Binding between endohedral Na atoms in Si clathrate I; a first principles study

We investigate the binding nature of the endohedral sodium atoms with the ensity functional theory methods, presuming that the clathrate I consists of a sheaf of one-dimensional connections of Na@Si$_{24}$ cages interleaved in three perpendicular directions. Each sodium atom loses 30% of the 3s$^1$ charge to the frame, forming an ionic bond with the cage atoms; the rest of the electron contributes to the covalent bond between the nearest Na atoms. The presumption is proved to be valid; the configuration of the two Na atoms in the nearest Si$_{24}$ cages is more stable by 0.189 eV than that in the Si$_{20}$ and Si$_{24}$ cages. The energy of the beads of the two distorted Na atoms is more stable by 0.104 eV than that of the two infinitely separated Na atoms. The covalent bond explains both the preferential occupancies in the Si$_{24}$ cages and the low anisotropic displacement parameters of the endohedral atoms in the Si$_{24}$ cages in the [100] directions of the clathrate I.Comment: First page: Affiliation added to PDF and PS versio

An Electron-Tracking Compton Telescope for a Survey of the Deep Universe by MeV gamma-rays

Photon imaging for MeV gammas has serious difficulties due to huge backgrounds and unclearness in images, which are originated from incompleteness in determining the physical parameters of Compton scattering in detection, e.g., lack of the directional information of the recoil electrons. The recent major mission/instrument in the MeV band, Compton Gamma Ray Observatory/COMPTEL, which was Compton Camera (CC), detected mere $\sim30$ persistent sources. It is in stark contrast with $\sim$2000 sources in the GeV band. Here we report the performance of an Electron-Tracking Compton Camera (ETCC), and prove that it has a good potential to break through this stagnation in MeV gamma-ray astronomy. The ETCC provides all the parameters of Compton-scattering by measuring 3-D recoil electron tracks; then the Scatter Plane Deviation (SPD) lost in CCs is recovered. The energy loss rate (dE/dx), which CCs cannot measure, is also obtained, and is found to be indeed helpful to reduce the background under conditions similar to space. Accordingly the significance in gamma detection is improved severalfold. On the other hand, SPD is essential to determine the point-spread function (PSF) quantitatively. The SPD resolution is improved close to the theoretical limit for multiple scattering of recoil electrons. With such a well-determined PSF, we demonstrate for the first time that it is possible to provide reliable sensitivity in Compton imaging without utilizing an optimization algorithm. As such, this study highlights the fundamental weak-points of CCs. In contrast we demonstrate the possibility of ETCC reaching the sensitivity below $1\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$ at 1 MeV.Comment: 19 pages, 12 figures, Accepted to the Astrophysical Journa

Ammonia (J,K) = (1,1) to (4,4) and (6,6) inversion lines detected in the Seyfert 2 galaxy NGC 1068

We present the detection of the ammonia (NH3) (J,K) = (1,1) to (4,4) and (6,6) inversion lines toward the prototypical Seyfert 2 galaxy NGC 1068, made with the Green Bank Telescope (GBT). This is the first detection of ammonia in a Seyfert galaxy. The ortho-to-para-NH3 abundance ratio suggests that the molecule was formed in a warm medium of at least 20 K. For the NH3 column density and fractional abundance, we find (1.09\pm0.23)\times10^14 cm^-2 and (2.9\pm0.6)\times10^-8, respectively, from the inner 1.2 kpc of NGC 1068. The kinetic temperature can be constrained to 80\pm20 K for the bulk of the molecular gas, while some fraction has an even higher temperature of 140\pm30 K.Comment: 5 pages, 3 figures, accepted by A&

Large Silicon Abundance in Photodissociation Regions

We have made one-dimensional raster-scan observations of the rho Oph and sigma Sco star-forming regions with two spectrometers (SWS and LWS) on board the ISO. In the rho Oph region, [SiII] 35um, [OI] 63um, 146um, [CII] 158um, and the H2 pure rotational transition lines S(0) to S(3) are detected, and the PDR properties are derived as the radiation field scaled by the solar neighborhood value G_0~30-500, the gas density n~250--2500 /cc, and the surface temperature T~100-400 K. The ratio of [SiII] 35um to [OI] 146um indicates that silicon of 10--20% of the solar abundance must be in the gaseous form in the photodissociation region (PDR), suggesting that efficient dust destruction is undergoing even in the PDR and that part of silicon atoms may be contained in volatile forms in dust grains. The [OI] 63um and [CII] 158um emissions are too weak relative to [OI] 146um to be accounted for by standard PDR models. We propose a simple model, in which overlapping PDR clouds along the line of sight absorb the [OI] 63um and [CII] 158um emissions, and show that the proposed model reproduces the observed line intensities fairly well. In the sigma Sco region, we have detected 3 fine-structure lines, [OI] 63um, [NII] 122um, and [CII] 158um, and derived that 30-80% of the [CII] emission comes from the ionized gas. The upper limit of the [SiII] 35um is compatible with the solar abundance relative to nitrogen and no useful constraint on the gaseous Si is obtained for the sigma Sco region.Comment: 25 pages with 7 figures, accepted in Astrophysical Journa

