High-Performance Silicon-Based Multiple Wavelength Source

We demonstrate a stable CMOS-compatible on-chip multiple-wavelength source by filtering and modulating individual lines from a frequency comb generated by a microring resonator optical parametric oscillator.. We show comb operation in a low-noise state that is stable and usable for many hours. Bit-error rate measurements demonstrate negligible power penalty from six independent frequencies when compared to a tunable diode laser baseline. Open eye diagrams confirm the fidelity of the 10 Gb/s data transmitted at the comb frequencies and the suitability of this device for use as a fully integrated silicon-based WDM source.Comment: 3 pages, 3 figure

Strong polarization mode coupling in microresonators

We observe strong modal coupling between the TE00 and TM00 modes in Si3N4 ring resonators revealed by avoided crossings of the corresponding resonances. Such couplings result in significant shifts of the resonance frequencies over a wide range around the crossing points. This leads to an effective dispersion that is one order of magnitude larger than the intrinsic dispersion and creates broad windows of anomalous dispersion. We also observe the changes to frequency comb spectra generated in Si3N4 microresonators due polarization mode and higher-order mode crossings and suggest approaches to avoid these effects. Alternatively, such polarization mode-crossings can be used as a novel tool for dispersion engineering in microresonators.Comment: Comments are very welcome (send to corresponding author

Discriminating the Progenitor Type of Supernova Remnants with Iron K-Shell Emission

Supernova remnants (SNRs) retain crucial information about both their parent explosion and circumstellar material left behind by their progenitor. However, the complexity of the interaction between supernova ejecta and ambient medium often blurs this information, and it is not uncommon for the basic progenitor type (Ia or core-collapse) of well-studied remnants to remain uncertain. Here we present a powerful new observational diagnostic to discriminate between progenitor types and constrain the ambient medium density of SNRs solely using Fe K-shell X-ray emission. We analyze all extant Suzaku observations of SNRs and detect Fe K alpha emission from 23 young or middle-aged remnants, including five first detections (IC 443, G292.0+1.8, G337.2-0.7, N49, and N63A). The Fe K alpha centroids clearly separate progenitor types, with the Fe-rich ejecta in Type Ia remnants being significantly less ionized than in core-collapse SNRs. Within each progenitor group, the Fe K alpha luminosity and centroid are well correlated, with more luminous objects having more highly ionized Fe. Our results indicate that there is a strong connection between explosion type and ambient medium density, and suggest that Type Ia supernova progenitors do not substantially modify their surroundings at radii of up to several parsecs. We also detect a K-shell radiative recombination continuum of Fe in W49B and IC 443, implying a strong circumstellar interaction in the early evolutionary phases of these core-collapse remnants.Comment: Accepted by ApJL; 5 pages with just 1 table and 1 figur

Iron and Nickel Line Diagnostics for the Galactic Center Diffuse Emission

We have observed the diffuse X-ray emission from the Galactic center (GC) using the X-ray Imaging Spectrometer (XIS) on Suzaku. The high-energy resolution and the low-background orbit provide excellent spectra of the GC diffuse X-rays (GCDX). The XIS found many emission lines in the GCDX near the energy of K-shell transitions of iron and nickel. The most pronounced features are FeI K alpha at 6.4 keV and K-shell absorption edge at 7.1 keV, which are from neutral and/or low ionization states of iron, and the K-shell lines at 6.7 keV and 6.9 keV from He-like (FeXXV K alpha) and hydrogenic (FeXXVI Ly alpha) ions of iron. In addition, K alpha lines from neutral or low ionization nickel (NiI K alpha) and He-like nickel (NiXXVII K alpha), and FeI K beta, FeXXV K beta, FeXXVI Ly beta, FeXXV K gamma and FeXXVI Ly gamma are detected for the first time. The line center energies and widths of FeXXV K alpha and FeXXVI Ly alpha favor a collisional excitation (CE) plasma for the origin of the GCDX. The electron temperature determined from the line flux ratio of FeXXV K alpha / FeXXV K beta is similar to the ionization temperature determined from that of FeXXV K alpha /FeXXVI Ly alpha. Thus it would appear that the GCDX plasma is close to ionization equilibrium. The 6.7 keV flux and temperature distribution to the galactic longitude is smooth and monotonic,in contrast to the integrated point source flux distribution. These facts support the hypothesis that the GCDX is truly diffuse emission rather than the integration of the outputs of a large number of unresolved point sources. In addition, our results demonstrate that the chemical composition of Fe in the interstellar gas near the GC is constrained to be about 3.5 times solar.Comment: 11 pages, 19 figures. Accepted for publication in PASJ Suzaku Special Issue (vol. 59 sp. 1

Characteristics of Diffuse X-Ray Line Emission within 20 pc of the Galactic Center

