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INTEGRAL AND BLAZARS
R. Hudec1,2 , F. Munz1,2 , P. Kubanek1,2 , and E. Pian3
1

Astronomical Institute, Academy of Sciences of the Czech Republic, CZ­251 65 Ondrejov, Czech Republic 2 ISDC, Versoix, Switzerland 3 INAF, Trieste, Italy

ABSTRACT We refer to analysis of the ESA INTEGRAL satellite data for specific class of active galactic nuclei - blazars. These objects represent promising sources to be observed by INTEGRAL, especially during their active states. Suitable strategy for the future analysis is proposed and discussed. The probability to see blazars with INTEGRAL instruments will increase with time as better limits will be achieved and the chance to have an observation during flaring episode will also correspondingly increase. Key words: INTEGRAL satellite, blazars. Figure 1. IBIS total exposure times for blazars, CP data, revolutions 1­250. 3. CORE PROGRAMME OBSERVATIONS

1. INTRODUCTION Blazars represent the most extreme class of active galaxies, they are powerful and variable. They are observed in all wavelength bands ­ from radio through VHE gamma frequencies, with maximum spectral output and largest variability often at gamma ray energies and contribution to the extragalactic background radiation across the electromagnetic spectrum (Giommi & Colafrancesco [2], Giommi [1]). 66 blazars were identified as sources of 100 MeV emission by EGRET aboard CGRO (Hartman [6]) and 6 blazars were identified as VHE gamma sources (350 GeV) by Cerenkov telescopes (Krawczynski [7]). It is obvious that blazars represent suitable targets for INTEGRAL satellite (Winkler et al. [14]) especially during active states (flares) and that the chance for that will increase gradually in time.

Blazars in the INTEGRAL Galactic Plane Scans (GPS) represent a promising group of objects for the study within the INTEGRAL CP. The GPS zone is usually neglected by extragalactic astronomers due to heavy obscuration: in optical, 20% of the sky is obscured by our Galaxy, while the gamma-ray telescopes on board INTEGRAL allow detection of up to few mCrabs in the most exposed GPS regions. Despite of the fact that blazars belong to extragalactic objects, seven optically bright (with V 17 mag, to be detected by the INTEGRAL OMC camera) blazars were identified within galactic scans of INTEGRAL, namely: 1ES 0647+250, PKS 0823­223 (no gamma from EGRET, gravitational lensing candidate), 1ES 2344+514 (TeV gamma ray source, very close), 8C 0149+710 (probably BL Lac candidate), 4C 47.08, 87GB 02109+5130 (poorly understood blazar, TeV candidate), and BL Lac (the prototype). While the prototype object BL Lac is well studied, most of the INTEGRAL GPS blazars have been poorly investigated and poorly understood so far. The study with Sonneberg Observatory Archival Plates reveals that most of these objects are optically variable, hence a gamma ray variability can be expected. The optical analyses of the objects are affected by the fact that they are in crowded

2. INTEGRAL OBSERVATIONS The INTEGRAL observations are basically divided into the following categories: (i) AO-1, 2, 3 Program (allocated pointed observations); (ii) Core Program CP (Galactic Plane Scans, Galactic Center Deep Exposure,...); and (iii) Objects inside FOV of AO-1, 2, 3 observations.


Figure 2. The position of the INTEGRAL CP blazar 4C47.08 on the IBIS mosaics.

Figure 4. IBIS total exposure times for blazars, all data, revolution 1­250.

Figure 5. The optical light curve of 3C454.3 around the time of the INTEGRAL AO-3 ToO observation. Figure 3. Top: Historical optical light curve of NRAO530. The source exhibits rare but large amplitude optical flares (4 mag). Bottom: Optical monitoring of GPS blazar 4C47.08 (Tosti et al., 2005).


4.

AO OBSERVATIONS

Additional blazars have been identified in the fields represented by the AO observations of other scheduled targets, covered by up to 400 ksec cumulative exposure time. Regarding the pointed observations of blazars by INTEGRAL, the AO-2 ToO blazar observation No. 220049 by Pian et al. ([8]) has provided promising results. This collaborative proposal was based on extended optical and/or X-ray monitoring (RXTE ASM and others) of flaring activity of a large list of blazars and, alternatively, on soft gamma-ray monitoring by INTEGRAL itself (serendipitous detection of a flaring blazar in the IBIS FOV). Then ToO INTEGRAL observation was activated meeting the trigger criteria (major flaring event). The program was coordinated with the XMM Newton ToO program. Blazar S5 0716+714 was the target of this ToO observation. This is a BL Lac object, intensively monitored at radio and optical wavelengths by Whole Earth Blazar Telescope (more than 40 telescopes, Villata et al. [12]). The ToO was triggered by optical activity - 2 outbursts up to the extreme level of R = 12.1 mag (historical maximum, light increase by 1 mag in 2 weeks and 2 mags in 4 months) and, consequently, the INTEGRAL ToO observation was performed in the time interval 2004 April 2­7 ([8], [9]). The object was observed at somewhat higher (2x) gamma-ray state when in Oct 2000 (BeppoSAX ToO, Tagliaferri et al. [10]) (R =12.5 versus 12.1). There was a low signal/noise ratio and hence no intra-orbit variability study was possible, and no spectrum extraction. Within the INTEGRAL observation, the S5 0716+714 was detected only by IBIS ISGRI at 4.5 sigma, in 30­ 60 keV band, for a count rate of 0.11 counts s-1 (exposure 280 ksec). No signal above 60 keV was detected. The target was better detected at the beginning (decline), hence the useful exposure was reduced to 84 ksec. There was no positive detection in IBIS/PICsIT, SPI and JEM­ X (less than 292, 6 and 6 mCrab, Pian et al. [8]). Within the ToO observation, another extragalactic sources were observed in the IBIS FOV (19 â 19 degrees at half response) - 3 additional AGNs with higher significance than the main blazar target. These 3 AGNs were observed up to 100 keV with no intra-orbit variability study possible. However, the spectrum extraction was possible for S5 0836+710 (high z blazar of the FSRQ sub-class): single power-law spectrum, for Mkn 6 (bright Seyfert): single power law spectrum, and for Mkn 3 (bright Seyfert): broken power-law with cutoff at 50 keV (Pian et al. [8]). This AO-2 blazar observation clearly confirmed that even with relatively short exposures, the INTEGRAL is an efficient tool to study bright AGNs at high Galactic latitudes and also has demonstrated the importance of high-energy instruments with large FOV and good angular resolution (like IBIS/INTEGRAL). In addition, the detection of two

