arXiv subjects

Kaya Mori

Publications and source records attributed to Kaya Mori.

105 records · Page 6Linked to original sources

arXiv2013

The pGAPS experiment: an engineering balloon flight of prototype GAPS

The General Anti-Particle Spectrometer (GAPS) project is being carried out to search for primary cosmic-ray antiparticles especially for antideuterons produced by cold dark matter. GAPS plans to realize the science observation by Antarctic long duration balloon flights in the late 2010s. In preparation for the Antarctic science flights, an engineering balloon flight using a prototype of the GAPS instrument, "pGAPS", was successfully carried out in June 2012 in Japan to verify the basic performance of each GAPS subsystem. The outline of the pGAPS flight campaign is briefly reported.

astro-ph.IM↗

arXiv2013

NuSTAR discovery of a 3.76-second transient magnetar near Sagittarius A*

We report the discovery of 3.76-s pulsations from a new burst source near Sgr A* observed by the NuSTAR Observatory. The strong signal from SGR J1745-29 presents a complex pulse profile modulated with pulsed fraction 27+/-3 % in the 3-10 keV band. Two observations spaced 9 days apart yield a spin-down rate of Pdot = (6.5+/-1.4)x10^{-12}. This implies a magnetic field B = 1.6x10^14 G, spin-down power Edot = 5x10^33 erg/s, and characteristic age P/2Pdot = 9x10^3 yr, for the rotating dipole model. However, the current Pdot may be erratic, especially during outburst. The flux and modulation remained steady during the observations and the 3-79 keV spectrum is well fitted by a combined blackbody plus power-law model with temperature kT_BB = 0.96+/-0.02 keV and photon index = 1.5+/-0.4, respectively. The neutral hydrogen column density (nH ~ 1.4x10^23 cm^{-2}) measured by NuSTAR and Swift suggests that SGR J1745-29 is located at or near the Galactic Center. The lack of an X-ray counterpart in the published Chandra survey catalog sets a quiescent 2-8 keV luminosity limit of Lx ~< 10^32 erg/s. The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3.5x10^35 erg/s for a distance of 8 kpc. SGR J1745-29 joins a growing subclass of transient magnetars, indicating that many magnetars in quiescence remain undetected in the X-ray band or have been detected as high-B radio pulsars. The peculiar location of SGR J1745-29 has important implications for the formation and dynamics of neutron stars in the Galactic Center region.

astro-ph.HE↗

arXiv2013

The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 13 June 2012, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 -- 79 keV, extending the sensitivity of focusing far beyond the ~10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low-background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than one-hundred-fold improvement in sensitivity over the collimated or coded-mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity, spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives, and will also undertake a broad program of targeted observations. The observatory consists of two co-aligned grazing-incidence X-ray telescopes pointed at celestial targets by a three-axis stabilized spacecraft. Deployed into a 600 km, near-circular, 6degree inclination orbit, the Observatory has now completed commissioning, and is performing consistent with pre-launch expectations. NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of ten years, we anticipate proposing a guest investigator program, to begin in Fall 2014.

astro-ph.IM↗

arXiv2010

The Nuclear Spectroscopic Telescope Array (NuSTAR)

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing hard X-ray (5 -- 80 keV) telescope to orbit. NuSTAR will offer a factor 50 -- 100 sensitivity improvement compared to previous collimated or coded mask imagers that have operated in this energy band. In addition, NuSTAR provides sub-arcminute imaging with good spectral resolution over a 12-arcminute field of view. After launch, NuSTAR will carry out a two-year primary science mission that focuses on four key programs: studying the evolution of massive black holes through surveys carried out in fields with excellent multiwavelength coverage, understanding the population of compact objects and the nature of the massive black hole in the center of the Milky Way, constraining explosion dynamics and nucleosynthesis in supernovae, and probing the nature of particle acceleration in relativistic jets in active galactic nuclei. A number of additional observations will be included in the primary mission, and a guest observer program will be proposed for an extended mission to expand the range of scientific targets. The payload consists of two co-aligned depth-graded multilayer coated grazing incidence optics focused onto solid state CdZnTe pixel detectors. To be launched in early 2012 on a Pegasus rocket into a low-inclination Earth orbit. Data will be publicly available at GSFC's High Energy Astrophysics Science Archive Research Center (HEASARC) following validation at the science operations center located at Caltech.

