The existence of free-free absorption in the nucleus of Cen A was suggested by Tingay & Murphy in 2001 as an explanation for the large value of the spectral index between 2.2 and 5 GHz. In this paper ...we present further evidence for this absorption, based on the observations of short time-scale variability at mm wavelengths and their lack of correlation with X-ray emission, as expected if they were both originated in the same physical process. To explain the short-term variability, we assumed that the mm-wave emission is produced near the base of the inhomogeneous parsec-scale jet that has a bulk velocity of about 0.5c. The presence of an ionized media between the jet and the observer will produce the observed 7-mm flux density variability, as different jet features move behind the absorbing material. We estimated that a region with 5 × 106 electrons cm−3 and 1.5 × 1015 cm radius can explain the intensity and the duration of the mm-flux variations, but the corresponding column density would not be enough to absorb significantly the X-ray flux, explaining the lack of correlation between the radio and X-ray variability. The upper limit for the cloud size inferred for the absorbing region shows that the media surrounding the core of Cen A must be clumpy.
The bright gamma-ray quasar 4C +55.17 is a distant source (z = 0.896) with a hard spectrum at GeV energies as observed by the Large Area Telescope (LAT) on board the Fermi satellite. This source is ...identified as a good source candidate for very high energy (VHE; >30 GeV) gamma-rays. In general, VHE gamma-rays from distant sources provide a unique opportunity to study the extragalactic background light (EBL) and underlying astrophysics. The flux intensity of this source in the VHE range is investigated. Then, constraints on the EBL are derived from the attenuation of gamma-ray photons coming from the distant blazar. We searched for a gamma-ray signal from this object using the 35 h observations taken by the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes between 2010 November and 2011 January. No significant VHE gamma-ray signal was detected. We computed the upper limits of the integrated gamma-ray flux at the 95 per cent confidence level of 9.4 × 10−12 and 2.5 × 10−12 cm−2 s−1 above 100 and 200 GeV, respectively. The differential upper limits in four energy bins in the range from 80 to 500 GeV are also derived. The upper limits are consistent with the attenuation predicted by low-flux EBL models on the assumption of a simple power-law spectrum extrapolated from LAT data.
We have sub-classified short and long-duration gamma-ray bursts (GRBs) into seven families according to the binary nature of their progenitors. Short GRBs are produced in mergers of neutron-star ...binaries (NS-NS) or neutron star-black hole binaries (NS-BH). Long GRBs are produced via the induced gravitational collapse (IGC) scenario occurring in a tight binary system composed of a carbon-oxygen core (COcore) and a NS companion. The COcore explodes as type Ic supernova (SN) leading to a hypercritical accretion process onto the NS: if the accretion is sufficiently high the NS reaches the critical mass and collapses forming a BH, otherwise a massive NS is formed. Therefore long GRBs can lead either to NS-BH or to NS-NS binaries depending on the entity of the accretion. We discuss for the above compact-object binaries: 1) the role of the NS structure and the nuclear equation of state; 2) the occurrence rates obtained from X and gamma-rays observations; 3) the predicted annual number of detections by the Advanced LIGO interferometer of their gravitational-wave emission.
Here, the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes observed S2 0109+22 in 2015 July during its flaring activity in high-energy gamma-rays observed by Fermi-Large Area ...Telescope. We analyse the MAGIC data to characterize the very high energy (VHE) gamma-ray emission of S2 0109+22, which belongs to the subclass of intermediate synchrotron peak (ISP) BL Lacertae (BL Lac) objects. We study the multifrequency emission in order to investigate the source classification. Finally, we compare the source long-term behaviour to other VHE gamma-ray emitting (TeV) blazars. We performed a temporal and spectral analysis of the data centred around the MAGIC interval of observation (MJD 57225–57231). Long-term radio and optical data have also been investigated using the discrete correlation function. The redshift of the source is estimated through optical host-galaxy imaging and also using the amount of VHE gamma-ray absorption. The quasi-simultaneous multifrequency spectral energy distribution (SED) is modelled with the conventional one-zone synchrotron self-Compton (SSC) model. MAGIC observations resulted in the detection of the source at a significance level of 5.3σ. The VHE gamma-ray emission of S2 0109+22 is variable on a daily time scale. VHE gamma-ray luminosity of the source is lower than the average of TeV BL Lacs. The optical polarization and long-term optical/radio behaviour of the source are different from the general population of TeV blazars. All these findings agree with the classification of the source as an ISP BL Lac object. As a result, we estimate the source redshift as z = 0.36 ± 0.07. The SSC parameters describing the SED are rather typical for blazars.
