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We present the spectral and timing results for two TeV-detected blazars of unknown redshift RGB J0136+391 and H1722+119, which are based on long-term multiwavelength (MWL) observations carried out with several space and ground-based instruments. The sources showed a strong variability in the 0.3–12 keV energy range. The X-ray spectra were generally soft with a photon index higher than 2.0 and exhibited a synchrotron peak position in the UV—soft X-ray range (Ep<2 keV). H1722+119 demonstrated a very weak harder-when-brighter spectral trend that could be related to the emergence of soft X-ray components during the 0.3–10 keV flares. While the source mostly showed power-law spectra in this band (possibly formed by the first-order Fermi mechanism), RGB J0136+391 demonstrated the importance of stochastic acceleration and logparabolic spectra with low curvatures (a curvature parameter b∼0.3), as well as a significantly stronger harder-when-brighter spectral evolution. The sources exhibited strong flaring activity in the 0.1–300 GeV energy range with various flaring profiles: symmetrical (variability is dominated by the disturbance’s crossing timescale), positive and negative asymmetries (gradual particle acceleration and jet radial expansion with non-uniformity of the Doppler factor, respectively), two-peak maxima (collision of two relativistic shocks or ‘blobs’ containing high-energy plasma, which move with different speeds) and plateau-like variations after the flux rising phase (the observed emission is generated in several emission zones). The lognormal function was well-fit with the distributions of the X-ray and MeV–GeV fluxes, hinting at the possible imprint of the accretion disc’s variability onto the jet. The 0.1–300 GeV spectra showed features of the hadronic contribution and inverse-Compton upscatter of synchrotron photons in the Klein–Nishina regime.

We report the detection by the Astrorivelatore Gamma a Immagini Leggero (AGILE) satellite of terrestrial gamma ray flashes (TGFs) obtained with the minicalorimeter (MCAL) detector operating in the energy range 0.3–100 MeV. We select events typically lasting a few milliseconds with spectral and directional selections consistent with the TGF characteristics previously reported by other space missions. During the period 1 June 2008 to 31 March 2009 we detect 34 high‐confidence events showing millisecond durations and a geographical distribution peaked over continental Africa and Southeast Asia. For the first time, AGILE‐MCAL detects photons associated with TGF events up to 40 MeV. We determine the cumulative spectral properties of the spectrum in the range 0.5–40 MeV, which can be effectively described by a Bremsstrahlung spectrum. We find that both the TGF cumulative spectral properties and their geographical distribution are in good agreement with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) results.

In this paper, we present the timing and spectral results for the seven distant and poorly investigated BL Lacertae sources with redshifts z≳0.5, using the archival data obtained with the Swift and other multifrequency instruments during 2006 August–2015 July. Our timing study has revealed a number of the 0.3–10 keV flux changes from the fluctuations observed within one day to the variabilities on timescales of a few months. Namely, the relatively densely-sampled observations of BZB J1517+6525 showed a strong X-ray flare by a factor of ∼5 during 2014 September-December. While this instance can be explained by the propagation of shock wave though the blazar jet which causes a strong heat-up of the relativistic plasma, the lower-amplitude flux fluctuations observed in higher X-ray states could be related to the interaction between the shock front and jet inhomogeneities. The curved 0.3–10 keV spectra showed the ranges of the photon index at 1 keV a=1.27(0.10)–2.30(0.09), curvature parameter b=0.12(0.08)–1.19(0.27), synchrotron SED peak position Ep=0.49(0.09)–3.39(0.97) keV, while some spectra do not exhibit spectral curvature and fit well with a simple power law model yielding the 0.3–10 keV photon-index Γ=1.48(0.15)–2.32(0.08). Large spectral curvatures observed for the majority of the log-parabolic spectra hint at the importance of the second-order Fermi mechanism for our targets. Our targets show very high isotropic luminosity L0.3–10keV∼1046ergs−1 compared to the nearby bright BL Lacertae objects.

We present the results based on the monitoring of the high energy peaked BL Lacertae object 1ES 2344+514 with the satellite Swift during 2005–2015. Our timing study shows that the source was highly variable on longer (weeks-to-months) time-scales with the 0.3–10 keV flux ranging by a factor of 13.3. The flux variability exhibited an erratic character, changing its amplitude and minimum flux level from flare to flare. In some epochs, an X-ray flare was accompanied by enhanced optical-UV activity, although the uncorrelated 0.3–10 keV and lower-frequency variabilities were also often seen. Our target was significantly passive on intra-day timescales compared to other HBLs. The logparabolic distribution of the X-ray emitting electrons and the underlying physical processes seem to be less important for this object in some epochs, since only seven out of the relatively rich spectra showed a curvature, while the majority of the 0.3–10 keV spectra fitted with a simple powerlaw well. The photon index varied on diverse timescales, and the source showed mainly a “harder-when-brighter” spectral evolution. In the hardness ratio–flux plane, 1ES 2344+514 showed both clockwise and counterclockwise loops, indicating a complex spectral evolution with the flux.

We report the detection of the first VLBI ring-structure around the core of an AGN - around the core of the quasar 3C454.3. This ring-structure starts being visible in VLBI maps around 1996. It expands with an apparent velocity between 0.11±0.01mas/yr and 0.18±0.01mas/yr and dominates the pc-scale structure for at least 14 years. This is the result of a re-analysis of 41 VLBA data sets at six different radio frequencies observed between 1995.57 and 2011.48. We observe a correlation between radio flaring, flux-density variability, a ring-structure and kinematic properties of the jet. Taken together, it is tempting to see a causal connection and to explain all of this geometrically. The kinematic changes as well as the changes in the flaring characteristics might be caused by a change of the angle to the line of sight towards the observer. This behaviour resembles our findings for 0735+178 - with 3C454.3 being the second AGN to reveal kinematic mode changes. These mode changes could be explained by the presence of a supermassive binary black hole. 3C454.3 had been modelled as a binary black hole before.

