Progress in laboratory performance verification of the R-process Isotope Observer

The source of galactic cosmic rays (GCR) remains unknown, despite many decades of research. It is generally accepted among high-energy astrophysicists that GCR are accelerated by supernova (SN) shocks in the interstellar medium, but the evidence for this remains indirect. However, the fact that the cosmic-ray spectrum continues far past the limit of SN shock acceleration is a serious problem for this model. Most SNae occur in superbubbles – regions of the interstellar medium containing massive stars and SN remnants. Thus, SNae should be accelerating the ejecta of other SNae. The local medium of the superbubble will be enhanced in freshly synthesized R-process material. We are currently developing the R-process Isotope Observer (RIO), which could make the first measurements of the isotopic abundances of the “ultraheavy” GCR (those in the range 32 ⩽ Z ⩽ 42) in order to determine the fractional contribution of freshly synthesized R-process material in GCR through the measurement of several key isotopic ratios. We have recently performed a series of tests of the RIO detector with 400 A MeV ⁸⁴Kr at the HIMAC accelerator in Japan. The results of these tests indicate that a mass resolution of ∼0.3 u can be achieved with RIO. We also discuss the prospects for accommodation of the RIO mission.     

The nature of nonthermal X-ray filaments near the galactic center

Recent Chandra and XMM-Newton observations reported evidence of two X-ray filaments G359.88−0.08 (SgrA-E) and G359.54+0.18 (the ripple filament) near the Galactic center. The X-ray emission from these filaments has a nonthermal spectrum and coincides with synchrotron emitting radio sources. Here, we report the detection of a new X-ray feature coincident with a radio filament G359.90−0.06 (SgrA-F) and show more detailed VLA, Chandra and BIMA observations of the radio and X-ray filaments. In particular, we show that radio emission from the nonthermal filaments G359.90−0.06 (SgrA-F) and G359.54+0.18 (the ripple) has a steep spectrum whereas G359.88−0.08 (SgrA-E) has a flat spectrum. The X-ray emission from both these sources could be due to synchrotron radiation. However, given that the 20 km s⁻¹ molecular cloud, with its intense 1.2 mm dust emission, lies in the vicinity of SgrA-F, it is possible that the X-rays could be produced by inverse Compton scattering of far-infrared photons from dust by the relativistic electrons responsible for the radio synchrotron emission. The production of X-ray emission from ICS allows an estimate of the magnetic field strength of 0.08 mG within the nonthermal filament. This should be an important parameter for any models of the Galactic center nonthermal filaments.     

Nonlinear processes in cosmic-ray precursor of strong supernova shock: Maximum energy and average energy spectrum of accelerated particles

The instability in the cosmic-ray precursor of a supernova shock is studied. The level of turbulence in this region determines the maximum energy of accelerated particles. The consideration is not limited by the case of weak turbulence. It is assumed that the Kolmogorov type nonlinear wave interactions together with the ion-neutral collisions restrict the amplitude of random magnetic field. As a result, the maximum energy of accelerated particles strongly depends on the age of a SNR. The average spectrum of cosmic rays injected in the interstellar medium in the course of adiabatic SNR evolution takes the approximate form E⁻² at energies larger than 10–30 GeV/nucleon with the maximum energy that is close to the position of the knee in cosmic-ray spectrum at 4 × 10¹⁵ eV. At an earlier stage of SNR evolution – the ejecta-dominated stage, the particles are accelerated to higher energies and have a rather steep power-law distribution. These results suggest that the knee may mark the transition from the ejecta-dominated to the adiabatic evolution of SNR shocks which accelerate cosmic rays.     

RIO: the R-process isotope observer

Our galaxy is filled with a “gas” of relativistic nuclei and electrons – the galactic cosmic rays (GCRs). The source of GCR nuclei is unknown, but there is a general consensus among high-energy astrophysicists that GCRs are accelerated by supernova (SN) shocks in the interstellar medium. The evidence in support of this picture is strong but indirect. However, the fact that the cosmic-ray spectrum extends continuously, without steps or peaks, to more than five orders of magnitude in energy beyond the limit of SN shock acceleration appears to be incompatible with this picture, at least without extreme fine-tuning of models. A “smoking gun” is needed to definitively establish that SN shocks are indeed the accelerator of GCRs. If GCRs are accelerated in SN shocks, they will be enhanced in freshly-synthesized r-process material. We are currently studying the R-process Isotope Observer (RIO) as a Mission of Opportunity for the International Space Station. RIO will make the first measurements of the isotopic abundances of the “ultraheavy” GCRs (those in the range 32 ⩽ Z ⩽ 42) and will determine the fractional contribution of freshly-synthesized r-process material in GCRs through the measurement of several key isotopic ratios.     

