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-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

• 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 · ctrot = 25 h – 4.2 d

Structure of a TeV Blazar

TeV blazarsrelativistic-moving blobsof Leptons or Hadrons

Spectrum of TeV Blazars

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

synchrotronemission IC emission

414MeV

4 keV 4 TeV

Emission Processes

e- + magnetic fields synchrotron radiation

e- + photons IC emission

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

IBISJEM-X

SPI sensitivityfor 3σ detection

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

Points in red are >3σ detections!

30 mCrab

Preliminary ISGRI Maps

100 – 150 keV

8.7 σ

50 – 100 keV

39.7 σ

20 – 50 keV

160 σ

NGC 4151

The Lightcurves of ISGRI (20-80 keV)[°

]

June 14 June 25

steep rise(2.9 h)

The Lightcurves of JEM-X (3-20 keV)

June 14 June 25

[°]

Optical Images of the OMCN

Mrk 421

nearbystar

(V=6)2`

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

~15 min

Lightcurves with a high resolutionare available from the OMC!

Preliminary Emission-Region Constraints

from INTEGRAL Observations

Shortest variability time scale: 2.9 hours size of emission region

z

tcl flare

1

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

l < 3 · 1015 cm = 203 AU

cos1

1

)cos1(

1 2

for Θ 0° β > 0.98

Time Variability of Mrk 421 at TeV Energies

CAT lightcurve at TeV energies (1999-2000)

Smallest variability timescale ~30 minutes

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

emission region < 3.6 AU

Lightcurves at X-Ray & TeV Energies

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

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

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

BeppoSAX observations

Whipple observations

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!

Δt 5 days of RXTE: 2-60 keV

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 agree-ment with pumping e- to higherenergies)! (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 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 Indices4-

15 k

eV

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.

hardening

softening

rising phase

decay phase

Preliminary JEM-X & ISGRI Spectrum

F = A·E-n·exp(-E/Ecut)

A = 0.28 ± 0.02n = 1.91 ± 0.03Ecut = (98 ± 16) keV

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

Correlation between bolometric luminosity and peak energy

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10

Ep [keV]

Fb

ol [

e-1

0 e

rg/(

cm

² s

)] x

Different Fits to the Spectral ShapeNeither 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

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

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

EbaEAdE

dN log χ2red = 6.3

0E

E

n eEAdE

dN χ2

red = 2.8

m

E

E

n eEAdE

dN

0χ2

red = 3.0

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

Analytical fit 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:

f

mnf

n

E

EEKEF

0

1)(m & n are the power-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:

)log(

0

0

)(E

Eba

E

EKEF

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

b

a

p EE 2

2

0 10

Calculation of the peak energy ofthe spectral energy density νFν

The spectral energydensity at Ep:

scm

erg

E

EEEKF

a

pppp 2

2

00

9106.1)(

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

FF ppbol

)(10ln

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:

p

NN

ilog

log

log

log00

p

N

Nlog

0

log

0

loglog

log

log

00)(

p

NN

Derivation of log-parabolic spectrumAssumption: p depends on energy:

qi

i

gp

g, q > 0 and

constant

The probability for particle acceleration decreases with increasing energy!

1

0

00

0211 i

j

qj

ii

iiiiiiii

gNpNNppNpN

Using γi = ε · γi-1 one obtains: )1(

2

1

0

1

10

1

0

iiqiqi

j

jqiqi

j

qj

)1(2

1

00

iiq

i

qi

gNN

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

)log(

00

0

)(

ba

NN

Comparison with BeppoSAX datashows good agreement (Fossati et al.,A&A 413,489, 2004)

Shift of Ep clearly seen!data from 1999

0.414 keV 41.4 keV

Still log-parabolic law)log(

00

0

)(

ba

NN

Most synchrotron spectra of BL lacs are curved (due to radiativelosses and escape of high-energy electrons from emitting region).

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)

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-

),(int

zEmeasuredrinsic eFF

),( zEemmeas eFF

Fmeas

0.41 1.31 4.14 TeV

13.1 TeV

Deabsorbed TeV spectrum of Mrk 421 for 2 states

measured spectrum

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

• 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)?

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 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)

Synchrotronradiation

Synchrotronself-Compton