4.4.1 Multiwavelength campaigns of blazars in outburst
Due to their low spatial density compared to that of Seyfert galaxies, blazars constitute a minority of the AGN detected by INTEGRAL, although their luminosity is on average larger than that of Seyferts (see Sect. 5.3). Only the brightest blazars are observable by INTEGRAL and are preferentially tar-gets when they undergo an outburst phase. Therefore, the best strategy consists in observing them as Targets of Opportunity (ToO). Furthermore, the X-ray band is crucial for the modelling of the spec-tral energy distributions of blazars and therefore simultaneous multiwavelength observations need to accompany the high-energy ones.
With this purpose, a group of colleagues led by Dr. Elena Pian have constituted a net to coordinate multiwavelength observations of blazars during flaring states. In particular, for every INTEGRAL AO since the beginning of the mission, a ToO proposal has been accepted for the observation of a blazar (out of a list of a few hundreds), triggered by a particularly bright state in the optical, in the X-rays or in the gamma-rays. On two occasions, for S0716+714 and 3C 454.3, the ToO observations have been successfully activated and I have contributed to the analysis of the INTEGRAL data that have been included in the Pian et al. (2005) and Pian et al. (2006) publications.
INTEGRAL ToO observation of S0716+714
The BL Lac S0716+714 underwent two outbursts in March 2004, after having increased steadily its optical flux since the beginning of the year. When the source reached an optical flux R= 12.1 mag, the INTEGRAL ToO observations were triggered and performed on April 2–6, 2004 (Pian et al. 2005).
S0716+714 was detected by ISGRI at a flux level twice as high as observed by BeppoSAX in 2000.
The detection significance was higher in the first part of the observation indicating that the source was already weakening. The signal-to-noise ratio was too low to extract a spectrum and the source was detected only up to 60 keV. Only upper limits could be obtained with SPI and JEM-X. Quasi-simultaneous observations with XMM-Newton allowed to construct the spectral energy distribution in the optical-UV and X-ray band (Foschini et al. 2006b). The XMM data confirmed that in this source the X-ray emission above 2 keV corresponds to the beginning of the Compton peak, whereas the soft X-rays are the tail of the synchrotron branch, as expected in LBL objects like S0716+714.
During this INTEGRAL observation, 2 Seyfert galaxies (Mkn 3 and Mkn 6) and the FSRQ S5 0836+
710 were serendipitously detected by ISGRI and hard X-ray spectra could be extracted (Pian et al.
2005). Mkn 6 and S5 0836+710 spectra were best fitted by a simple power law with photon index Γ =1.5 and 1.3, respectively, consistent with those found by BeppoSAX/PDS and CGRO/BATSE. A hint of a break or a cut-off was found in the Mkn 3 spectrum but the low statistic of the data above 70 keV did not allow a firm conclusion.
INTEGRAL ToO observation of 3C 454.3
3C 454.3 is one of the brightest FSRQ in the blazar class. It has been extensively observed at all wavelengths from radio to the gamma-rays and presents the two-peaks spectral energy distribution (SED) typical of blazars. In April-May 2005, the millimeter to X-ray emission of 3C 454.3 increased
Figure 4.4: SEDs of 3C 454.3 at different epochs: in each panel the simultaneous data are highlighted in colours and the corresponding fitted model is represented (Ghisellini et al. 2007). The grey points indicate the data taken at other epochs. Top panel: BeppoSAXdata from 2000 plus other non-simultaneous historical data (red points). Middle panel:two sets of simultaneous data (red and blue points) collected during the 2005 flare, including INTEGRAL data.Bottom panel: SwiftandAGILEdata collected during the 2007 outburst (red points).
dramatically, reaching R = 12 in the optical band and becoming the brightest AGN at X-rays2. An INTEGRAL ToO observation was performed for 200 ks in May 2005 and the source was found to be 2–3 times brighter than observed in the past (Pian et al. 2006). A 20% decrease in the ISGRI flux and a possible drop by 10–15% of the JEM-X flux during the INTEGRAL observations indicate that the source was probably already in the declining phase of the flare. JEM-X, ISGRI and SPI combined spectrum is best fitted by a power law with Galactic absorption and photon indexΓ =1.8 up to 200 keV. This slope indicates that the inverse Compton branch of the spectrum is rising in the hard X-rays and that the peak is located above 200 keV. Simultaneous millimeter, near-infrared and optical observations were performed and allowed to reconstruct the SED during this flare. Pian et al. (2006) suggest that this SED is well represented by a strong contribution of synchrotron emission in the mm-UV and that the Compton peak is dominated by the contribution of synchrotron self-Compton (SSC) processes. Instead, the Compton emission of the historical SED would be dominated by inverse Compton on the BLR photons.