Figure 6. INTEGRAL IBIS/ISGRI images of 3C454.3, 20­40 (left) and 100­200 keV (right).

galacic fields. Below the detection limit of the INTEGRAL OMC on board camera is blazar NRAO530 (1730­130), which is an example of blazar with violent optical activity (4 mags within 1 month). In flare, the object is expected to be much brighter also in gamma. This strengthens the role of optical monitoring and ToO program ­ the flare can be recognized by optical monitoring with small (D 50 cm) telescopes. The short hard X­ray flare from this object has beeen indeed reported by Foschini et al. [4], although the independent inspection of IBIS data by our group has not lead to the confirmation of this event. The historical optical light curve of NRAO530 (Fig. 3) however shows very prominent optical flares with amplitude about 4 magnitudes indicating that flaring activity in this target can indeed be expected. Unfortunately, there are no optical observations around the time of the flare reported by Foschini et al. [4]. All the above mentioned blazars in INTEGRAL GPS have been investigated with INTEGRAL CP data (IBIS and JEM­X telescopes). We have no positive detection by high energy instruments on board INTEGRAL yet (except marginal detection of 1ES 0647+250). The targets quiet level is still below the sensitivity threshold of the instruments. However, the positive detection may be possible in the future as (i) there will be more cumulative time available and (ii) the probability to see a blazar during a flare (and hence much brighter) will also increase with time.


high z blazars (S5 0836+710 at z = 2.17 and PKS 1830­ 21 at z = 2.51), the most distant objects seen by INTEGRAL so far shows that INTEGRAL can also play a role in investigation of high z Universe. More recently, an INTEGRAL AO-3 ToO observation of 3C454.3 (z = 0.859) was performed, with preliminary results given by L. Foschini et al. ([3], PI E. Pian with a large collaboration) and more detailed results by Pian et al. [5]. This ToO was triggered by high optical (T. Balonek, VSNET alert) and X-ray (BAT Swift) activity of the source. The INTEGRAL observation started 2005 May 15, at 18:40 UT, with exposure of 200 ksec. The source was clearly detected by IBIS/ISGRI in the 20­ 40 keV and 40­100 keV energy bands, with a significance of 20 and 15 sigma. The observed fluxes were 1.02±0.05 cts s-1 = 9.4 ± 0.5 mCrab in 20­40 keV, 1.00±0.08 cts s-1 = 13 ± mCrab in 40­100 keV, and 1.6 â 10-10 erg cm-2 s-1 in 20­100 keV band. The observed spectrum was flat, with photon index 2.2±0.2 and normalization 0.13 (+0.10 -0.06) ph cm-2 s-1 keV-1 . The allocated 200 ks were however not enough to detect the object up to 400 keV, more than 400 ks would be necessary for that. It is important to continue the future INTEGRAL AO observations of blazars with longer exposures in order to fully show the importance of scientific study of blazars with this satellite. We further stress the importance of ground-based monitoring of potential targets for observations by INTEGRAL, including flaring and violently variable blazars, since the early recognition of starting flare can be essential for scheduling and organizing the high­energy observation.

not allow to see the blazars in quiescence. The efforts to predict active states of blazars, based both on theoretical modelling as well as on observational data, as described recently by Valtonen et al. [15] may be very useful to schedule ToO observations in time.

ACKNOWLEDGMENTS We acknowledge by the ESA PECS project 98023. Some parts of this study are linked to the project A3003206 by the GA of the AS CR. The International GammaRay Astrophysics Laboratory (INTEGRAL) is an European Space Agency mission with instruments and science data center funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Spain, Switzerland), Czech Republic and Poland, and with the participation of Russia and the USA.

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5. CONCLUSIONS For blazars in GPS, no positive detection by high-energy instruments on board INTEGRAL has been confirmed yet (except marginal detection of 1ES 0647+250, the suspected flare of NRAO530 [4] and the newly confirmed PKS 1830­211). The targets quiet level is still below the sensitivity threshold of the instruments. On the other hand, the positive detection may be possible in the future as: (i) there will be more cumulative time available, and (ii) the probability to see a blazar during a flare (and hence much brighter) will also increase with time. For the AO blazar observation, it has been confirmed that with relatively short exposures the INTEGRAL is an efficient tool to study bright AGNs at high Galactic latitudes. This also confirms the importance of high-energy instruments with large FOV and good angular resolution. Furthermore, it is also an excellent proof that the approach by Pian et al. is the right one for variable sources and INTEGRAL-extended monitoring and ToO observations of blazars in active states. We suggest to recognize the scientific value of ToO observations of flaring blazars (and highly variable objects in general) by the INTEGRAL satellite since the observational bias mostly do