arXiv subjects

M. Murgia

Publications and source records attributed to M. Murgia.

116 records · Page 7Linked to original sources

arXiv2001

Four Extreme Relic Radio Sources in Clusters of Galaxies

(Abridged) We describe the results of the highest-resolution radio observations yet made of four relic radio sources in the Abell clusters A13, A85, A133 and A4038. Our VLA images at 1.4 GHz with 4" resolution show a remarkable variety of fine structure in the form of spectacular arcs, wisps, plumes and loops. Their integrated radio flux densities fall very rapidly with frequency, with power-law slopes between 2.1 and 4.4 near 1.4 GHz The relics possess linear polarization levels ranging between 2.3 % (A133) and 35 % (A85); the higher polarization fractions imply a highly ordered magnetic field in the fine structure and low differential Faraday rotation in the intervening cluster gas. The optical identification of host galaxies remains problematic. In A85, A133 and A4038 the travel times for the brightest cluster galaxies are significantly longer than the modeled ages of the relics and nearby bright ellipticals provide a better match. Excess X-ray emission in the 0.5 keV-to-2 keV band was found near the relics in A85 and A133. The surface brightness was too high to be attributed to the inverse-Compton mechanism alone. We found excellent fits to the broad-band radio spectra using the anisotropic (KGKP) model of spectral ageing, and we have extended the model to include diffusion of particles between regions of different field strength (the Murgia-JP, or MJP, model). The steep radio spectra imply ages for the relics of ~ 10^8 yr, at the start of which period their radio luminosities would have been ~ 10^25 W/Hz at 1.4 GHz.

astro-ph↗

arXiv2001

Spin polarised injection in sexithienyl thin films

Electroluminescence in organic semiconductors strongly depends on the relative population of excited singlet and triplet excitonic states resulting from recombination of injected carriers. In conventional organic light-emitting diodes (OLED) optical emission is usually based on fluorescence from excited singlet states1, although phosphorescence from triplets is also investigated2. Governing the spin statistics of the injected carriers would open the way to amplify a chosen electronic transition increasing therefore the OLED efficiency. Direct injection of carriers with high degree of spin polarisation along a given axis should lead to preferential population of either singlet or triplet excited states, depending on the relative electrodes polarisation. Here we report a first experimental evidence of direct spin polarised (SP) injection in sexithienyl (T6), a prototypical organic semiconductor, from half-metallic manganites. The spin penetration depth in T6 is about 250-300 nm at room temperature. The results are discussed taking into account possible spin-flip mechanisms in organic material and interface effects.

cond-mat.str-el↗

arXiv2000

A multi-frequency study of the radio galaxy NGC326

We present preliminary results of a multi-frequency study of the inversion symmetric radio galaxy NGC326 based on VLA observations at 1.4, 1.6, 4.8, 8.5, and 14.9 GHz. These data allow us to investigate in detail the morphological, spectral and polarization properties of this peculiar object at different levels of spatial resolution.

astro-ph↗

arXiv2000

The giant radio galaxy 8C0821+695 and its environment

We present new VLA and Effelsberg observations of the radio galaxy 8C0821+695. We have obtained detailed images in total intensity and polarization of this 2 Mpc sized giant. The magnetic field has a configuration predominantly parallel to the source main axis. We observe Faraday rotation at low frequencies, most probably produced by an ionized medium external to the radio source. The spectral index distribution is that typical of FR II radio galaxies, with spectral indices gradually steepening from the source extremes towards the core. Modeling the spectrum in the lobes using standard synchrotron loss models yields the spectral age of the source and the mean velocity of the jet-head with respect to the lobe material. The existence of a possible backflow in the lobe is considered to relate spectral with dynamical determinations of the age and the velocity with respect to the external medium. Through a very simple model, we obtain a physical characterization of the jets and the external medium in which the radio galaxy expands. The results in 8C0821+695 are consistent with a relativistic jet nourishing the lobes which expand in a hot, low density halo. We infer a deceleration of the source expansion velocity which we explain through a progressive increase in the hot-spot size.

astro-ph↗

arXiv1999

The lives of FR I radio galaxies

After a brief introduction to the morphological properties of FRI radio sources, we discuss the possibility that FRI jets are relativistic at their bases and decelerate quickly to non-relativistic velocities. From two-frequency data we determine spectral index distributions and consequently the ages of FRI sources. We show that in the large majority of cases synchrotron theory provides unambiguous and plausible answers; in a few objects re-acceleration of electrons may be needed. The derived ages are of the order 10^7-10^8 years, 2-4 times larger than the ages inferred from dynamical arguments and a factor 5-10 larger than the ages of FRII sources. The linear sizes of FRI and FRII sources make it unlikely that many FRII's evolve into FRI's. A brief discussion is given of the possibility that radio sources go through different cycles of activity.

astro-ph↗

arXiv1999

The size of electron-hole pairs in pi conjugated systems

We have performed momentum dependent electron energy-loss studies of the electronic excitations in sexithiophene and compared the results to those from parent oligomers. Our experiment probes the dynamic structure factor S(q,omega)and we show that the momentum dependent intensity variation of the excitations observed can be used to extract the size of the electron-hole pair created in the excitation process. The extension of the electron-hole pairs along the molecules is comparable to the length of the molecules and thus maybe only limited by structural constraints. Consequently, the primary intramolecular electron-hole pairs are relatively weakly bound. We find no evidence for the formation of excitations localized on single thiophene units.

cond-mat.soft↗

arXiv1999

Synchrotron Spectra and Ages of Compact Steep Spectrum Radio Sources

The high-frequency integrated spectra of Compact Steep Spectrum (CSS) sources show breaks with a moderate spectral steepening well fitted by continuous injection synchrotron spectra. In lobe-dominated CSS sources the radiative ages deduced by the synchrotron theory are in the range of up to 0.1 Myears, if equipartition magnetic fields are assumed. These radiative ages are well correlated with the source size indicating that the CSS sources are young. In order to maintain the frustration scenario, in which the sources' lifetimes are about 10 Myears, their equipartition magnetic field would be systematically decreased by a factor of more than 20. To complete the sample used in this work, we conducted observations at 230 GHz with the IRAM 30-m telescope of those sources which did not have such high-frequency observations up to now.

astro-ph↗

arXiv1998

Radiative ages in a representative sample of low luminosity radio galaxies

Two frequency observations, mainly at 1.4 and 5 GHz from the VLA, have been used to study spectral variations along the lobes of some nearby low luminosity radio galaxies that constitute a representative sample selected from the B2 catalogue. The variations of the spectral index have been interpreted as being due to synchrotron and inverse Compton losses and characteristic spectral ages are deduced for the relativistic electrons. The radiative ages are in the range of several 10^7 years. These ages correlate well with the source sizes. They also appear to be consistent with dynamical ages determined from ram-pressure arguments, if we make reasonable assumptions about the ambient gas density and allow for very moderate deviations from the equipartition conditions. There appears to be a significant difference between the radiative ages of sources in our sample and those of more powerful 3CR radio sources. We briefly discuss the possibility of re-acceleration processes and indicate some objects where these may occur.

astro-ph↗