2The high activity of 3C 454.3 is actually still ongoing. In July 2007, another bright optical flare (R = 12.8 mag) triggered a repointing of the gamma-ray satellite AGILE that found the source in a bright gamma-ray state with a flux F(>100 MeV)=2.8×10−6ph cm−2s−1(1.2 Crab). Several other gamma-ray detections at a flux of about F(>100 MeV)= 10−6ph cm−2s−1(0.44 Crab) followed since July 2007, the last one in June 2008 reported by Vittorini et al. (2008).
INTEGRAL observations of blazars 39 Katarzy´nski & Ghisellini (2007) have proposed an alternative interpretation based on the internal shock model. In this model, relativistic blobs of plasma travel through the jet and collide with different speeds giving rise to internal shocks. These shocks accelerate electrons to relativistic energies, which in turn are responsible for the observed multiwavelength radiation. Katarzy´nski & Ghisellini (2007) propose that the location of the shock and the compactness of this region depends on the bulk Lorentz factorΓof the blob, i.e. lower-Γblobs collide in a region closer to the base of the jet and with larger magnetic field. Therefore, for smallΓ, the emission is dominated by synchrotron radiation and the inverse Compton contribution is rather due to SSC. For larger Γ, the external photons are seen as significantly boosted (by a factorΓ2) and the external Compton emission dominates over the SSC, while the synchrotron peak is decreased due to the lower magnetic field. When a constant power of the jet is assumed, Katarzy´nski & Ghisellini (2007) succeed to model the SEDs of 3C 454.3 at different epochs explaining the differences only as a function of the Lorentz factor adopted. A new flare was observed in July 2007 and allowed Ghisellini et al. (2007) to model the SEDs of 3 different epochs (in 2007, 2005 and the historical one with BeppoSAX data from 2000; Fig. 4.4), obtaining results in qualitative agreement with those of Katarzy´nski & Ghisellini (2007). The jet power emitted during these periods results to be similar and the 2000 and 2007 SEDs requireΓ = 16, whereas the 2005 one is best fitted withΓ = 7−8 (for the two sets of observations in this period). This series of works clearly shows the importance of the X-ray and gamma-ray measurements to constrain the physics of the jets in blazars.
4.4.2 INTEGRAL and XMM-Newton observations of 3C 273
Since the beginning of the mission, INTEGRAL has repeatedly observed the radio loud quasar 3C 273 (see Chapter 7 for a more detailed analysis of the properties of this object). In particular, about 850 ks of data were accumulated between 2003 and 2005, and some of these observations were performed quasi-simultaneously to XMM-Newton ones. Chernyakova et al. (2007) have analysed these data and studied the broad-band X-ray emission of 3C 273 and its connection to the optical-UV emission (using the XMM/OM data). The INTEGRAL-XMM combined spectrum in 5 different epochs is best fitted by a hard power law plus a soft cut-off power law (describing the soft-excess) or two power laws below and above 2 keV. Haardt et al. (1998) first, and later Grandi & Palumbo (2004) fitted 3C 273 BeppoSAX spectra with a model that is the superposition of a non-thermal contribution from the jet and a Seyfert-like contribution, i.e. a power-law continuum modified by reflection and an iron emission line. This spectral decomposition did not apply to these XMM-INTEGRAL data. No correlation was found between the X-ray photon index and the ratio of the UV and X-ray fluxes, contrarily to what has been found by Walter & Courvoisier (1992) and interpreted as evidence that the X-rays are the result of thermal Comptonisation of the UV photons. Chernyakova et al. (2007) argue instead that the soft X-rays could be the high-energy tail of the synchrotron emission. On the other hand, a correlation between the X-ray and UV flux is found that was not observed before for this source. Chernyakova et al. (2007) suggest that this peculiar behaviour of the source compared to that seen in past observations could be attributed to a real change of its spectral properties during the last decades. In fact, the X-ray data of the last 40 years, available from our 3C 273 database (see Chapter 8), show that there has been an evolution of the photon index towards a softer spectrum, ranging from Γ = 1.4 in the late 1970s up to a maximum of Γ = 1.82 in June 2003 (Fig. 4.5).
This spectral evolution is considered an important element to unveil the origin of the X-ray emission.
Chernyakova et al. (2007) argue that the hard photon index of 1.4 observed in the past can be explained neither with X-ray emission from the jet nor with emission from the nuclear regions, in a compact, pair-production dominated source, whereas the states with Γ > 1.5 can be accounted for in both
1970 1980 1990 2000 1
1.2 1.4 1.6 1.8 2
Time RXTE
XMM-Newton/PN BeppoSAX/MECS EXOSAT ASCA GINGA
HEAO2/MPC UHURU
OSO8/A HEAO-1 ARIEL5/C
BALL/AIT
XMM-Newton/PN This Work 1
2 3