94,926 research outputs found
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
New insights on accretion in supergiant fast X-ray transients from XMM-Newton and INTEGRAL observations of IGR J17544-2619
No full textXMM–Newton observations of the supergiant fast X-ray transient IGR J17544?2619 are reported and placed in the context of an analysis of archival INTEGRAL/IBIS data that provide a refined estimate of the orbital period at 4.9272?±?0.0004?d. A complete outburst history across the INTEGRAL mission is reported. Although the new XMM–Newton observations (each lasting ?15 ks) targeted the peak flux in the phase-folded hard X-ray light curve of IGR J17544?2619, no bright outbursts were observed, the source spending the majority of the exposure at intermediate luminosities of the order of several 1033?erg?s?1 (0.5–10 keV) and displaying only low level flickering activity. For the final portion of the exposure, the luminosity of IGR J17544?2619 dropped to ?4?×?1032?erg?s?1 (0.5–10 keV), comparable with the lowest luminosities ever detected from this source, despite the observations being taken near to periastron. We consider the possible orbital geometry of IGR J17544?2619 and the implications for the nature of the mass transfer and accretion mechanisms for both IGR J17544?2619 and the supergiant fast X-ray transients (SFXTs) population. We conclude that accretion under the ‘quasi-spherical accretion’ model provides a good description of the behaviour of IGR J17544?2619 and suggests an additional mechanism for generating outbursts based upon the mass accumulation rate in the hot shell (atmosphere) that forms around the neutron star under the quasi-spherical formulation. Hence, we hope to aid in explaining the varied outburst behaviours observed across the SFXT population with a consistent underlying physical model
Experimental Measurements and Computer Simulation of Fission Product Gamma-Ray Spectra
No full textAirborne gamma ray spectrometry using high volume scintillation detectors, optionally in conjunction with Ge detectors, has potential for making rapid environmental measurements in response to nuclear accidents. An experimental investigation and computer simulation have been used to characterise the response of such detectors to short lived fission products. Small samples of 235U were irradiated in a research reactor for short periods, to generate fission product sources. Gamma ray spectra were recorded over a period of one year using both high volume scintillation detectors and semiconductor detectors. The main gamma emitting nuclides have been identified, and their associated signals in each detector defined. Simulation work has been used to calculate the corresponding spectra which would have been observed at airborne survey heights, or from fission product sources irradiated for a prolonged period. While Ge detectors can record a fuller range of isotopes, of interest in characterising release terms, the NaI spectra also have potential for providing data on a range of radiologically important isotopes at all stages
Tetrathiafulvalene-2,3,6,7-tetrathiolate Linker Redox-State Elucidation via S K-edge X-ray Absorption Spectroscopy
No full textSulfur K-edge XAS data provide a unique tool to examine
oxidation states and covalency in electronically complex Sbased
ligands. We present sulfur K-edge X-ray absorption
spectroscopy on a discrete redox-series of Ni-based
tetrathiafulvalene tetrathiolate (TTFtt) complexes as well as on a
1D coordination polymer (CP), NiTTFtt. Experiment and theory
suggest that Ni—S covalency decreases with oxidation which
has implications for charge transport pathways. Finally, a
characteristic peak for doubly oxidized TTFtt verifies this
formal redox state in the CP, NiTTFtt
The nature of the X-ray transient MAXI J0556−332
No full textPhase-resolved spectroscopy of the newly discovered X-ray transient MAXIJ0556-332 has revealed the presence of narrow emission lines in the Bowen region that most likely arise on the surface of the mass donor star in this low-mass X-ray binary. A period search of the radial velocities of these lines provides two candidate orbital periods (16.43 ± 0.12 and 9.754 ± 0.048h), which differ from any potential X-ray periods reported. Assuming that MAXIJ0556-332 is a relatively high-inclination system that harbours a precessing accretion disc in order to explain its X-ray properties, it is only possible to obtain a consistent set of system parameters for the longer period. These assumptions imply a mass ratio of q≃ 0.45, a radial velocity semi-amplitude of the secondary of K 2≃ 190kms -1 and a compact object mass of the order of the canonical neutron star mass, making a black hole nature for MAXIJ0556-332 unlikely. We also report the presence of strong Niii emission lines in the spectrum, thereby inferring a high N/O abundance. Finally, we note that the strength of all emission lines shows a continuing decay over the ≃1 month of our observations
Determination of K shell XRF parameters and K to L shell vacancy transfer probabilities of ferromagnetic 3d transition metals
No full textIn this study, K shell X-ray fluorescence cross-sections (sigma K alpha, sK beta and sigma K), K shell fluorescence yields (omega K) and K to L shell vacancy transfer probabilities (eta KL) of ferromagnetic 3d transition metals (Fe, Co, Ni) were investigated to understand how the relationship between ferromagnetism and these values changes with atomic number by using energy dispersive X-ray fluorescence (EDXRF) technique. The all obtained experimental and theoretical results were comparatively given as a function of the atomic number. It was clearly observed that all of the investigated parameters depend on the atomic number and the ferromagnetism state of the atom
