DOI 10.1393/ncc/i2011-10869-x
Colloquia: Scineghe2010
IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011
Fermi-LAT view of Intermediate Synchrotron Peaked blazars
C. Monteon behalf of the Fermi-LAT CollaborationDipartimento di Fisica “M. Merlin” dell’Universit`a e del Politecnico di Bari I-70126 Bari, Italy
INFN, Sezione di Bari - 70126 Bari, Italy
(ricevuto il 25 Febbraio 2011; pubblicato online il 12 Maggio 2011)
Summary. — After the first year of scientific activity with the Large Area Tele-scope (LAT), i.e. the primary instrument onboard Fermi, a catalog of Active Galac-tic Nuclei has been published. The first LAT AGN Catalog (1LAC) includes 671 gamma-ray sources located at high galactic latitudes, detected with a test statistic (T S) > 25. Among these sources, an interesting group of objects is the one com-posed by Intermediate Synchrotron Peaked (ISP) blazars, i.e. sources for which the synchrotron emission peaks at intermediate frequencies (from 1014Hz to 1015Hz). These objects are expected to have their Spectral Energy Distribution (SED) high-energy peak centered on the Fermi-LAT band (from 20 MeV to 300 GeV). For the brightest 1LAC ISP sources the analysis has been extended to 22 months data in order to constrain better their spectral features and characterize the temporal evo-lution of their gamma-ray spectra.
PACS 07.85.Fv – X- and γ-ray sources, mirrors, gratings, and detectors. PACS 29.40.-n – Radiation detectors.
PACS 07.85.-m – X- and γ-ray instruments.
PACS 98.54.Cm – Active and peculiar galaxies and related systems (including BL Lacertae objects, blazars, Seyfert galaxies, Markarian galaxies, and active galactic nuclei).
1. – Introduction
The Gamma-ray Large Area Space Telescope was launched on June 11, 2008 into a low Earth circular orbit at an altitude of 550 km and an inclination of 25.6◦. It began its scientific operation two months later, and shortly thereafter, it was renamed the Fermi
Gamma-ray Space Telescope.
The Large Area Telescope (LAT), the main instrument onboard F ermi, is a pair-production telescope with large effective area (∼ 8000 cm2, on axis for E > 1 GeV) and field of view (∼ 2.4 sr at 1 GeV), sensitive to gamma rays in the energy range from 20 MeV to more than 300 GeV [1].
c
258 C. MONTE on behalf of the FERMI-LAT COLLABORATION
Fig. 1. – (Colour on-line) Locations of the associated sources in the 1LAC. Red full circles: FSRQs, blue full circles: BL Lacs, magenta stars: radio galaxies, green triangles: AGNs of unknown type [2].
One of the major scientific goals of the Fermi mission is to investigate high-energy emission in Active Galactic Nuclei (AGNs). Although AGNs are probably one of the better-known class of gamma-ray emitting objects, there are many questions that are yet unanswered, as the mechanisms by which the particles are accelerated, the precise site of the gamma-ray emission and the origin of AGN variability [2].
2. – The first Large Area Telescope AGN Catalog (1LAC)
The 1st LAT Catalog of Active Galactic Nuclei (1LAC) [2], includes 671 sources detected in 11 months (from August 4, 2008 to July 4, 2009) with galactic latitudes
|B| > 10◦and a test statistic greater than 25 and statistically associated with 709 AGNs
(due to some multiple associations). The 1LAC includes 300 BL Lacertae (BL Lacs) blazars, 296 Flat Spectrum Radio Quasar (FSRQs) blazars, 41 AGNs of other types (including radio galaxies) and 72 AGNs of unknown type, distributed in the sky as shown in fig. 1.
3. – The SED-based classification of blazars
AGNs are highly variable objects, therefore simultaneous observations in different energy bands are essential to understand the emission processes that take place in these objects. For 48 sources of the LBAS (LAT Bright AGN Sample) [3] it has been possible to combine Fermi -LAT three months (August 4, 2008 to October 31, 2008) gamma-ray data with simultaneous (or quasi-simultaneous) observations at other wavelengths, in order to assemble high-quality and quasi-simultaneous Spectral Energy Distributions (SEDs) [4]. All the SEDs clearly show the typical two-bump signature attributed to synchrotron and inverse Compton emission. The study of this unprecedented collection of blazars has allowed one to estimate a certain number of parameters characterizing the SED of blazars, as the frequency of the synchrotron peak (νSpeak) and of the inverse Compton peak (νICpeak). The determination of νSpeak for a large number of blazars has permit-ted to introduce a new SED-based classification scheme [4] for all blazars: Low
Syn-chrotron Peaked (LSP) blazars are sources with the synSyn-chrotron peak at low energy, i.e.