astro-ph.IM↗

arXiv2007

Modeling Phase-resolved Observations of the Surfaces of Magnetic Neutron Stars

Recent observations by XMM-Newton detected rotational pulsations in the total brightness and spectrum of several neutron stars. To properly interpret the data, accurate modeling of neutron star emission is necessary. Detailed analysis of the shape and strength of the rotational variations allows a measurement of the surface composition and magnetic field, as well as constrains the nuclear equation of state. We discuss our models of the spectra and light curves of two of the most observed neutron stars, RX J1856.5-3754 and 1E 1207.4-5209, and discuss some implications of our results and the direction of future work.

astro-ph↗

arXiv2007

Spectral and Rotational Changes in the Isolated Neutron Star RX J0720.4-3125

RX J0720.4-3125 is an isolated neutron star that, uniquely in its class, has shown changes in its thermal X-ray spectrum. We use new spectra taken with Chandra's Low Energy Transmission Grating Spectrometer, as well as archival observations, to try to understand the timescale and nature of these changes. We construct lightcurves, which show both small, slow variations on a timescale of years, and a larger event that occurred more quickly, within half a year. From timing, we find evidence for a `glitch' coincident with this larger event, with a fractional increase in spin frequency of 5x10^{-8}. We compare the `before' and `after' spectra with those from RX J1308.6+2127, an isolated neutron star with similar temperature and magnetic field strength, but with a much stronger absorption feature in its spectrum. We find that the `after' spectrum can be represented remarkably well by the superposition of the `before' spectrum, scaled by two thirds, and the spectrum of RX J1308.6+2127, thus suggesting that the event affected approximately one third of the surface. We speculate the event reflects a change in surface composition caused by, e.g., an accretion episode.

astro-ph↗

arXiv2007

Ionization and dissociation equilibrium in strongly-magnetized helium atmosphere

Recent observations and theoretical investigations of neutron stars indicate that their atmospheres consist not of hydrogen or iron but possibly other elements such as helium. We calculate the ionization and dissociation equilibrium of helium in the conditions found in the atmospheres of magnetized neutron stars. For the first time this investigation includes the internal degrees of freedom of the helium molecule. We found that at the temperatures and densities of neutron star atmospheres the rotovibrational excitations of helium molecules are populated. Including these excitations increases the expected abundance of molecules by up to two orders of magnitude relative to calculations that ignore the internal states of the molecule; therefore, if the atmospheres of neutron stars indeed consist of helium, helium molecules and possibly polymers will make the bulk of the atmosphere and leave signatures on the observed spectra from neutron stars. We applied our calculation to nearby radio-quiet neutron stars with B_{dipole}~10^{13}-10^{14} G. If helium comprises their atmospheres, our study indicates that isolated neutron stars with T_{BB}~10^6 K such as RXJ0720.4-3125 and RXJ1605.3+3249 will have He^+ ions predominantly, while isolated neutron stars with lower temperature (T_{BB}~5x10^5 K) such as RXJ1856.5-3754 and RXJ0420.0-5022 will have some fraction of helium molecules. If helium molecules are abundant, their spectroscopic signatures may be detected in the optical, UV and X-ray band. (Abridged)

astro-ph↗

arXiv2007

Modelling mid-Z element atmospheres for strongly-magnetized neutron stars

We construct models for strongly-magnetized neutron star atmospheres composed of mid-Z elements (carbon, oxygen and neon) with magnetic fields B=10^{12}-10^{13} G and effective temperatures Teff=(1-5)*10^6 K; this is done by first addressing the physics relevant to strongly-magnetized plasmas and calculating the equation of state and polarization-dependent opacities. We then obtain the atmosphere structure and spectrum by solving the radiative transfer equations in hydrostatic and radiative equilibrium. In contrast to hydrogen opacities at the relevant temperatures, mid-Z element opacities are dominated by numerous bound-bound and bound-free transitions. Consequently, temperature profiles are closer to grey profiles, and photosphere densities are lower than in the hydrogen case. Mid-Z element atmosphere spectra are significantly softer than hydrogen atmosphere spectra and show numerous absorption lines and edges. The atmosphere spectra depend strongly on surface composition and magnetic field but weakly on surface gravity. Absorption lines are primarily broadened by motional Stark effects and the (unknown) surface magnetic field distribution. Given the multiple absorption features observed from several isolated neutron stars, it is possible to determine, with existing X-ray data, the surface composition, magnetic field, temperature, and gravitational redshift; we present qualitative comparisons between our model spectra and the neutron stars 1E1207.4-5209 and RX J1605.3+3249. Future high-resolution X-ray missions such as Constellation-X will measure the gravitational redshift with high accuracy by resolving narrow absorption features, and when combined with radius measurements, it will be possible to uniquely determine the mass and radius of isolated neutron stars. (Abridged)

