Engineering topics

Raiteri, C. M.

Publications and source records attributed to Raiteri, C. M..

8 records

NASA NTRS2014

The Most Powerful Flaring Activity from the NLSy1 PMN J0948+0022

We report on multifrequency observations performed during 2012 December-2013 August of the first narrow-line Seyfert 1 galaxy detected in gamma rays, PMN J0948+0022 (z equal to 0.5846). A gamma-ray flare was observed by the Large Area Telescope on board Fermi during 2012 December-2013 January, reaching a daily peak flux in the 0.1-100 GeV energy range of (155 plus or minus 31)×10(exp −8) ph cm (exp −2) s (exp −1) on 2013 January 1, corresponding to an apparent isotropic luminosity of approximately 1.5×10)exp 48) erg s(exp −1). The gamma-ray flaring period triggered Swift and VERITAS observations in addition to radio and optical monitoring by OVRO, MOJAVE, and CRTS. A strong flare was observed in optical, UV, and X- rays on 2012 December 30, quasi-simultaneously to the gamma-ray flare, reaching a record flux for this source from optical to gamma rays. VERITAS observations at very high energy (E greater than 100 GeV) during 2013 January 6-17 resulted in an upper limit of F(sub greater than 0.2 TeV) less than 4.0 × 10(exp −12) ph cm(exp −2) s(exp −1). We compared the spectral energy distribution (SED) of the flaring state in 2013 January with that of an intermediate state observed in 2011. The two SEDs, modelled as synchrotron emission and an external Compton scattering of seed photons from a dust torus, can be modelled by changing both the electron distribution parameters and the magnetic field.

powerful↗

NASA NTRS2012

SBS 0846+513: a New Gamma-ray Emitting Narrow-line Seyfert 1 Galaxy

We report Fermi-LAT observations of the radio-loud AGN SBS 0846+513 (z=0.5835), optically classified as a Narrow-Line Seyfert 1 galaxy, together with new and archival radio-to-X-ray data. The source was not active at γ-ray energies during the first two years of Fermi operation. A significant increase in activity was observed during 2010 October-2011 August. In particular a strong gamma-ray flare was observed in 2011 June reaching an isotropic γ-ray luminosity (0.1-300 GeV) of 1.0×10(sup 48) erg s(sup −1), comparable to that of the brightest flat spectrum radio quasars, and showing spectral evolution in gamma rays. An apparent superluminal velocity of (8.2+/-1.5)c in the jet was inferred from 2011-2012 VLBA images, suggesting the presence of a highly relativistic jet. Both the power released by this object during the flaring activity and the apparent superluminal velocity are strong indications of the presence of a relativistic jet as powerful as those of blazars. In addition, variability and spectral properties in radio and gamma-ray bands indicate blazar-like behaviour, suggesting that, except for some distinct optical characteristics, SBS 0846+513 could be considered as a young blazar at the low end of the blazar's black hole mass distribution.

emitting↗

NASA NTRS2002

The Relationship Between X-Rays and Relativistic Jets

We present recent multiwaveband observations centered on X-ray monitoring of blazars and the radio galaxy 3C 120 with the RXTE satellite, In 3C 120, we observed four X-ray dips, each followed by ejections of superluminal radio knots down the jet. This behavior, similar to that of the microquasar GRS 1915+105, is interpreted as infall of a piece of the inner accretion disk causing ejection of energy into the relativistic jet. The X-ray emission from the quasars PKS 1510-089, 3C 279, and 3C 273 is highly variable on timescales as short as approximately 1 day. Over 2 years, X-ray flares in PKS 1510-089 occurred about 2 weeks after radio outbursts, which can be explained by light-travel delays. In 3C 279 the X-ray and optical variations are usually well correlated, with very little, if any, time delay. We conclude that the X-ray and optical emission from blazars occurs near the radio core rather than close to the black hole.

Marscher, A. P.↗

NASA NTRS2000

The 3C 279 Campaign of Winter 1999: A Gamma-Optical Correlation?

Preliminary results are presented from the gamma-optical campaign of January-February 1999 on 3C 279. During this period we obtained good optical sampling of the source, the best ever for a gamma-bright OVV quasar. Its large and fast variations have been compared with the gamma-ray fluxes obtained simultaneously by Energy Gamma Ray Experiment Telescope (EGRET), on Compton Gamma Ray Observatory (CGRO). Despite rather poor counting statistics in the gamma-ray data, a fair correlation is found, with the gamma variations following those in the optical by 3-4 days. This is the first time such a significant day-scale correlation has been observed between the optical and gamma emissions from a OVV quasar. Its implications are currently under study.

Hartman, R. C.↗

NASA NTRS1999

Multiwavelength Observations of Mkn 501 During the 1997 High State

During the observation period 1977, the close Blazar Mkn 501 showed extremely strong emission and high variability. We examine multiwavelength aspects of this event using Radio optical, soft and hard X-ray and TeV data. We concentrate on the medium-timescale variability of the broadband spectra averaged over 1-week intervals. We confirm the previously found correlation between soft and hard X-ray emission and the at TeV energies while the source shows only minor variability at radio and optical wavelengths. The non-linear correlation between hard X-ray and TeV fluxes is consistent with a simple analytic estimate based on an SSC model in which Klein-Nishina effects are important for the highest-energy electrons in the jet and flux; variations are caused by variations of the electron density and/or the spectral index of the electron injection spectrum. The time-averaged spectra are fitted with an SSC dominated leptonic jet model using the full Klein-Nishina cross section and following the self-consistent evolution of relativistic particles along the jet accounting for gamma gamma absorption and pair production within the source as well as due to the intergalactic infrared red background radiation. The contribution from external inverse-Compton scattering is tightly constrained by the low maximum EGRET flux and found co be negligible.

