AGILE data on Cygnus X-3 are reviewed focussing on the correlation between the production of gamma-ray transient emission and spectral state changes of the source. AGILE clearly establishes a relation between enhanced gamma-ray emission and the "quenched" radio/hard X-ray states that precede in general major radio flares. We briefly discuss the theoretical implications of our findings.
Cygnus X-3 is a microquasar discovered, as a bright X-ray source, in 1966 [1]. The companion star is a Wolf-Rayet star with a strong stellar helium wind ( Ṁ ∼ 10 -5 M y -1 , v wind ∼ 1000 km s -1 ). The system is located at a distance of about 7-10 kpc and the orbital period is 4.8 hours, inferred from infrared [2], X-rays [3] and γ-rays [4]. Due to the very tight orbit (d ≈ 3 • 10 11 cm), the compact object is totally enshrouded in the wind of the companion star. The nature of the compact object is still unknown [5]: published results suggest either a neutron star of 1.4 M [6] or a black hole with a mass 10 M [7]. In the radio band the system shows strong radio flares ("major flares") reaching up to few tens of Jy. Radio observations at milliarcsec scales confirm emissions (at cm wavelengths) from both a core and a one-sided relativistic jet (v ∼ 0.3 -0.7c), with an inclination to the line-of-sight of 14 • [8]. The radiation from the jet dominates the radio emission from the core during (and soon after) the major flares [9].
Cygnus X-3 exhibits a clear and repetitive pattern of (anti)correlations between radio and X-ray emission, and an overall anticorrelation between soft and hard X-ray fluxes [10,11].
Gamma-ray detections of Cygnus X-3 were reported in the 1970s and in the 1980s at TeV [12][13][14] and PeV energies [15,16]. However, subsequent observations * e-mail: giovanni.piano@iasf-roma.inaf.it by more sensitive ground-based telescopes did not confirm TeV and PeV emission from the source [17]. Furthermore, the COS-B satellite could not find any clear emission from Cygnus X-3 at MeV-GeV energies [18] and CGRO/EGRET observations of the Cygnus region (1991)(1992)(1993)(1994), even confirming a γ-ray detection above 100 MeV consistent with the position of Cygnus X-3 [19], could not demonstrate a solid association with the microquasar. The firm γ-ray detection of Cygnus X-3 was announced at the end of 2009: on December 2, 2009 the AGILE Team claimed the discovery of strong γ-ray flares above 100 MeV [20], and on December 11, 2009 the Fermi -LAT collaboration confirm the AGILE-GRID results, by announcing the firm detection in γ-rays of the 4.8-hour orbital period of Cygnus X-3 [4].
Here we present an extended analysis on Cygnus X-3 with respect to the work published by the AGILE Team in 2009 [20], taking into account the AGILE-GRID data collected between 2007-November-02 and 2009-July-29, during the AGILE “pointing” mode data-taking. We note the temporal repetitive coincidence of the γ-ray major flares with peculiar soft X-ray spectral states and pre-flaring radio states. We briefly discuss the theoretical implications of our findings in the perspective of the spectral modeling of this microquasar.
Between November 2007 and July 2009 AGILE repeatedly pointed at the Cygnus region for a total of eConf C110509 arXiv:1110.6043v2 [astro-ph.HE] 28 Oct 2011 ∼275 days, corresponding to a net exposure time of ∼11 Ms. Seven γ-ray flares, each lasting 1-2 days, were detected (Table I). The analysis was carried out by using a Multi-Source maximum-Likelihood Analysis (MSLA), to take into account the strong emission of the nearby γ-ray pulsars (PSR J2021+3651, PSR J2021+4026 and PSR J2032+4127). By integrating all the flaring episodes, we found a γ-ray source detected at 5.9σ ( √ T S = 5.9) at the average Galactic coordinate (l, b) = (80.0 • , 0.9 • ) ± 0.5 • (stat) ± 0.1 • (syst), with a photon flux of [131 ± 27 (stat) ± 10% (syst)]•10 -8 photons cm -2 s -1 . The average differential spectrum between 100 MeV and 3 GeV is well described by a power law with a photon index Γ = 1.93 ± 0.23 (Figure 1). By using this photon index, a Multi-Source maximum-Likelihood Analysis (MSLA), applied in the deep integration of the AGILE-GRID data (between November 2007 and July 2009), found a weak persistent emission (significance:
√ T S = 5.17) from a position consistent with Cygnus X-3 with a photon flux of [14 ± 3 (stat) ± 10% (syst)]•10 -8 photons cm -2 s -1 . So the average “flaring” flux is about 10 times the steady flux associated to Cygnus X-3, and the photon flux of a single γ-ray flare can be as high as ∼20 times greater than the steady flux (Table I).
In order to analyze the pattern of emission, the plot in Figure 2 shows the comprehensive multi-wavelength light curve of Cygnus X-3. The γ-ray emission of the flaring episodes is compared with hard X-ray fluxes from Swift/BAT (15-50 keV), soft X-ray fluxes from RXTE /ASM (3-5 keV) and radio flux density (when available) from AMI-LA (15 GHz) and RATAN-600 (2.15, 4.8, 11.2 GHz) radio telescopes.
Observing the light curve in Figure 2, we can notice that: • every time we detect γ-ray activity, Cygnus X-3 is in a soft spectral state (the 3-5 keV RXTE /ASM count rate 3 counts s -1 );
• every time we detect γ-ray flaring episodes (Table I and red points in the AGILE-GRID light curve in Figure 2) -with exception of the γ-ray flare of 11-12 February 2008 (MJD: 54507.76-54508.46)
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