XMM-Newton observations of the Mouse,SLX 1744–299 and SLX 1744–300

We observed the radio and X-ray source G359.23–0.82, also known as “the Mouse”, with XMM-Newton. The X-ray image of this object shows a point-like source at the Mouse’s “head”, accompanied by a “tail” that extends for about 40″ westward. The morphology is consistent with that observed recently with Chandra [Gaensler, B.M., van der Swaluw, E., Camilo, F., et al. The Mouse that soared: high resolution X-ray imaging of the pulsar-powered bow shock G359.23–0.82, ApJ 616, 383–402, 2004]. The spectrum of the head can be described by a power-law model with a photon index Γ ≃ 1.9. These results confirm that the Mouse is a bow-shock pulsar wind nebula (PWN) powered by PSR J1747–2958. We found that the hydrogen column density toward the Mouse, N_H = (2.60 ± 0.09) × 10²² cm⁻², is 20%–40% lower than those toward two serendipitously detected X-ray bursters, SLX 1744–299 and SLX 1744–300. At a plausible distance of 5 kpc, the X-ray luminosity of the Mouse, L(0.5–10 keV) = 3.7 × 10³⁴ erg s⁻¹, is 1.5% of the pulsar’s spin-down luminosity. We detected a Type I X-ray burst from SLX 1744–300 and found a possible decrease of N_H and persistent luminosity for this source, in comparison with those observed with ROSAT in 1992.     

Giant radio pulses from the Crab pulsar

Individual giant radio pulses (GRPs) from the Crab pulsar last only a few microseconds. However, during that time they rank among the brightest objects in the radio sky reaching peak flux densities of up to 1500 Jy even at high radio frequencies. Our observations show that GRPs can be found in all phases of ordinary radio emission including the two high frequency components (HFCs) visible only between 5 and 9 GHz [Moffett, D.A., Hankins, T.H. Multifrequency radio observations of the Crab pulsar. Astrophys. J. 468, 779–783, 1996]. This leads us to believe that there is no difference in the emission mechanism of the main pulse (MP), inter pulse (IP) and HFCs. High resolution dynamic spectra from our recent observations of giant pulses with the Effelsberg telescope at a center frequency of 8.35 GHz show distinct spectral maxima within our observational bandwidth of 500 MHz for individual pulses. Their narrow band components appear to be brighter at higher frequencies (8.6 GHz) than at lower ones (8.1 GHz). Moreover, there is an evidence for spectral evolution within and between those structures. High frequency features occur earlier than low frequency ones. Strong plasma turbulence might be a feasible mechanism for the creation of the high energy densities of ∼6.7 × 10⁴ erg cm⁻³ and brightness temperatures of ∼10³¹ K.     

The temperature map of the loop and coronal sources of an occulted solar flare

An occulted solar flare occurred at about 06:07 UT on 2002, November 2. The RHESSI X-ray images show two separate parts. The lower part consists of a complete loop and the upper part a coronal source which well extends above the solar limb. The loop source shrank for about 3 min with a speed of ∼24 km s⁻¹ during the early impulsive phase and then expanded at ∼7 km s⁻¹, while the coronal source presented an upward motion at about 6 km s⁻¹. We obtained the temperature map of the loop source from RHESSI image spectrum. The temperature of the loop increases with altitude, indicating that the reconnection X-point of this flare is located above the loop source. However, the apparent coronal source is the top of another independent large-scale loop.     

Signatures of magnetic reconnection in solar radio observations?

Magnetic reconnection occurs during eruptive processes (flares, CMEs) in the solar corona. This leads to a change of magnetic connectivity. Nonthermal electrons propagate along the coronal magnetic field thereby exciting dm- and m-wave radio burst emission after acceleration during reconnection or other energy release processes in heights of some Mm to ⩾700 Mm. We summarize the results of some case studies which can be interpreted as radio evidence of magnetic reconnection: under certain conditions, simple spectral structures (pulsation pulses, reverse drift bursts) are formed by simultaneously acting but widely spaced radio sources. Narrowband spikes are emitted as a side-effect during large-scale coronal loop collisions. In dynamic radio spectra, the lower fast mode shock formed in the reconnection outflow appears as type II burst-like but nondrifting emission lane. It has been several times observed at the harmonic mode of the local plasma frequency between 250 and 500 MHz and at heights of ≈200 Mm.     