in the far IR or IR band (or νSpeak ≤ 1014Hz); Intermediate Synchrotron Peaked (ISP) blazars are sources for which the synchrotron emission peaks at intermediate frequencies
FERMI-LAT VIEW OF INTERMEDIATE SYNCHROTRON PEAKED BLAZARS 259
Fig. 2. – (Colour on-line) PKS 0426-380 SED: unfolding method (filled black squares), gtlike with the source modeled with a PL in each energy bin (filled magenta triangles) and gtlike with the source modeled with a LP in the whole energy band (continuous cyan line). The upper limits cut has been fixed at a value of T S = 10.
(from 1014Hz to 1015Hz); High Synchrotron Peaked (HSP) blazars have the synchrotron peak at UV or higher energies (νSpeak≥ 1015Hz).
4. – The Intermediate Synchrotron Peaked (ISP) blazars analysis
For ISP blazars, the inverse Compton (high energy) peak in the SED is expected to be exactly within the Fermi -LAT energy band. Consequently, looking at the gamma-ray spectra of 1LAC blazars [2], a preliminary list of about 30 sources showing the SED high-energy peak in the Fermi -LAT high-energy band has been built. For these sources, that are ISP candidates, LAT 22 months data (from August 4, 2008 to June 4, 2010) have been analyzed in the energy range from 100 MeV to 300 GeV, in order to obtain the spectral energy distributions in the gamma-ray band and the light curves.
For the spectral analysis, different methods have been used. In the first case (unfolding method) the spectra have been evaluated using a deconvolution technique [5] based on the Bayes theorem, that allows one to take into account the energy dispersion. In the second case, spectra have been reconstructed using Fermi science tool gtlike [6], where a parametric model (Power Law, PL) is assumed in each individual energy bin for the source spectrum and for the background components and a maximum-likelihood fit is performed for each energy bin. In the third method of analysis, the source under investigation has been modeled with a Log-Parabola (LP) function in the whole energy band (from 100 MeV to 300 GeV) and then a maximum-likelihood fit has been performed. As an example, fig. 2 shows the gamma-rays SED for the PKS 0426-380 (1FGL J0428.6-3756 [7], celestial coordinates: RA = 67.1567◦, DEC =−37.94121◦), obtained with the different methods of analysis: the unfolding method (filled black squares), gtlike with the source modeled with a PL in each energy bin (filled magenta triangles) and gtlike with the source modeled with a LP in the whole energy band (continuous cyan line). lt is evident that the results of the different methods are consistent.
The plots in fig. 3 show the light curve for the integral flux greater than 100 MeV for the PKS 0426-380 obtained dividing the entire 22 month data set in time intervals, respectively, of 1 month each (top) and 15 days each (bottom). For each time interval, a maximum-likelihood fit has been performed considering a region of interest of 15 degrees around the source under investigation and modeling the source under study with a power
260 C. MONTE on behalf of the FERMI-LAT COLLABORATION
Fig. 3. – Light curve for the integral flux greater than 100 MeV for the PKS 0426-380 obtained dividing the entire 22 month data set in time intervals of 1 month each (top) and 15 days each (bottom).
law with all spectral parameters free. It is worth to underline that in our ISP candidates sample there are sources showing variability (like that whose light curves are shown in fig. 3) and others that seem to be almost stable during the 22 months period.
5. – Conclusions
A sample of about 30 ISP candidates has been selected from the 1LAC. Preliminary spectral analysis performed over a period of 22 months seems to show that the log-parabola function can be used to model the shape of the SED high-energy peak in the Fermi -LAT gamma-ray energy band for these sources. The study of the temporal evolution has started with the construction of the light curves with 1 month and 15 days time intervals. The analysis results shown here are still preliminary: a further analysis will be performed, especially to investigate any spectral features of these blazars and possible connections between the variability of the sources and eventual changes in their spectral parameters.
∗ ∗ ∗
The Fermi -LAT Collaboration acknowledges support from a number of agencies and institutes for both development and the operation of the LAT as well as scien-tific data analysis. These include NASA and DOE in the United States, CEA/Irfu and IN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the National Space Board in Sweden. Additional support from INAF in Italy and CNES in France for science analysis during the operations phase is also gratefully acknowledged.
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[5] Mazziotta M. N., Proceeding of the 31 ICRC, L´od´z, Poland (2009).
[6] http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicerone/ [7] Abdo A. A. et al., Astrophys. J. Suppl. Ser., 188 (2010) 405.