Long-duration γ-ray bursts (GRBs) are the most luminous sources of electromagnetic radiation known in the Universe. They arise from outflows of plasma with velocities near the speed of light that are ...ejected by newly formed neutron stars or black holes (of stellar mass) at cosmological distances
. Prompt flashes of megaelectronvolt-energy γ-rays are followed by a longer-lasting afterglow emission in a wide range of energies (from radio waves to gigaelectronvolt γ-rays), which originates from synchrotron radiation generated by energetic electrons in the accompanying shock waves
. Although emission of γ-rays at even higher (teraelectronvolt) energies by other radiation mechanisms has been theoretically predicted
, it has not been previously detected
. Here we report observations of teraelectronvolt emission from the γ-ray burst GRB 190114C. γ-rays were observed in the energy range 0.2-1 teraelectronvolt from about one minute after the burst (at more than 50 standard deviations in the first 20 minutes), revealing a distinct emission component of the afterglow with power comparable to that of the synchrotron component. The observed similarity in the radiated power and temporal behaviour of the teraelectronvolt and X-ray bands points to processes such as inverse Compton upscattering as the mechanism of the teraelectronvolt emission
. By contrast, processes such as synchrotron emission by ultrahigh-energy protons
are not favoured because of their low radiative efficiency. These results are anticipated to be a step towards a deeper understanding of the physics of GRBs and relativistic shock waves.
Currently the detection of Very High Energy gamma-rays for astrophysics rely on the measurement of the Extensive Air Showers (EAS) either using Cherenkov detectors or EAS arrays with larger field of ...views but also larger energy thresholds. In this talk we present a novel hybrid detector concept for a EAS array with an improved sensitivity in the lower energies (~ 100 GeV). We discuss its main features, capabilities and present preliminary results on its expected perfomances and sensitivities.This wide field of view experiment is planned to be installed at high altitude in South America making it a complementary project to the planned Cherenkov telescope experiments and a powerful tool to trigger further observations of variable sources and to detect transients phenomena.
Long-duration γ-ray bursts (GRBs) originate from ultra-relativistic jets launched from
the collapsing cores of dying massive stars. They are characterized by an initial phase
of bright and highly ...variable radiation in the kiloelectronvolt-to-megaelectronvolt band, which is probably produced within the jet and lasts from milliseconds to minutes, known as the prompt emission. Subsequently, the interaction of the jet with the surrounding medium generates shock waves that are responsible for the afterglow emission, which lasts from days to months and occurs over a broad energy range from the radio to the gigaelectronvolt bands. The afterglow emission is generally well explained as synchrotron radiation emitted by electrons accelerated by the external shock. Recently, intense long-lasting emission between 0.2 and 1 teraelectronvolts was observed from GRB 190114C. Here we report multifrequency observations of GRB 190114C, and study the evolution in time of the GRB emission across 17 orders of magnitude in energy, from 5 × 10^(−6) to 10^(12) electronvolts. We find that the broadband spectral energy distribution is double-peaked, with the teraelectronvolt emission constituting a distinct spectral component with power comparable to the synchrotron component. This component is associated with the afterglow and is satisfactorily explained by inverse Compton up-scattering of synchrotron photons by high-energy electrons. We find that the conditions required to account for the observed teraelectronvolt component are typical for GRBs, supporting the possibility that inverse Compton emission is commonly produced in GRBs.