Petry, D.↗

NASA NTRS1995

Injection of freshly synthesized Ca-41 in the early solar nebula by an asymptotic giant branch star

We show that ejecta from the envelope of one asymptotic giant branch star of M is approximately 3 solar mass may account for many of the short-lived nuclei in the early solar system and also for the recent evidence of the presence of Ca-41 (bar-tau(sub 41) = 1.50 x 10(exp 5) yr) in early solar nebular condensates. This would require that the injection into the protosolar molecular cloud took place within a narrow time interval of (5-7) x 10(exp 5) yr before the formation of the solar system. If true, this places extremely tight constraints on the whole process of injection mixing and collapse. The timescales for both Ca-41 and Al-26 require that the placental medium be a dense molecular cloud (2 x 10(exp 3) -8 x 10(exp 3( H/cc). If the observed residual Ca-41 is instead produced by a proton bombardment mechanism within the early solar system, similar to what appears necessary to explain Mn-53, then the time interval is relaxed but would still be (1-2) x 10(exp 6) yr from consideration of Al-26.

Wasserburg, G. J.↗

NASA NTRS1994

Asymptotic Giant Branch stars as a source of short-lived radioactive nuclei in the solar nebula

We carried out a theoretical evaluation of the contribution of Asymptotic Giant Branch (AGB) stars to some short-lived (10(exp 6) less than or equal to Tau-bar less than or equal to 2 x 10(exp 7) yr) isotopes in the Interstellar Medium (ISM) and in the early solar system using stellar model calculations for thermally pulsing evolutionary phases of low-mass stars. The yields of s-process nuclei in the convective He-shell for different neutron exposures tau(sub 0) were obtained, and AGB stars were shown to produce several radioactive nuclei (especially Pd-107, Pb-205, Fe-60, Zr-93, Tc-99, Cs-135, and Hf-182) in diferent amounts. Assuming either contamination of the solar nebula from a single AGB star or models for continuous injection and mixing from many stars into the ISM, we calculate the ratios of radioactive to stable nuclei at the epoch of the Sun's formation. The dilution factor between the AGB ejecta and the early solar system matter is obtained by matching the observed Pd-107/Pd-108 and depends on the value of tau(sub 0). It is found that small masses M(sub He) of He-shell material (10(exp -4)-10(exp -7) solar mass) enriched in s-process nuclei are sufficient to contaminate 1 solar mass of the ISM to produce the Pd-107 found in the early solar system. Predictions are made for all of the other radioactive isotopes. The optimal model to explain several observed radioactive species at different states of the proto-solar nebula involves a single AGB star with a low neutron exposure (tau(sub 0) = 0.03 mbarn(sup -1)) which contaminated the cloud with a dilution factor of M(sub He)/solar mass approximately 1.5 x 10(exp -4). This will also contribute newly synthesized stable s-process nuclei in the amount of approximately 10(exp -4) of their abundances already present in the proto-solar cloud. Variations in the degree of homogenization (approximately 30%) of the injected material may account for some of the small general isotopic anomalies found in meteorites. It is also found that Fe-60 is produced in small but significant quantities that may be sufficient to explain the observations if the time elapsed delta from the contamination of the ISM to the formation of protoplanetary bodies is not higher than delta = 5 x 10(exp 6) yr. If delta is longer, up to 10 x 10(exp 6) yr, this would require the single AGB star to experience enhanced neutron densities (n(sub n) approximately 3 x 10(exp 9)n/cu cm) in the s-processing zone in order to compensate for the branching at Fe-59. The alternative model of long-term continuous ejection of matter from many AGB stars does not appear to match the observations. We also estimate the Al-26 production from the H-shell and find that the Al-26 abundance in the early solar system may be readily explained in a self-consistent manner. Moreover, Al-26 from AGB stars may contribute substantially to the galactic Al-26 gamma-source, while no significant gamma-flux from Co-60 (deriving from Fe-60 decay) is to be expected.

Wasserburg, G. J.↗

NASA NTRS1993

AGB stars as a source of short-lived radioactive nuclei in the solar nebula

The purpose is to estimate the possible contribution of some short-lived nuclei to the early solar nebula from asymptotic giant branch (AGB) sources. Low mass (1 to 3 solar mass) AGB stars appear to provide a site for synthesis of the main s process component for solar system material with an exponential distribution of neutron irradiations varies as exp(-tau/tau(sub 0)) (where tau is the time integrated neutron flux with a mean neutron exposure tau(sub 0)) for solar abundances with tau(sub 0) = 0.28 mb(sup -1). Previous workers estimated the synthesis of key short-lived nuclei which might be produced in AGB stars. While these calculations exhibit the basic characteristics of nuclei production by neutron exposure, there is need for a self-consistent calculation that follows AGB evolution and takes into account the net production from a star and dilution with the cloud medium. Many of the general approaches and the conclusions arrived at were presented earlier by Cameron. The production of nuclei for a star of 1.5 solar mass during the thermal pulsing of the AGB phase was evaluated. Calculations were done for a series of thermal pulses with tau(sub 0) = 0.12 and 0.28 mb(sup -1). These pulses involve s nucleosynthesis in the burning shell at the base of the He zone followed by the ignition of the H burning shell at the top of the He zone. After about 10-15 cycles the abundances of the various nuclei in the He zone become constant. Computations of the abundances of all nuclei in the He zone were made following Gallino. The mass of the solar nebula was considered to consist of some initial material of approximately solar composition plus some contributions from AGB stars. The ratios of the masses required from the AGB He burning zone to the ISM necessary to produce the observed value of Pd-107/Pd-108 in the early solar system were calculated and this dilution factor was applied to all other relevant nuclei.

Wasserburg, G. J.↗