The well-known Crab Nebula is at the center of the SN1054 supernova remnant. It consists of a rotationally powered pulsar interacting with a surrounding nebula through a relativistic particle wind. The emissions originating from the pulsar and nebula have been considered to be essentially stable. Here, we report the detection of strong gamma-ray (100 mega-electron volts to 10 giga-electron volts) flares observed by the AGILE satellite in September 2010 and October 2007. In both cases, the total gamma-ray flux increased by a factor of three compared with the non-flaring flux. The flare luminosity and short time scale favor an origin near the pulsar, and we discuss Chandra Observatory x-ray and Hubble Space Telescope optical follow-up observations of the nebula. Our observations challenge standard models of nebular emission and require power-law acceleration by shock-driven plasma wave turbulence within an approximately 1-day time scale.

Since its launch in April 2007, the AGILE satellite detected with the Gamma-Ray Imaging Detector several blazars in high γ-ray activity: 3C 279, 3C 454.3, PKS 1510–089, S5 0716+714, 3C 273, W Comae and Mrk 421. Thanks to the rapid dissemination of our alerts, we were able to obtain multiwavelength ToO data from other observatories such as Spitzer, Swift, RXTE, Suzaku, INTEGRAL, MAGIC, VERITAS, as well as radio-to-optical coverage by means of the GASP Project of the WEBT and the REM Telescope. This large multifrequency coverage gave us the opportunity to study truly simultaneous spectral energy distributions of these sources from radio to γ-ray energy bands and to investigate the different mechanisms responsible for their emission. We present an overview of the AGILE results on these γ-ray blazars and the relative multifrequency data.

Supergiant Fast X-ray Transients (SFXT) are High Mass X-ray Binaries displaying X-ray outbursts reaching peak luminosities of 10\\(^{38}\\) erg/s and spend most of their life in more quiescent states with luminosities as low as 10\\(^{32}\\)-10\\(^{33}\\) erg/s. The main goal of our comprehensive and uniform analysis of the SFXT Swift triggers is to provide tools to predict whether a transient which has no known X-ray counterpart may be an SFXT candidate. These tools can be exploited for the development of future missions exploring the variable X-ray sky through large FoV instruments. We examined all available data on outbursts of SFXTs that triggered the Swift/BAT collected between 2005-08-30 and 2014-12-31, in particular those for which broad-band data, including the Swift/XRT ones, are also available. We processed all BAT and XRT data uniformly with the Swift Burst Analyser to produce spectral evolution dependent flux light curves for each outburst. The BAT data allowed us to infer useful diagnostics to set SFXT triggers apart from the general GRB population, showing that SFXTs give rise uniquely to image triggers and are simultaneously very long, faint, and `soft' hard-X-ray transients. The BAT data alone can discriminate very well the SFXTs from other fast transients such as anomalous X-ray pulsars and soft gamma repeaters. However, to distinguish SFXTs from, for instance, accreting millisecond X-ray pulsars and jetted tidal disruption events, the XRT data collected around the time of the BAT triggers are decisive. The XRT observations of 35/52 SFXT BAT triggers show that in the soft X-ray energy band, SFXTs display a decay in flux from the peak of the outburst of at least 3 orders of magnitude within a day and rarely undergo large re-brightening episodes, favouring in most cases a rapid decay down to the quiescent level within 3-5 days (at most). [Abridged]

Recently, there has been renewed interest in the Be X-ray binary (Be/XRB) SAX J1324.4-6200 because of its spatial coincidence with a gamma-ray source detected by Fermi/LAT. To explore more thoroughly its properties, new observations were carried out in 2023 by NuSTAR, XMM-Newton, and Swift, jointly covering the energy range 0.2-79 keV. The X-ray spectrum of SAX J1324.4-6200 fits well with an absorbed power law with a high energy cut-off. We measured a NuSTAR spin period of 175.8127 +/- 0.0036 s and an XMM-Newton spin period of 175.862 +/- 0.025 s. All the available spin period measurements of SAX J1324.4-6200, spanning 29 years, are correlated with time, resulting in a remarkably stable spin-down of dP/dt=(6.09 +/- 0.06)*1E-9 s/s. If SAX J1324.4-6200 hosts an accretion powered pulsar, accretion torque models indicate a surface magnetic field of ~1E12-1E13 G. The X-ray properties emerging from our analysis strenghten the hypothesis that SAX J1324.4-6200 belongs to the small group of persistent Be/XRBs. We also performed radio observations with the Parkes Murriyang telescope, to search for radio pulsations. However, no radio pulsations compatible with the rotational ephemeris of SAX J1324.4-6200 were detected. We rule out the hypothesis that SAX J1324.4-6200 is a gamma-ray binary where the emission is produced by interactions between the pulsar and the companion winds. Other models commonly used to account for the production of gamma-rays in accreting pulsars cannot reproduce the bright emission from SAX J1324.4-6200. We examined other mechanisms for the gamma-ray emission and noted that there is a ~0.5% chance probability that an unknown extragalactic AGN observed through the Galactic plane may coincidentally fall within the Fermi/LAT error circle of the source and be the responsible of the gamma-ray emission. [Abridged]