Context. On March 28, 2011, the BAT instrument on board the Swift satellite detected a new transient event that in the very beginning was classified as a gamma ray burst (GRB). However, the unusual ...X-ray flaring activity observed from a few hours up to days after the onset of the event made a different nature seem to be more likely. The long-lasting activity in the X-ray band, followed by a delayed brightening of the source in infrared and radio activity, suggested that it is better interpreted as a tidal disruption event that triggered a dormant black hole in the nucleus of the host galaxy and generated an outflowing jet of relativistic matter. Aims. Detecting a very high energy emission component from such a peculiar object would be enable us to constrain the dynamic of the emission processes and the jet model by providing information on the Doppler factor of the relativistic ejecta . Methods. The MAGIC telescopes observed the peculiar source Swift J1644+57 during the flaring phase, searching for gamma-ray emission at very-high energy (VHE, E > 100 GeV), starting observations nearly 2.5 days after the trigger time. MAGIC collected a total of 28 h of data during 12 nights. The source was observed in wobble mode during dark time at a mean zenith angle of 35°. Data were reduced using a new image-cleaning algorithm, the so-called sum-cleaning, which guarantees a better noise suppression and a lower energy threshold than the standard analysis procedure. Results. No clear evidence for emission above the energy threshold of 100 GeV was found. MAGIC observations permit one to constrain the emission from the source down to 100 GeV, which favors models that explain the observed lower energy variable emission. Data analysis of simultaneous observations from AGILE, Fermi and VERITAS also provide negative detection, which additionally constrain the self-Compton emission component.
Context. The high frequency peaked BL Lac PKS 2155-304 with a redshift of z = 0.116 was discovered in 1997 in the very high energy (VHE, E > 100 GeV) γ-ray range by the University of Durham Mark VI ...γ-ray Cherenkov telescope in Australia with a flux corresponding to 20% of the Crab Nebula flux. It was later observed and detected with high significance by the southern Cherenkov observatory H.E.S.S. establishing this source as the best studied southern TeV blazar. Detection from the northern hemisphere is difficult due to challenging observation conditions under large zenith angles. In July 2006, the H.E.S.S. collaboration reported an extraordinary outburst of VHE γ-emission. During the outburst, the VHE γ-ray emission was found to be variable on the time scales of minutes and with a mean flux of ~7 times the flux observed from the Crab Nebula. Follow-up observations with the MAGIC-I standalone Cherenkov telescope were triggered by this extraordinary outburst and PKS 2155-304 was observed between 28 July to 2 August 2006 for 15 h at large zenith angles. Aims. We studied the behavior of the source after its extraordinary flare. Furthermore, we developed an analysis method in order to analyze these data taken under large zenith angles. Methods. Here we present an enhanced analysis method for data taken at high zenith angles. We developed improved methods for event selection that led to a better background suppression. Results. The quality of the results presented here is superior to the results presented previously for this data set: detection of the source on a higher significance level and a lower analysis threshold. The averaged energy spectrum we derived has a spectral index of (−3.5 ± 0.2) above 400 GeV, which is in good agreement with the spectral shape measured by H.E.S.S. during the major flare on MJD 53 944. Furthermore, we present the spectral energy distribution modeling of PKS 2155-304. With our observations we increased the duty cycle of the source extending the light curve derived by H.E.S.S. after the outburst. Finally, we find night-by-night variability with a maximal amplitude of a factor three to four and an intranight variability in one of the nights (MJD 53 945) with a similar amplitude.
Polarimetric tomography of blazar jets Barres de Almeida, Ulisses; Tavecchio, Fabrizio; Mankuzhiyil, Nijil
Monthly notices of the Royal Astronomical Society,
07/2014, Letnik:
441, Številka:
4
Journal Article
Recenzirano
Odprti dostop
In this paper, we propose a way of using optical polarization observations to provide independent constraints and to guide the modelling of the spectral energy distribution (SED) of blazars. This is ...particularly useful when two-zone models are required to fit the observed SED. As an example, we apply the method to the 2008 multiwavelength campaign of PKS 2155–304, for which the required polarization information was already available. We find that this approach is able to simultaneously describe the SED and the variability of the source, which is otherwise difficult to interpret. More generally, by using polarization data to disentangle different active regions within the source, the method reveals otherwise unseen correlations in the multiwavelength behaviour, which are important for SED modelling.
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