The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002
The extra-galactic source blazar 3C 66A was observed in 2002 with the aid of the air Cherenkov telescope GT-48 dedicated for stereoscopic measurements of extensive air showers (EAS). The analysis of the observational data obtained during the period November–December with a total exposure time of 6h4...
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Головна астрономічна обсерваторія НАН України
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irk-123456789-796442015-04-04T03:02:30Z The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 Fidelis, V.V. MS3: Physics of Stars and Galaxies The extra-galactic source blazar 3C 66A was observed in 2002 with the aid of the air Cherenkov telescope GT-48 dedicated for stereoscopic measurements of extensive air showers (EAS). The analysis of the observational data obtained during the period November–December with a total exposure time of 6h40m revealed a γ-ray flux atE >1 TeV ≈ 93% of the steady flux from the Crab Nebula at 5.03 σ level. The observations have shown variations of flux intensity from source in the very-high energy (VHE) band which may be interpreted as low and high states of the object during the observing period. The temporal properties of object are correlated well with quasi-simultaneous observations in the 2–10 keV energy band as measured by the All Sky Monitor onboard RXTE. 2005 Article The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 / V.V. Fidelis // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 205-210. — Бібліогр.: 21 назв. — англ. 0233-7665 http://dspace.nbuv.gov.ua/handle/123456789/79644 en Кинематика и физика небесных тел Головна астрономічна обсерваторія НАН України |
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MS3: Physics of Stars and Galaxies MS3: Physics of Stars and Galaxies Fidelis, V.V. The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 Кинематика и физика небесных тел |
| description |
The extra-galactic source blazar 3C 66A was observed in 2002 with the aid of the air Cherenkov telescope GT-48 dedicated for stereoscopic measurements of extensive air showers (EAS). The analysis of the observational data obtained during the period November–December with a total exposure time of 6h40m revealed a γ-ray flux atE >1 TeV ≈ 93% of the steady flux from the Crab Nebula at 5.03 σ level. The observations have shown variations of flux intensity from source in the very-high energy (VHE) band which may be interpreted as low and high states of the object during the observing period. The temporal properties of object are correlated well with quasi-simultaneous observations in the 2–10 keV energy band as measured by the All Sky Monitor onboard RXTE. |
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Fidelis, V.V. |
| author_facet |
Fidelis, V.V. |
| author_sort |
Fidelis, V.V. |
| title |
The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 |
| title_short |
The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 |
| title_full |
The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 |
| title_fullStr |
The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 |
| title_full_unstemmed |
The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 |
| title_sort |
variations of very-high energy γ-quanta flux from the blazar 3c 66a in 2002 |
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Головна астрономічна обсерваторія НАН України |
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2005 |
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MS3: Physics of Stars and Galaxies |
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http://dspace.nbuv.gov.ua/handle/123456789/79644 |
| citation_txt |
The variations of very-high energy γ-quanta flux from the blazar 3C 66A in 2002 / V.V. Fidelis // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 205-210. — Бібліогр.: 21 назв. — англ. |
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Кинематика и физика небесных тел |
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2025-07-06T03:40:14Z |
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2025-07-06T03:40:14Z |
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1836867350974955520 |
| fulltext |
THE VARIATIONS OF VERY-HIGH ENERGY γ-QUANTA FLUX
FROM THE BLAZAR 3C 66A IN 2002
V. V. Fidelis
Scientific-Research Institute “Crimean Astrophysical Observatory”
Nauchny, 98409 Crimea, Ukraine
e-mail: fedelis@crao.crimea.ua
The extra-galactic source blazar 3C 66A was observed in 2002 with the aid of the air Cherenkov tele-
scope GT-48 dedicated for stereoscopic measurements of extensive air showers (EAS). The analysis
of the observational data obtained during the period November–December with a total exposure
time of 6h40m revealed a γ-ray flux at E > 1 TeV ≈ 93% of the steady flux from the Crab Nebula at
5.03σ level. The observations have shown variations of flux intensity from source in the very-high
energy (VHE) band which may be interpreted as low and high states of the object during the ob-
serving period. The temporal properties of object are correlated well with quasi-simultaneous
observations in the 2–10 keV energy band as measured by the All Sky Monitor onboard RXTE.