astro-ph↗

arXiv2006

Detailed atmosphere modelling for the neutron star 1E1207.4-5209: Evidence of Oxygen/Neon atmosphere

We present a comprehensive investigation of the two broad absorption features observed in the X-ray spectrum of the neutron star 1E1207.4-5209 based on a recent analysis of the 260 ksec XMM-Newton data by Mori et al. 2005. Expanding on our earlier work (Hailey & Mori 2002) we have examined all previously proposed atmospheric models for 1E1207.4-5209. Using our atomic code, which rapidly solves Schrodinger's equation for arbitrary ion in strong magnetic field (Mori & Hailey 2002), we have systematically constructed atmospheric models by calculating polarization-dependent LTE opacities and addressed all the physics relevant to strongly-magnetized plasmas. We have been able to rule out virtually all atmospheric models because they either do not sustain an ionization balance consistent with the claimed atmosphere composition or because they predict line strengths and line widths which are inconsistent with the data. Only Oxygen or Neon atmospheres at B~10^{12} G provide self-consistent atmospheric solutions of appropriate ionization balance and with line widths, strengths and energies consistent with the observations. The observed features are likely composed of several bound-bound transition lines from highly-ionized oxygen/neon and they are broadened primarily by motional Stark effects and magnetic field variation over the line-emitting region. Further considerations of plausible mechanisms for the formation of a mid-Z atmosphere likely rule out Neon atmospheres, and have important implications for the fallback mechanism in supernova ejecta. Future high resolution spectroscopy missions such as Constellation-X will be able to resolve predicted substructure in the absorption features and will measure magnetic field strength and gravitational redshift independently to better than 10% accuracy.

astro-ph↗

arXiv2005

Detailed spectral analysis of the 260 ksec XMM-Newton data of 1E1207.4-5209 and significance of a 2.1 keV absorption feature

We have reanalyzed the 260 ksec XMM-Newton observation of 1E1207.4-5209. There are several significant improvements over previous work. Firstly, a much broader range of physically plausible spectral models was used. Secondly, we have utilized a more rigorous statistical analysis. The standard F-distribution was not employed, but rather the exact finite statistics F-distribution was determined by Monte-Carlo simulations. This approach was motivated by the recent work of Protassov et al. (2002) and Freeman et al. (1999). They demonstrated that the standard F-distribution is not even asymptotically correct when applied to assess the signficance of additional absorption features in a spectrum. With our improved analysis we do not find a third and fourth spectral feature (Bignami et al. (2003), De Luca et al. (2004)) in 1E1207.4-5209, but only the two broad absorption features previously reported (Sanwal et al. (2002), Hailey and Mori (2002)). Two additional statistical tests, one line model dependent and the other line model independent confirmed our modified F-test analysis. For all physically plausible continuum models where the weak residuals are strong enough to fit, the residuals occur at the instrument Au-M edge. As a sanity check we confirmed that the residuals are consistent in strength and position with the instrument Au-M residuals observed in 3C273.

astro-ph↗

arXiv2003

Isolated magnetar spin-down, soft X-ray emission and RXJ1856.5-3754

When an isolated magnetar with magnetic dipole field B ~ 10^15 G moves at high velocity (v > 10^7 cm/s) through the ISM, its transition into propeller effect driven spin-down may occur in less than 10^6 years. We propose that the nearby neutron star RXJ1856.5-3754 is such a magnetar, and has spun down by the propeller effect to a period greater than 10^4 sec within ~5*10^5 years. This magnetar scenario is consistent with observed thermal X-ray emission properties and the absence of detectable spin-modulations of them. Detection of other rapidly moving long period (> 100 sec) magnetars with known ages would strongly constrain the very great variety of predicted propeller-effect torque magnitudes.

