A multi-wavelength view of VHE gamma ray flares from
Mrk421 and Mrk 501 observed by ARGO-YBJ experiment
S.Z Chen (IHEP/CAS/China, [email protected])
On behalf of the ARGO-YBJ collaboration
 1. Introduction
 2. ARGO-YBJ experiment
 3. Flares of Mrk 421
 4. Flares of Mrk 501
 5. Summary and outlook
4300m a.s.l.
Tibet/China
1. Introduction
Leptonic SSC, EC? Hadronic?
Acceleration? Location?
Production of flares?
？
The flux and radiation spectra are highly variation. Thus, longterm continuously multi-wavelength observations,
particularly at X-ray and TeV band, are crucial to understand
the emission mechanisms and underline processes of the
outbursts.
VHE gamma-ray detectors
IACTs, e.g. H.E.S.S., VERITAS and MAGIC, ∼1% ICrab. FOV 3~5deg
and only work on moonless night duty cycle ~10%, IACTs cannot
long-term monitor on AGN


EAS arrays, e.g. Tibet ASr, Milagro, ARGO-YBJ, are essential to
long-term monitor on AGN due to FOV ~2sr and >86% duty cycle,
even with lower sensitivity than IACTs.
ARGO-YBJ
MAGIC
VERITAS
HESS
FOV
Long-term multi-wavelength data
• X-ray(2-50keV)：
RXTE (1995-2011)
MAXI (2009.8-now)
Swift (2005-now)
• Gamma-ray(0.1-300GeV)
Fermi/LAT (2008.08now)
• VHE gamma-ray (0.3-20TeV)
ARGO-YBJ
(2007.112013.01)
Swift
Fermi
ARGO
2. ARGO-YBJ experiment
4300m a.s.l.
74 m
1 CLUSTER = 12 RPC
(43 m2)‫‏‬
78 m
111 m
Number of Fired Strips
99 m
Tibet/China
Start from July 2006
Rate：3.5kHz
Threshold：~ 300 GeV
FOV: 2 sr
RPC
8 Strips
10 Pads
2
(56 x 62 cm2)‫( ‏‬6.5 x 62 cm )
for each RPC for each Pad
Long-term stability
-87 σ


Stable operation: since November 2007
The average duty cycle > 86%
Moon shadow is detected at 10σper month,
the position and significance are stable.

Moon shadow
Duty cycle >86%
VHE gamma ray sources observation
• Crab is a standard candle at VHE
gamma-ray.
• The SED obtained by ARGO-YBJ
is in good agreement with other
experiments
• Mrk 421 and Mrk 501 are bright
Blazars in ARGO-YBJ sky.
3. Flares of Mrk 421
Mrk 421 is an
excellent candidate for
the study of the jets
of AGN.

With frequent major
outbursts about once
every two years.

Fermi
RXTE/2-12keV
Swift/15-50keV
ARGO-YBJ
ARGO-YBJ observation
• A good several years long-term correlation
between X-ray and TeV gamma-ray including both
active and quiet phases.
Bartoli et al. ApJ 734:110 (2011)
X-ray and TeV gamma-ray correlation
• No significant lag longer than 1 day is found. The tight
correlation between X-ray and TeV emission
indicates their same origin!
• It is contrary to the result obtained by Whipple that TeV
lags behind X-ray 1.8±0.4 days (Blazejowski et al. 2005)
RXTE/ASM
&ARGO-YBJ
Bartoli et al. ApJ 734:110 (2011)
Evolution
The spectra harden when the flux increases in both Xray and TeV.

Quadratic relation between TeV and X-ray fluxes seems
to be the best one.

y=ax2+b
y=ax+b
Krennrich et al. 2002
Spectral Modelling
In the framework of one-zone SSC model, The flux
variation seems to be caused by the variation of the
maximum energy and density of the electron injection
spectrum.

Bartoli et al. ApJ 734:110 (2011)
Flare in June 2008
The moonlight hampered the IACTs observations during the
most intense part of the emission in June 11-13.

The gamma-ray flux is higher than that predicted in the
analysis of Donnarumma et al. (2009).

Aielli et al. ApJ 714:L208 (2010)
Flare in Feb. 2010
injected electron SED should
be changed during the flare.

peak
After peak
Flare in Feb. 2012
The largest GeV gammaray flare, while the GeV
spectral index is constant.

The TeV SED is very soft.

4. Flares of Mrk 501
RXTE/2-12keV
During the 20112012 flare, Mrk501
appeared in day
time. Only EAS
array can
1997observe it.
Swift/15-50keV
2011-2012 flare
• Flares only in X-ray and TeV band.
ARGO-YBJ TeV
Bartoli et al. ApJ 758:2 (2012)
2011-2012 flare (epoch A)
• One-zone SSC model can fit steady SED well, but
cannot fit flare SED. The radiation mechanism
may be different during steady and flare states.
8.4 TeV
Bartoli et al. ApJ 758:2 (2012)
Summary
• ARGO-YBJ has continuously monitored Mrk 421 and Mrk
501at TeV bands since Nov. 2007, and 4.5 years longterm simultaneous X-ray/GeV/TeV observation is
achieved by combining ARGO-YBJ , Fermi and X-ray
satellites.
• Many flares from Mrk 421 and Mrk 501 are
simultaneously observed by large FOV detectors at multiwavelength bands. Different SED evolution types are
observed, which will be essential to study the underline
mechanism of production of the flares and process within
jet .
• More detailed study of the flares are still going on!
Outlook of ground-based EAS arrays
•
•
•
Past EAS arrays (before 2013)
Tibet ASr: 1990-2008
Milagro:
1999-2008
ARGO-YBJ: 2006-2013
50~200% Icrab
Nearly future EAS arrays (~2014)
Tibet ASr+MD,
HAWC
LAWCA
~10% Icrab
Future EAS arrays (~2018?)
LHAASO
~1% Icrab, 0.3~1000 TeV
~100% Icrab
~10% Icrab
~1% Icrab