INTRODUCTION
Observation of VHE cosmic γ-rays at different redshifts is a means to probe the intergalactic infrared radiation
field (IIRF) and to test cosmological models [16, 20]. Propagating in intergalactic medium, VHE γ-rays interact
with IR photons through the pair-production mechanism and attenuate the high-energy ends of blazar spectra.
Thus, looking for cutoffs in the γ-ray spectra of distant blazars, one may judge about the energy density of IIRF.
Another aspect of VHE cosmic γ-rays observations arises with the problems of empirical and theoretical
research of radiation processes in blazars and of the understanding their structure. Blazars emit an essential
part of their power in the high energy and VHE bands. Their fundamental feature is the beamed continuum,
caused by relativistic jets ejected from the central engine [5].
According to commonly adopted models, the γ-ray emission from blazars may be explained by two main
mechanisms – the synchrotron and inverse Compton (IC) radiation. Among them, the most popular model
is the Synchrotron-Self Compton one (SSC) in which the Compton scattered photons by relativistic electrons
emitting the synchrotron radiation have energies in some cases in the TeV range [13]. Due to the synchrotron and
IC radiation, the spectral energy distribution (SED) of γ-bright blazars is marked by two broad peaks, the first
of them is extending from the radio to UV/X-ray band and the second one may reach TeV energies [18].
Emission in two peaks, correlated in different ways, may be studied by quasi-simultaneous X-ray and TeV
observations of blazars. Usually, these radiation processes are studied in the extreme states of blazars such as
flaring with the highest seed photon densities and highest energy of electrons. Timing analysis of observing data
and resolving characteristic time-scales of blazars may be used to constrain the geometry and spatial structure
of emitting regions on different scales.
VHE γ-rays may be observed with the aid of the ground-based technique detecting Cherenkov light from
induced EAS. Some γ-ray sources which have been detected by the EGRET detector onboard the Compton
Gamma-Ray Observatory, covering the energy band from about 30 MeV to above 20 GeV [19], have displayed
the fluxes in the VHE band too. Among them are Mrk 421 at z = 0.031 [14], Mrk 501 at z = 0.033 [15],
1ES 2344+514 at z = 0.044 [4], 1ES 1959+650 at z = 0.047 [2], BL Lac at z = 0.069 [11], H 1426+428 at
z = 0.129 [1], and 3C 66A at z = 0.444 [12].
The last object has the highest redshift in comparison with the others which makes the increasing order
of redshift. Thus, its observations are of great interest. The blazar 3C 66A was also classified as a BL Lac
object owing to its high optical polarization and the brightness variability with an amplitude of about 2 mag [9].
Its flux in the 0.2–4 keV range reached 10−11 erg cm−2 s−1 in the flaring period according to X-ray observations
onboard the Einstein Observatory [8].
In this paper, we present our observations of 3C 66A at E > 1 TeV in comparison with flux variations
in X-rays.
c© V. V. Fidelis, 2004
205
CHERENKOV TELESCOPE AND OBSERVATION
The second generation γ-ray telescope GT-48 is situated at an altitude of 600 m above sea level. It consists
from two identical alt-azimuth mountings, or sections, separated apart by 20 m in the North–South direction.
Four telescopes, which we call “elements”, are mounted on each section. The optics of each element consists of
four mirrors with diameters of 1.2 m with a common 5-meter focus. In the focal plane of each element, there is
a camera consisting of 37 photomultipliers (cells) with a hexagonal grid pattern which organize 37 channels. Each
camera has field of view equal to 2.6◦ and records images of Cherenkov flashes in the visible band (300–560 nm).
The signals from the cells from the four elements on each section are linearly added, channel by channel.
