8.8  3.3) keV
This is an averaged value over the whole observation.
It is the highest peak energy ever measured for Mrk 421!
Total bolometric luminosity:
~1045 erg/s
80
Fbol [e-10 erg/(cm² s)]
Ep correlates nicely with the
bolometric energy (BeppoSAX
data from Massaro et al., Ap. J.
413, 489, 2004):
Correlation between bolometric luminosity and
peak energy
x
70
60
50
40
30
20
10
0
0
1
2
3
4
5
Ep [keV]
6
7
8
9
10
Different Fits to the Spectral Shape
Neither the spectra at X-ray nor at TeV energies can be fitted with a simple power law
 complexer spectral shapes have to be used
X-ray data of BeppoSAX
(Massaro et al., A&A 413, 489, 2004)
TeV data
(Aharonian et al., A&A 350, 757, 1999)
Analytical fit functions at TeV energies
Krennrich et al. (Ap. J. 560, L45, 2001) performed fits to TeV data
of Mrk 421 with different analytical functions:
Power laws do not fit the spectra very well: χ2red = 41
dN
 A  E  a blog E
dE
dN
 A  E n  e
dE
dN
 A  E n  e
dE

 E

 E0
E
E0



m
χ2red = 6.3
χ2red = 2.8
χ2red = 3.0
The TeV data of Mrk 421
can be well fitted by a
power law with an exponential cut off!
Analytical fit functions at X-rays
However at X-rays a power law with exponential cut off does
not fit the data well! Fossati et al. (Ap. J. 541, 166, 2000) used
a continuous combination of 2 power laws:
 E
F ( E )  K  E  1   
  E0 
n
f



nm
f
m & n are the power-law indices for
E >> E0 and E << E0, respectively.
Since this model has 5 parameters which cannot simply be related to physical quantities a log-parabolic function was used:
E
F ( E )  K   
 E0 
 ( a  blog
E
)
E0
Fixing the energy E0 at a useful energy
(in the middle of the considered energy
range) this function has only 3 parameters.
Properties of the log-parabolic function
Calculation of the peak energy of
the spectral energy density νFν
E p  E0 10
2a
2b
The spectral energy
 Ep 
9
 p F ( p )  1.6 10  K  E0  E p   
density at Ep:
E

0

a
2
 erg 
 cm 2 s 


 p F ( p )
The log-parabolic function can be analytically
F    ln 10 
integrated leading to the bolometric luminosity: bol
b
Limitation of the function: restricted to symmetric distributions!
Relation of spectral shape with acceleration process
Assumption:
p = probability of a particle to gain an energy ε in an acceleration step i
γi = ε · γi-1 = ε · ε · γi-2 = · · · · · · = εi · γo (ε > 1 & independent of energy)
Ni = p · Ni-1 = p · p · Ni-2 = · · · = pi · No (p < 1 & independent of energy)
Eliminating i yields:

N
log
log
0
N0
i

log 
log p
 N 

log 
 N0 
log
 
 log  
0 
 
N (  )  N 0   
0 
log p
log p
log
Derivation of log-parabolic spectrum
pi 
Assumption: p depends on energy:
g
g, q > 0 and
constant
 iq
The probability for particle acceleration decreases with increasing energy!
i
gi
i 0
q

 j 0 j
N i  pi  N i 1  pi  pi 1  N i 2      N 0   pi  N 0 
Using γi = ε · γi-1 one obtains:
i 1

j 0
i 1
q
j
   
iq
0
jq
i 1
 
  
iq
0
1
q 2 i( i 1)
j 1
i
 g 
q


Ni  N0   q   
0 
 
1
i (1i )
2
Log-parabolic law
Inserting i = log(γ/γ0)/logε one
gets after some lengthy calculations a log-parabolic law:
 
N (  )  N 0   
0 

)
0
 ( a  blog
Comparison with BeppoSAX data
shows good agreement (Fossati et al.,
A&A 413,489, 2004)
data
from
1999
Shift of Ep clearly seen!
0.414 keV
41.4 keV
Still log-parabolic law
 
N (  )  N 0   
0 

)
0
 ( a  blog
It can be shown analytically that
a and b are linearly correlated.
This is supported by BeppoSAX data!
(Massaro et al., A&A 413, 489, 2004)
Most synchrotron spectra of BL lacs are curved (due to radiative
losses and escape of high-energy electrons from emitting region).
Absorption of TeV γ-rays at
intergalactic photon field
300 GeV – 20 TeV γ-rays interact most
efficiently with IR photons (1 – 50 µm)
via γTeV + γIR  e+ e-
Fmeas  Fem  e  ( E , z )
Deabsorbed TeV spectrum
of Mrk 421 for 2 states
Fint rinsic  Fmeasured  e ( E , z )
Fmeas
measured spectrum
0.41
1.31
4.14 TeV
13.1 TeV
Cosmic Infrared Background
• Absorption effects of TeV quanta allow the
measurement of the cosmic IR/optical background radiation
– closely related to the total electromagnetic luminosity
of the universe since decoupling time (~380 000 years)
• cut-off energy of Mrk 501 and Mrk 421 is
different (6.2 & 3.1 TeV, respectively) 
cut off in Mrk 421 intrinsic and not due to cosmic
IR absorption (since both at same z)!
Some Open Questions
• How is the spectral transition from the low-energy
component to the high-energy component?
• What are the seed photons for the inverse Compton effect?
– internal seed photons or
– external seed photons?
Information from spectral shape at X-ray & TeV energies!
• What is the nature of the accelerated particles (Leptons or
Hadrons)? (different Larmor radii for e- & p lead to
different time scales)
• Are the X- and -ray flaring events related to optical
variations?
• Why is the spectral shape at TeV energies different at the
various activity states (intensity-spectral-shape correlation)?
The End
Goal of the Proposal
• Measurement of the drop off at X-ray & TeV
energies 
– hint about source of seed photons
– shape yields information about radiative energy losses
– traces maximum energy of accelerated particles
thus yielding information about the acceleration
processes
• Measurement of the variability time scale 
distinction between hadronic and leptonic models
• organisation of simultaneous measurements at all
wavelengths
Multiwavelength Spectrum of Mrk 421
Synchrotron
radiation
Synchrotron
self-Compton
Energy range of IBIS and SPI (~4.8 • 1018 Hz <  < ~1.9 • 1021 Hz)
Sensitivity limit of SPI (for 800 ks)