Hall effect studies and magnetic behaviour in Fe-nanoparticle embedded multi wall CNTs
No full textWith reference of our previous report [JNN 9, (2009) 6799], here we extended our work on ironnanoparticles embedded multiwall carbon nanotubes (MWCNTs) that were grown at 850 °C using chemical vapour deposition process that shows ferromagnetic as well as magnetic anisotropic behaviour due to the non-uniform distribution of 'Fe' particles in CNTs. The separation of 'Fe' nanoparticles from each other by the nonmagnetic carbon nanotube walls is preferably suitable for the application of magnetic storage devices, magnetic recording media and electromagnetic devices. The spin-related phenomenon in MWCNTs is demonstrated through a temperature dependent resistance curve through metal-insulator transition at ∼12.6 K and field effect magneto-resistance (MR). The minimal resistance is observed at 12.6 K. This result could provide the fabrication of nanometric-scale electronic devices and opening new opportunities to uncover deeper aspects of negative magnetoresistance effect
Energy conversion of X-ray, ultraviolet and infrared radiation in Gd2O3 crystals doped with Er3+ ions
No full textSpectra of photoluminescence (PL) and X-ray excited luminescence (XRL) in region of 1.5-5.0 eV, PL excitation spectra (2.8-5.8 eV), PL decay kinetics were measured in Gd2O3 crystals doped both with Er3+ and Zn2+ ions. Synchrotron radiation (VEPP-3 storage ring, Novosibirsk, Russia) were used for XRL measurements. PL spectra were studied at room temperature and T= 88 K under excitation with energy Eexc: a) in fundamental absorption region (Eexc≥Eg); b) in intracenter excitation region (Eexc<Eg); c) in infrared region (Eexc<<Eg) with using laser 980 nm diode as exciting photons (up-conversion processes). The probability of radiative transitions from excited states of the impurity center depends on Eexc. The photoluminescence efficiency in Gd2O3 doped with both Er3+ and Zn2+ ions in the green region decreases, 4F9/2→4I15/2 transitions dominate in the red region both in XRL and PL spectra. This effect is also clearly manifested in the spectra of anti-Stokes luminescence. Energy transfer between the excited Er3+ states of the impurity center and the crystal lattice defect is observed. Decrease of the lifetime of 4S3/2 excited state in Gd2O3:Er3+ + Zn2+ from 120 to 10 μs is observed. This fact indicated s a nonradiative energy transfer between impurity center and defect. Otherwise, the defect concentration variation (by means of doping with heterovalent ions) provides the ability to control the energy conversion efficiency as well as the phosphorus color. © 2017 Author(s).This work was partially supported by the Ministry of Education and Science of the Russian Federation (Government task No. 3.1485.2017/4.6 and basic part of the government mandate, project No. 3.8302.2017/8.9). One of the authors (PVA) thanks Dr. S. Omelkov for their help in PL time-resolved experiments
The sensitivity of the Durham mk6 ground-based atmospheric cherenkov telescope to very high energy gamma-ray sources
No full textThe subject of this thesis is a determination of the sensitivity of the Durham Mk6 ground based Imaging Atmospheric Cherenkov Telescope (lACT), an instrument which uses the imaging atmospheric Cherenkov technique to detect Very High Energy (VHE) gamma-rays. The first three chapters are introductory: Chapter 1 describes the basics of Very High Energy (VHE) gamma ray astronomy. Chapter 2 describes the properties of Extensive Ak Showers (EAS). Chapter 3 describes the detection of these EAS on the ground by lACTs, Chapter 4 details the Durham Mk6 lACT and includes a description of the Cherenkov imaging technique for background discrimination. Chapter 5 describes the MOCCA and SOLMK simulation codes. Chapter 6 contains a description of the details of the simulations produced for this diesis. This chapter continues to its logical conclusion and presents a revised VHE gamma-ray flux of 2.5 ± 0.7(_stat) [+0.5 or -1.6](_syst x lO(_7) photons m(^-2) s(^-1) for a sub set of a previously published data set which gave a 6.8σ detection of the close X-ray selected BL Lac PKS 2155-304. Chapter 7 discusses the importance of PKS 2155- 304 and presents 3σ flux limits for another seven Southern hemisphere AGN observed between 1996 and 1999 using the Durham Mk6 IACT. Finally there is a short discussion on the future of VHE gamma-ray astronomy
Optical spectroscopy of the low-mass X-ray binary GX 9+9
No full textPhase-resolved medium-resolution VLT spectroscopy of the low-mass X-ray binary GX 9+9 has revealed narrow C ill emission lines that move in phase relative to our new estimate of the ephemeris, and show a velocity amplitude of 230 +/not superset of 35 km s(-1). We identify the origin of these lines as coming from the surface of the donor star, thereby providing the first estimate of the mass function of f(M-1) >= 0.22 M-circle dot. Rotational broadening estimates together with assumptions for the mass donor give 0.07 <= q <= 0.35 and 182 <= K-2 <= 406 km s(-1). Despite a low-mass ratio, there is no evidence for a superhump in our data set. Doppler maps of GX 9+9 show the presence of a stream overflow, either in the form of material flowing downward along the accretion disc rim or in a similar fashion as occurs in high mass transfer rate cataclysmic variables known as the SW Sex stars. Finally, we note that the Bowen region in GX 9+9 is dominated by C in instead of N III emission as has been the case for most other X-ray binaries
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