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Register a new userOptimizing Multi-Wavelength Blazar Studies through Fermi-LAT and Swift Synergy
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Blazar flares seen by the Fermi Gamma-Ray Space Telescope Large Area Telescope (Fermi LAT) are often followed up by Target of Opportunity (ToO) requests to the Neil Gehrels Swift Observatory (Swift). Using flares identified in the daily light curves of Fermi LAT Monitored Sources, we investigated which follow-up Swift ToO requests resulted in refereed publications. The goal was to create criteria of what Swift should look for in following up a Fermi-LAT gamma-ray flare. Parameters tested were peak gamma-ray flux, flare duration (based on a Bayesian Block analysis), type of AGN (BL Lac or FSRQ), and pattern of activity (single flare or extensive activity). We found that historically active sources and high-photon-flux sources result in more publications, deeming these successful Swift ToOs, while flare duration and type of AGN had little or no impact on whether or not a ToO led to a publication.
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We report the results of a multi-band observing campaign on the famous blazar 3C 279 conducted during a phase of increased activity from 2013 December to 2014 April, including first observations of it with NuSTAR. The $gamma$-ray emission of the source measured by Fermi-LAT showed multiple distinct flares reaching the highest flux level measured in this object since the beginning of the Fermi mission, with $F(E > 100,{rm MeV})$ of $10^{-5}$ photons cm$^{-2}$ s$^{-1}$, and with a flux doubling time scale as short as 2 hours. The $gamma$-ray spectrum during one of the flares was very hard, with an index of $Gamma_gamma = 1.7 pm 0.1$, which is rarely seen in flat spectrum radio quasars. The lack of concurrent optical variability implies a very high Compton dominance parameter $L_gamma/L_{rm syn} > 300$. Two 1-day NuSTAR observations with accompanying Swift pointings were separated by 2 weeks, probing different levels of source activity. While the 0.5$-$70 keV X-ray spectrum obtained during the first pointing, and fitted jointly with Swift-XRT is well-described by a simple power law, the second joint observation showed an unusual spectral structure: the spectrum softens by $DeltaGamma_{rm X} simeq 0.4$ at $sim$4 keV. Modeling the broad-band SED during this flare with the standard synchrotron plus inverse Compton model requires: (1) the location of the $gamma$-ray emitting region is comparable with the broad line region radius, (2) a very hard electron energy distribution index $p simeq 1$, (3) total jet power significantly exceeding the accretion disk luminosity $L_{rm j}/L_{rm d} gtrsim 10$, and (4) extremely low jet magnetization with $L_{rm B}/L_{rm j} lesssim 10^{-4}$. We also find that single-zone models that match the observed $gamma$-ray and optical spectra cannot satisfactorily explain the production of X-ray emission.
The flat-spectrum radio quasar 4C $+$71.07 is a high-redshift ($z=2.172$), $gamma$-loud blazar whose optical emission is dominated by the thermal radiation from accretion disc. 4C $+$71.07 has been detected in outburst twice by the AGILE $gamma$-ray satellite during the period end of October - mid November 2015, when it reached a $gamma$-ray flux of the order of $F_{rm E>100,MeV} = (1.2 pm 0.3)times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$ and $F_{rm E>100,MeV} = (3.1 pm 0.6)times 10^{-6}$ photons cm$^{-2}$ s$^{-1}$, respectively, allowing us to investigate the properties of the jet and of the emission region. We investigated its spectral energy distribution by means of almost simultaneous observations covering the cm, mm, near-infrared, optical, ultra-violet, X-ray and $gamma$-ray energy bands obtained by the GASP-WEBT Consortium, the Swift and the AGILE and Fermi satellites. The spectral energy distribution of the second $gamma$-ray flare (the one whose energy coverage is more dense) can be modelled by means of a one-zone leptonic model, yielding a total jet power of about $4times10^{47}$ erg s$^{-1}$. During the most prominent $gamma$-ray flaring period our model is consistent with a dissipation region within the broad-line region. Moreover, this class of high-redshift, large-mass black-hole flat-spectrum radio quasars might be good targets for future $gamma$-ray satellites such as e-ASTROGAM.
The long-term optical, X-ray and $gamma$-ray data of blazar 3C 279 have been compiled from $Swift$-XRT, $RXTE$ PCA, $Fermi$-LAT, SMARTS and literature. The source exhibits strong variability on long time scales. Since 1980s to now, the optical $R$ band light curve spans above 32 yr, and a possible 5.6-yr-long quasi-periodic variation component has been found in it. The optical spectral behavior has been investigated. In the optical band, the mean spectral index is -1.71. The source exhibits an obvious special spectral behavior. In the low state, the source shows a clear bluer-when-brighter behavior in a sense that the optical spectrum turns harder (flatter) when the brightness increases. While in the high state, the optical spectrum is stable, that means the source spectral index does not vary with the brightness. The correlation analysis has been performed among optical, X-ray and $gamma$-ray energy bands. The result indicates that the variations of $gamma$-ray and X-ray bands are well correlated without time delay on the time scale of days, and their variations exhibit weak correlations with those of optical band. The variations, especial outbursts, are simultaneous, but the magnitude of variations is disproportionate. The detailed analysis reveals that the main outbursts exhibit strong correlations in different $gamma$-ray, X-ray and optical bands.
The Swift Burst Alert Telescope (BAT) hard X-ray transient monitor tracks more than 700 galactic and extragalactic sources on time scales ranging from a single Swift pointing (approximately 20 minutes) to one day. The monitored sources include all objects from the Fermi LAT bright source list which are either identified or which have a 95% error confidence radius of less than eight arc minutes. We report on the detection statistics of these sources in the BAT monitor both before and after the launch of Fermi.
We present six-year multi-wavelength monitoring result for broad-line radio galaxy 3C 120. The source was sporadically detected by Fermi-LAT and after the MeV/GeV gamma-ray detection the 43 GHz radio core brightened and a knot ejected from an unresolved core, implying that the radio-gamma phenomena are physically connected. We show that the gamma-ray emission region is located at sub-pc distance from the central black hole, and MeV/GeV gamma-ray emission mechanism is inverse-Compton scattering of synchrotron photons. We also discuss future perspective revealed by next-generation X-ray satellite Astro-H.
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