Over the last 3 yrs, the Galactic center (GC) region has been monitored with the Chandra X-Ray Observatory. With 11 Chandra observations through 2002 June, the total effective exposure reaches ~590 ks, providing significant photon statistics on the faint, filamentary, diffuse X-ray emission. The true-color X-ray image and the equivalent width (EW) images for the detected elemental species demonstrate that the diffuse X-ray features have a broad range of spatio-spectral properties. Enhancements of the low-ionization-state, or ``neutral'' Fe line emission (E~6.4 keV) to the northeast of Sgr A* can be interpreted as fluorescence within the dense ISM resulting from irradiation by hard, external X-ray sources. They may also be explained by emission induced by the bombardments by high energy particles on the ISM, such as unresolved supernova (SN) ejecta intruding into dense ISM. The detection of molecular cloud counterparts to the 6.4 keV Fe line features indicates that these Fe line features are associated with dense GC clouds and/or active star-forming regions, which supports the X-ray reflection and/or SN ejecta origins for the Fe line emission. We detect highly ionized S and Si lines which are generally coincident with the neutral Fe line emission and the dense molecular clouds in the northeast of Sgr A*. These hot plasmas are likely produced by massive star-forming activities and/or SNRs. In contrast, we find that highly ionized He-like Fe line emission (E~6.7 keV) is primarily distributed along the plane instead of being concentrated in the northeast of Sgr A*. The implied high temperature and the alignment along the plane are consistent with the magnetic confinement model.Comment: 13 pages (ApJ emulator style) including 4 figures (2 color figs). Accepted by ApJ. For full-quality figures, contact [email protected]

Further studies of 1E 1740.7-2942 with ASCA

We report the ASCA results of the Great Annihilator 1E 1740.7-2942 obtained with five pointing observations in a time span of 3.5 years. The X-ray spectrum for each period is well fitted with a single power-law absorbed by a high column of gas. The X-ray flux changes by a factor of 2 from period to period, but the other spectral parameters show no significant change. The photon index is flat with \Gamma = 0.9--1.3. The column densities of hydrogen N_H is $\sim$ 1.0 x 10^{23} H cm^{-2} and that of iron N_{Fe} is $\sim$ 10^{19} Fe cm^{-2}. These large column densities indicate that 1E 1740.7-2942 is near at the Galactic Center. The column density ratio leads the iron abundance to be 2 times larger than the other elements in a unit of the solar ratio. The equivalent width of the K\alpha-line from a neutral iron is less than 15 eV in 90% confidence. This indicates that the iron column density within several parsecs from 1E 1740.7-2942 is less than 5 x 10^{17} Fe cm^{-2}. In addition, the derived hydrogen column density is about 1/6 of that of giant molecular clouds in the line of sight. All these facts support that 1E 1740.7-2942 is not in a molecular cloud, but possibly in front of it; the X-rays are not powered by accretion from a molecular cloud, but from a companion star like ordinary X-ray binaries.Comment: To appear in ApJ July 20, 1999 issue, Vol. 520 #1, 23 pages LaTeX files, uses aasms4.sty and psfig.sty, also available at http://www-cr.scphys.kyoto-u.ac.jp/member/sakano/work/paper/index-e.htm

ASCA X-ray source catalogue in the Galactic Center region

The ASCA satellite made 107 pointing observations on a 5 x 5 deg^2 region around the center of our Milky Way Galaxy (the Galactic Center) from 1993 to 1999. In the X-ray images of the 0.7--3 keV or 3--10 keV bands, we found 52 point sources and a dozen diffuse sources. All the point sources are uniformly fitted with an absorbed power-law model. For selected bright sources, Sgr A*, AX J1745.6-2901, A 1742-294, SLX 1744-300, GRO J1744-28, SLX 1737-282, GRS 1734-292, AX J1749.2-2725, KS 1741-293, GRS 1741.9-2853, and an unusual flare source XTE J1739-302, we present further detailed spectral and timing analyses, and discuss their nature. The dozen extended X-ray sources comprise radio supernova remnants, giant molecular clouds, and some new discoveries. Most show emission lines from either highly ionized atoms or low-ionized irons. The X-ray spectra were fitted with either a thin thermal or power-law model. This paper summarizes the results and provides the ASCA X-ray source catalogue in the Galactic Center region.Comment: 33 pages, 8 figures, Accepted for publication in ApJS, also found in http://www.star.le.ac.uk/~mas/research/paper/#Sakano2001apj

Discovery of a possible X-ray counterpart to HESS J1804-216

Suzaku deep observations have discovered two highly significant nonthermal X-ray sources, Suzaku J1804$-$2142 (Src 1) and Suzaku J1804$-$2140 (Src 2), positionally coincident with the unidentified TeV $\gamma$-ray source, HESS J1804$-$216. The X-ray sources are not time variable and show no counterpart in other wavebands, except for the TeV source. Src 1 is unresolved at Suzaku spatial resolution, whereas Src 2 is extended or composed of multiple sources. The X-ray spectra are highly absorbed, hard, and featureless, and are well fitted by absorbed power-law models with best-fit photon indices and absorption columns of $-0.3_{-0.5}^{+0.5}$ and $0.2_{-0.2}^{+2.0}\times 10^{22}$ cm$^{-2}$ for Src 1, and $1.7_{-1.0}^{+1.4}$ and $1.1_{-0.6}^{+1.0}\times 10^{23}$ cm$^{-2}$ for Src 2. The measured X-ray absorption to the latter source is significantly larger than the total Galactic neutral hydrogen column in that direction. The unabsorbed 2--10 keV band luminosities are $7.5\times 10^{32}(d/{\rm 5 kpc})^2$ ergs s$^{-1}$ (Src 1) and $1.3\times 10^{33}(d/{\rm 5 kpc})^2$ ergs s$^{-1}$ (Src 2), where $d$ is the source distance. Among the handful of TeV sources with known X-ray counterparts, HESS J1804$-$216 has the largest ratio of TeV $\gamma$-ray to hard X-ray fluxes. We discuss the nature of the emission and propose the Suzaku sources as plausible counterparts to the TeV source, although further observations are necessary to confirm this.Comment: 12 pages, 5 figures, Publications of the Astronomical Society of Japan, in pres