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observations of mrk 421 with integral

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
Aim of the Proposal
  • To perform simultaneous or quasi-simulta-neous 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
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 d l 130 mpc

Schwarzschildradius:Rs = (3.9 - 15.8) AU

M = (2-8) · 108 M

radius of last stable orbit:r = 3Rs = (11.8 – 47.4) AU

Parameters of Mrk 421(z = 0.031 dl 130 Mpc)

Keplervelocity at r:v = 0.41 · ctrot = 25 h – 4.2 d

spectrum of tev blazars


IC emission


4 keV

4 TeV

Spectrum of TeV Blazars

X-ray and TeV emission time variability correlates  same e- population

emission processes

e- + magnetic fields synchrotron radiation

e- + photons IC emission

Emission Processes

Origin of photons forIC scattering:- synchrotron photons- thermal photons from disk- scattered photons from clouds

However:Lack of strong emissionlines in BL Lac favourSSC models

the transition region when mrk 421 is active




SPI sensitivityfor 3σ detection

The transition region when Mrk 421 is active

JEM-X will detect Mrk 421 with 10σ in ~5000 s!

SPI detects the active Mrk 421 in the40-100 keV band with 10σ in <104 s

lightcurves of mrk 421 from the asm of rxte
Lightcurves of Mrk 421 from the ASM of RXTE

30 mCrab

Points in red are >3σ detections!

preliminary isgri maps

50 – 100 keV

20 – 50 keV

100 – 150 keV

160 σ

8.7 σ

39.7 σ

Preliminary ISGRI Maps

NGC 4151

optical images of the omc


Mrk 421



~15 min

Optical Images of the OMC

Although a bright star is closeby the photometry of Mrk 421can be performed with theOSA analysis tools.

Lightcurves with a high resolutionare available from the OMC!

preliminary emission region constraints from integral observations
Preliminary Emission-Region Constraintsfrom INTEGRAL Observations

Shortest variability time scale: 2.9 hours  size of emission region

δ = Doppler factor ( 10)z = 0.031 (redshift)c = speed of light

l < 3 · 1015 cm = 203 AU

for Θ  0°  β > 0.98

time variability of mrk 421 at tev energies

Smallest variability time

scale ~30 minutes

Time Variability of Mrk 421 at TeVEnergies

CAT lightcurve at TeV energies (1999-2000)

Mrk 421 shows a very errratic timingbehaviour and a strong flaring activity

emission region < 3.6 AU

lightcurves at x ray tev energies

Whipple observations

X-ray & TeV flares occurred simul-taneously to within 1.5 hours

BeppoSAX observations

Lightcurves at X-Ray & TeV Energies

Flare at TeV and X-ray energiesnearly coincident same electron population respon-sible for X-ray and γ-ray radiation?

from Maraschi et al., Ap. J. 526, L81, 1999

correlation between x ray and tev ray lightcurves after blazejowski et al ap j 630 130 2005

Δt  5 days

of RXTE: 2-60 keV

Correlation between X-ray and TeV γ-ray lightcurves(after Blazejowski et al., Ap. J. 630, 130, 2005)

X-ray and TeV γ-ray intensitycorrelate, yet only loosely!

However:The X-rays lag behind the TeV γ-rays in contradiction to the SSC model!

time lags
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-rayslag the softer ones (in agree-ment with pumping e- to higherenergies)! (Fossati et al., Ap. J.541, 153, 2000)
energy dependence of time lags at x rays
Energy Dependence of Time Lags at X-Rays

ASCA data show:

soft X-rays (0.5-2 keV)lag the X-rays from2-7.5 keV

Interpretation:radiative cooling of e-!

However reality of thesetime lags questioned!

from Takahashi et al.,Ap. J. 470, L89, 1996

evolution of the x ray photon indices

4-15 keV

rising phase



decay phase

Evolution of the X-Ray Photon Indices

Higher fluxes haveflatter (harder) spectra!

The evolution of the spectrumis dictated by the interplay of acceleration, cooling and con-finement times. The clockwiseevolution is consistent withstochastic Fermi accelerationand synchrotron cooling.

preliminary jem x isgri spectrum
Preliminary JEM-X & ISGRI Spectrum

F = A·E-n·exp(-E/Ecut)A = 0.28 ± 0.02n = 1.91 ± 0.03

Ecut = (98 ± 16) keV

emission maximum at x ray energies


Emission Maximum at X-ray Energies

For a spectrum with exponential cut off theemission-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!

Ep correlates nicely with the bolometric energy (BeppoSAX data from Massaro et al., Ap. J. 413, 489, 2004):

Total bolometric luminosity:~1045 erg/s

different fits to the spectral shape
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
Analyticalfit functions at TeV energies

Krennrich et al. (Ap. J. 560, L45, 2001) performed fits to TeV dataof Mrk 421 with different analytical functions:

Power laws do not fit the spectra very well: χ2red = 41

χ2red = 6.3

The TeV data of Mrk 421can be well fitted by apower law with an expo-nential cut off!

χ2red = 2.8

χ2red = 3.0

analytical fit functions at x rays
Analyticalfit functions at X-rays

However at X-rays a power law with exponential cut off doesnot fit the data well! Fossati et al. (Ap. J. 541, 166, 2000) used a continuous combination of 2 power laws:

m & n are thepower-law indices forE >> E0 and E << E0, respectively.

Since this model has 5 parameters which cannot simply be re-lated to physical quantities a log-parabolic function was used:

Fixing the energy E0 at a useful energy(in the middle of the considered energyrange) this function has only 3 parameters.

properties of the log parabolic function
Properties of the log-parabolic function

Calculation of the peak energy ofthespectral energy density νFν

The spectral energydensity at Ep:

The log-parabolic function can be analyticallyintegrated leading to the bolometric luminosity:

Limitation of the function: restricted to symmetric distributions!

relation of spectral shape with acceleration process
Relation of spectral shape with acceleration process

Assumption:p = probability of a particle to gain an energy ε in an acceleration step iγi = 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:

derivation of log parabolic spectrum
Derivation of log-parabolic spectrum

g, q > 0 andconstant

Assumption: p depends on energy:

The probability for particle acceleration decreases with increasing energy!

Using γi = ε · γi-1 one obtains:

log parabolic law

data from 1999

0.414 keV

41.4 keV

Log-parabolic law

Inserting i = log(γ/γ0)/logε onegets after some lengthy calcula-tions a log-parabolic law:

Comparison with BeppoSAX datashows good agreement (Fossati et al.,A&A 413,489, 2004)Shift of Ep clearly seen!

still log parabolic law
Still log-parabolic law

It can be shown analytically thata 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 radiativelosses and escape of high-energy electrons from emitting region).

absorption of tev rays at intergalactic photon field

Deabsorbed TeV spectrum of Mrk 421 for 2 states

measured spectrum

0.41 1.31 4.14 TeV

13.1 TeV

Absorption of TeV γ-rays atintergalactic photon field

300 GeV – 20 TeV γ-rays interact mostefficiently with IR photons (1 – 50 µm)via γTeV + γIR e+ e-


cosmic infrared background
Cosmic Infrared Background
  • Absorption effects of TeV quanta allow themeasurement of the cosmic IR/optical back-ground 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
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 correla-tion)?
goal of the proposal
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 particlesthus 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



Energy range of IBIS and SPI (~4.8 • 1018 Hz <  < ~1.9 • 1021 Hz)

Sensitivity limit of SPI (for 800 ks)

Multiwavelength Spectrum of Mrk 421