Flashes are registered only when the amplitudes of the signals for any two of the 37 channels exceed a certain
threshold. The time resolution for the coincidence scheme is 15 ns. Flashes are recorded in a digitized form.
The total area of mirrors on both sections is 36 m2. The mountings are geared by a control system with
a pointing accuracy of 0.05◦. The effective threshold energy for the detection of γ-rays is approximately 1 TeV.
The telescope was described in detail in [21].
Observations were made in the ON/OFF mode, when ON runs (source exposure) are followed by OFF
runs of the background registration with a 30 minutes shift on right ascension. Both ON and OFF runs were
25 minutes in duration and carried out in the same range of elevation. In all 21 ON/OFF pairs were taken in two
dark moon periods. By processing of observing data, the weather conditions and apparatus noises were taken
into account, and 5 ON/OFF pairs were disregarded. 16 pairs with a total exposure time of 6h40m and the same
time of background observations were taken for processing of Cherenkov light images from EAS produced by
γ-ray primaries and cosmic ray (CR) primaries.
DATA REDUCTION AND ANALYSIS
Parameters of the remaining flashes were processed using formal mathematical methods. The first and second
moment of the brightness distribution was calculated and used for deriving of the centres of the brightness
distribution and effective dimensions of flash images. The parameters of flashes recorded simultaneously at each
section were determined independently using the data for each section.
All the registered showers, coming from the source direction (ON runs) as well as from the surrounding sky
region (OFF runs) are induced by a hadronic component of CR, mainly protons. Candidates for γ-rays are
selected using differences in the shape and dimensions of images and their orientation concerning the source
position in the field of view of camera. The images of γ-ray showers have the less effective dimensions and
compact forms and are aligned in the direction of source, while the images from proton showers are greater,
have more fragmented forms and an isotropic orientation.
Firstly, for the shape cut, we have used the so-called IPR parameter. It was assigned to the zero value for
images having the most compact form and to 1–7 for more fragmented images. Usually, the IPR parameter did
not exceed 1, in other words, is was assigned to 0, 1. The IPR parameter set may be different for each section
and, also, for each object under analysis. For the 3C 66A data, parameter IPR was equal to 0 for both sections.
For selection of γ-like events from their images, their effective dimensions – length, width, orientation pa-
rameters miss (the perpendicular distance of the centre of the field (the source) from the major image axis),
and dist (the distance of image centroid from source) – were used [6] (Fig. 1).
Flashes were also selected taking into account amplitudes of detected signals because the parameters of
flashes with low amplitudes are defined with high errors.
Figure 1. The main selection parameters of flash image. O is the centre of the brightness distribution, S is source
position in the field of view of camera
206
THE RESULTS OF ANALYSIS
The selection statistics is summarized in Table 1. Results are given in terms of the parameter miss accom-
panied with selection of events by parameter dist. The mean counting rate for observing period is equal
to (0.163 ± 0.032) γ-quanta min−1. To convert this counting rate to flux, we have expressed it as a fraction
of the steady counting rate of VHE γ-quanta from the Crab Nebula observed in the same season, equal to
(0.176 ± 0.033) γ-quanta min−1 [10]. Comparison of two quantities for count rates gives a value of flux of VHE
γ-quanta for 3C 66A, which is equal to (0.926 ± 0.355) Crab.
Table 1. Recorded and selected events
Selection Number of Number of Resulting events Significance,
algorithm events on events on number σ
source background
No selection 5811 5709 102 0.95
Selection by amplitude,
form and dimension 402 347 55 2.01
Selection by
miss < 0.175◦ 116 51 65 5.03
During four years of observations in CrAO (1996–2000), the blazar 3C 66A had exhibited an averaged flux
of VHE γ-quanta equal to (2.8 ± 0.4) · 10−11 cm−2 s−1 at E > 1 TeV [17]. From the calculations of the flux
from the Crab Nebula with the same energy threshold we derived the value of (1.5 ± 0.3) · 10−11 cm−2 s−1.