ΛN\Lambda N correlations from the stopped K−K^- reaction on 4{}^4He

We have investigated correlations of coincident $\Lambda N$ pairs from the stopped $K^-$ reaction on ${}^4$He, and clearly observed $\Lambda p$ and $\Lambda n$ branches of the two-nucleon absorption process in the $\Lambda N$ invariant mass spectra. In addition, non-mesonic reaction channels, which indicate possible exotic signals for the formation of strange multibaryon states, have been identified.Comment: 5 pages, 3 figures, submitted to Physical Review Letter

Herschel-PACS Observations of Far-IR CO Line Emission in NGC 1068: Highly Excited Molecular Gas in the Circumnuclear Disk

We report the detection of far-IR CO rotational emission from the prototypical Seyfert 2 galaxy NGC 1068. Using Herschel-PACS, we have detected 11 transitions in the J_upper=14-30 (E_upper/k_B = 580-2565 K) range, all of which are consistent with arising from within the central 10" (700 pc). The detected transitions are modeled as arising from 2 different components: a moderate excitation (ME) component close to the galaxy systemic velocity, and a high excitation (HE) component that is blueshifted by ~80 km s^{-1}. We employ a large velocity gradient (LVG) model and derive n_H2~10^{5.6} cm^{-3}, T_kin~170 K, and M_H2~10^{6.7} M_sun for the ME component, and n_H2~10^{6.4} cm^{-3}, T_kin~570 K, and M_H2~10^{5.6} M_sun for the HE component, although for both components the uncertainties in the density and mass are plus/minus (0.6-0.9) dex. We compare the CO line profiles with those of other molecular tracers observed at higher spatial and spectral resolution, and find that the ME transitions are consistent with these lines arising in the ~200 pc diameter ring of material traced by H_2 1-0 S(1) observations. The blueshift of the HE lines may also be consistent with the bluest regions of this H_2 ring, but a better kinematic match is found with a clump of infalling gas ~40 pc north of the AGN. We discuss the prospects of placing the HE component near the AGN, and conclude that while the moderate thermal pressure precludes an association with the ~1 pc radius H_2O maser disk, the HE component could potentially be located only a few parsecs more distant from the AGN, and might then provide the N_H~10^{25} cm^{-2} column obscuring the nuclear hard X-rays. Finally, we also report sensitive upper limits extending up to J_upper=50, which place constraints on a previous model prediction for the CO emission from the X-ray obscuring torus. [Abridged]Comment: 20 pages, 10 figures. Accepted for publication in Ap

Probing X-ray irradiation in the nucleus of NGC 1068 with observations of high-J lines of dense gas tracers

With the incorporation of high-J molecular lines, we aim to constrain the physical conditions of the dense gas in the central region of the Seyfert 2 galaxy NGC 1068 and to determine signatures of the AGN or the starburst contribution. We used the James Clerk Maxwell Telescope to observe the J=4-3 transition of HCN, HNC, and HCO+, as well as the CN N_J=2_{5/2}-1_{3/2} and N_J=3_{5/2}-2_{5/2}, in NGC 1068. We estimate the excitation conditions of HCN, HNC, and CN, based on the line intensity ratios and radiative transfer models. We find that the bulk emission of HCN, HNC, CN, and the high-J HCO+ emerge from dense gas n(H_2)>=10^5 cm^-3). However, the low-J HCO+ lines (dominating the HCO+ column density) trace less dense (n(H_2)<10^5 cm^-3) and colder (T_K30 K) gas than the other molecules. The HCO+ J=4-3 line intensity, compared with the lower transition lines and with the HCN J=4-3 line, support the influence of a local XDR environment. The estimated N(CN)/N(HCN)~1-4 column density ratios are indicative of an XDR/AGN environment with a possible contribution of grain-surface chemistry induced by X-rays or shocks.Comment: Main text: 8 pages, 5 tables, 1 figure. Appendix: 7 pages, 1 table, 8 figures. Accepted for publication in A&