astro-ph↗

arXiv2003

XMM-Newton observations of the Vela pulsar

We present spectral analysis from XMM-Newton observations of the Vela pulsar. We analyzed thermal emission from the pulsar dominating below ~ 1 keV since extracted spectra are heavily contaminated by nebular emission at higher energy. Featureless high-resolution spectra of the Reflection Grating Spectrometer aboard XMM-Newton suggest the presence of a hydrogen atmosphere, as previously indicated by Chandra results. Both the temperature and radius are consistent with those values deduced from Chandra. The derived Chandra and XMM-Newton temperature of T^{\infty} ~ (6.4-7.1)*10^5 K at its age of ~10^4 years is below the standard cooling curve.

astro-ph↗

arXiv2002

Evidence for a Mid-Atomic-Number Atmosphere in the Neutron Star 1E1207.4-5209

Recently Sanwal et al. (2002) reported the first clear detection of absorption features in an isolated neutron star, 1E1207.4-5209. Remarkably their spectral modeling demonstrates that the atmosphere cannot be Hydrogen. They speculated that the neutron star atmosphere is indicative of ionized Helium in an ultra-strong (~1.5x10^{14} G) magnetic field. We have applied our recently developed atomic model (Mori & Hailey 2002) for strongly-magnetized neutron star atmospheres to this problem. We find that this model, along with some simp le atomic physics arguments, severely constrains the possible composition of the atmosphere. In particular we find that the absorption features are naturally associated with He-like Oxygen or Neon in a magnetic field of ~10^{12} G, comparable to the magnetic field derived from the spin parameters of the neutron star. This interpretation is consistent with the relative line strengths and widths and is robust. Our model predicts possible substructure in the spectral features, which has now been reported by XMM-Newton (Mereghetti et al. 2002). However we show the Mereghetti et al. claim that the atmosphere is Iron or some comparable high-Z element at ~ 10^{12} G is easily ruled out by the Chandra and XMM-Newton data.

astro-ph↗

arXiv2001

A novel antimatter detector based on X-ray deexcitation of exotic atoms

We propose a novel antiparticle detector. The gaseous antiparticle spectrometer (GAPS) effects particle identification through the characteristic X-rays emitted by antiparticles when they form exotic atoms in gases. GAPS obtains particularly high grasp (effective area--solid angle product) at lower particle energies, where conventional schemes are most limited in their utility. The concept is simple and lightweight, so it can be readily employed on balloon and space-based missions. An extremely powerful potential application of GAPS is a space-based search for the neutralino through the detection of a neutralino annihilation by-product -- the antideuteron. Paradoxically, this space-based search for the neutralino is capable of achieving comparable sensitivity to as yet unrealized third generation, underground dark matter experiments. And GAPS can obtain this performance in a very modest satellite experiment. GAPS can also provide superior performance in searches for primary antiprotons produced via neutralino annihilation, black hole evaporation and in probing subdominant contributions to the antiproton flux at low energies. In a deep space mission GAPS will obtain higher sensitivity for a given weight and power than BGO calorimeters.

astro-ph↗

arXiv2001

Atomic calculation for the atmospheres of strongly-magnetized neutron stars

Complete modeling of radiative transfer in neutron star atmospheres is in progress, taking into account the anisotropy induced by magnetic fields, non-ideal effects and general relativity. As part of our modeling, we present a novel atomic calculation method producing an extensive atomic data set including energy values and oscillator strengths in the so-called Landau regime ($B > 4.7\times10^9Z^2$ G). Conventional atmosphere models for B=0 are not applicable to typical field strengths of cooling neutron stars ($B \sim10^{12}-10^{13}$ G), since an atom no longer keeps its spherical shape. The elemental composition and the configuration of the magnetic field in the atmosphere are presently unknown, so that atomic data must be produced for ground and excited states of several ions as a function of magnetic field. To accomplish this efficiently, we minimized the iterations in the Hartree equation and treated exchange terms and higher Landau states by perturbation methods. This method has the effect of reducing the computation time significantly. Inclusion of higher Landau states gives us much more accurate data for inner orbitals unlike other methods based on the adiabatic approximation. While existing atomic data in the Landau regime are available only for low $Z$ atoms, our method can be used in elements up to Fe with sufficient accuracy to be of use for spectroscopic missions such as {\it Chandra}, {\it XMM-Newton} and next-generation X-ray telescopes.

astro-ph↗