This quantity is in accordance with our observations of the Crab Nebula in 2002 [10] and a commonly accepted
value [7]. Thus, the average flux of VHE γ-quanta from 3C 66A in 1996–2000 is equal to (1.9 ± 0.6) Crab.
On the basis of this reanalysis of the early reported data, we may conclude that the source was in 2002 in full
in a low state.
Using the method of trial sources [3] based on the orientation difference of images from γ-ray and CR
primaries, the distribution of selected flashes over the detector’s field of view was calculated, and the position
of the true γ-ray source was determined. Figure 2 shows a three-dimensional histogram, and Figure 3 shows
isophotes of this distribution. The precision of the determination of coordinates from the Cherenkov data
is ±0.1◦. (Δα, Δδ) are deviations from the source in right ascension and declination. Nγ is the number of
selected events. The form of histogram confirms that the detected VHE γ-quanta were emitted by a point
source in the direction of the object with the coordinates of 3C 66A.
Figure 2. Stereo “map” of the distribution of γ-ray arrival directions from 3C 66A
207
Figure 3. The “map” of isophotes for γ-ray source 3C 66A. The highest excess of γ-like events appears at (−0.1, −0.1).
The external isophote corresponds to 28 events. The isophotes step is 8 events
SOURCE VARIABILITY
The flux from the blazar 3C 66A at E > 1 TeV in observing period was variable. There may be seen two
small flares (Fig. 4). In the first flare, occurred on November 29 (MJD52607.8), the flux rose up to 2 Crab at
2.7σ level. In the next night, the flux decreased up to 0.9 Crab at 1.51σ level. In the second flare, occurred on
December 2 (MJD52611), the source come to the flux maximum for the observed period, equal to 2.3 Crab at
3.43σ. In the next night, the source also was in the high state approximately on the level of 1.7 Crab (2.69σ).
The summed flux of these two nights approached to 2 Crab (4.34σ).
If we represent the behaviour of the source by two states – below and above the Crab level, – then the summed
flux in the high state will have value of ≈ 2 Crab at 5.05σ level and a low one – ≈ 0.43 Crab at 2.2σ level
(Fig. 4).
Figure 4. Light curve for 3C 66A at E > 1 TeV. Error bars are purely statistical. The short dash line shows the average
count rate of VHE γ-quanta from the Crab Nebula
So that, the flux of VHE γ-quanta at high state was two times greater than the average flux in observing
period and ≈ 4.7 times greater than the flux in the low state. This is a good accordance with the common
property of γ-bright blazars – a strong variability in the VHE band.
208
X-RAY/γ-RAY CORRELATION
From Fig. 5 it can be seen that the X-ray/γ-ray flux variations are similar. Their most dominant feature is
a simultaneous rising on November 28–29 (MJD52607– 52608) and decreasing by the end of observations. These
flux features may also be seen more distinctly in Fig. 6.
The keV counterpart of the second TeV flare coincides with it within 2 days. In both flares, the leading role
of the IC emission is seen.
Figure 5. Light curves for 3C 66A at E > 1 TeV and in the 2–10 keV energy band (quick-look results provided by
the ASM/RXTE team)
Figure 6. TeV/keV light curves in the dark moon period from November 28 till December 5, 2002
DISCUSSION
The analysis of our data obtained in 2002 with the GT-48 telescope confirms the existence of the TeV emis-
sion from the blazar 3C 66A comparable with the flux from the Crab Nebula at a confidential level ≈ 5σ.
The comparison with our early reported data for the 1996–2000 epoch shows the variability of the source in
the VHE band on the yearly scale.
In the observing period, the behaviour of the source at E > 1 TeV may be represented by two states
with a flux ratio of about 4.6. The increasing keV activity which is nearly coincided with the occurrence of
the TeV flares and the light curves in both energy bands display the common tendency to decrease the activity
by the end of observations.
Thus, our observations reveal the similar behaviour of 3C 66A in both energy bands.
209
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