The evolution of a complex solar radio burst corresponding to special configuration of microwave sources

A complex radio burst associated with periodic (∼1 and 6 min) pulsations and several kinds fine structures, e.g., normal- and reverse-drifting type III bursts, zebra patterns, and slowly drifting structure was observed with the radio spectrometers (1.0–2.0, 2.6–3.8, 5.2–7.6, and 0.65–1.5 GHz) at the National Astronomical Observatories of China (NAOC) in Beijing and Yunnan on 19 October 2001. In combination with the images of 17 and 34 GHz from NoRH and the magnetograms from MDI we reveal the existence and evolution of preexisting and new emerging sources, and find the horseshoe-shaped structure of microwave sources intensity during the late phase of the burst. Through the detailed comparison of the evolution of each source with the time profiles of radio bursts corresponding to these sources we indicate that the intimate correlation between the microwave sources evolution and the generation of the radio burst associated fine structures. Some fine structures can be considered as the MHD turbulence and plasma emission mechanism, based on the anisotropic beam instability and hybrid waves generations. From the characteristics of observations we may presume that the coronal magnetic structures should contain an extended coronal loop system and multiple discrete electrons acceleration/injection sites. The mechanisms of this complex radio burst are deal with the incoherent gyrosynchrotron emission from the trapped electrons and the coherent plasma emission from the non trapped electrons.     

The superbubble origin for galactic cosmic rays

The source of energy for cosmic-ray acceleration appears to be shock waves driven by supernova (SNe) ejecta. The great majority (80–90%) of SNe (SNII and SNIb) are formed by the core collapse of young, massive O and B stars. However, it has been known for more than forty years that the births of such massive stars in stellar clumps, termed OB associations, are correlated in space and time. The combined ejecta of core-collapse SNe, occurring at the deaths of these massive stars, create low-density (∼5.0 × 10⁻³ cm⁻³) superbubbles that reach dimensions of several hundred pc. The occurrence of correlated SNe in superbubbles affects not just the source of cosmic-ray energy, SNe shock waves, it impinges as well on the elemental and isotopic source abundances of cosmic-ray nuclei. We argue that the well-known anomalous cosmic-ray ²²Ne/²⁰Ne ratio, a factor of five times the Solar System ratio, results from a mixing of freshly synthesized nucleosynthetic material in supernova active cores of superbubbles. Although diluted by mixing with older, lower metallicity interstellar gas, the mean metallicities in the superbubble, SNe-dominated cores are high ∼3 times the Solar System value.     

Supernova remnant 1987A: The latest report from the Chandra X-ray Observatory

We continue monitoring supernova remnant (SNR) 1987A with the Chandra X-ray Observatory. As of 2004 January, bright X-ray spots in the northwest and the southwest are now evident in addition to the bright eastern ring. The overall X-ray spectrum, since 2002 December, can be described by a planar shock with an electron temperature of ∼2.1 keV. The soft X-ray flux is now 8 × 10⁻¹³ ergs cm⁻² s⁻¹, which is about five times higher than four years ago. This flux increase rate is consistent with our prediction based on an exponential density distribution along the radius of the SNR between the HII region and the inner ring. We still have no direct evidence of a central point source, and place an upper limit of L_X = 1.3 × 10³⁴ ergs s⁻¹ on the 3–10 keV band X-ray luminosity.     

Relationship of solar flare accelerated particles to solar energetic particles (SEPs) observed in the interplanetary medium

Observations of hard X-ray (HXR)/γ-ray continuum and γ-ray lines produced by energetic electrons and ions, respectively, colliding with the solar atmosphere, have shown that large solar flares can accelerate ions up to many GeV and electrons up to hundreds of MeV. Solar energetic particles (SEPs) are observed by spacecraft near 1 AU and by ground-based instrumentation to extend up to similar energies as in large SEP events, but it appears that a different acceleration process, one associated with fast coronal mass ejections is responsible. Much weaker SEP events are observed that are generally rich in electrons, ³He, and heavy elements. The energetic particles in these events appear to be similar to those accelerated in flares. The Ramaty high energy solar spectroscopic imager (RHESSI) mission provides high-resolution spectroscopy and imaging of flare HXRs and γ-rays. Such observations can provide information on the location, energy spectra, and composition of the flare accelerated energetic particles at the Sun. Here, preliminary comparisons of the RHESSI observations with observations of both energetic electron and ion near 1 AU are reviewed, and the implications for the particle acceleration and escape processes are discussed.     

The hard X-ray emission from the Vela pulsar wind nebula

We present the results of a preliminary spectral analysis performed on the BeppoSAX and XMM observations of the Vela plerion. The broad energy range covered by the instruments on board the two observatories allows an evaluation of the spectral parameters of the high energy emission model and provides an indication on the morphology of the source emission above 10 keV. We confirm the softening of the PWN spectrum (3–10 keV band) at distances greater than 4′ from the pulsar and estimate the diameter of the high energy (>10 keV) emission region to be on the order of 25′–30′.     

The plerionic supernova remnant G21.5-0.9: In and out

The absence of a supernova remnant (SNR) shell surrounding the Crab and other plerions (pulsar wind nebulae) has been a mystery for three decades. G21.5-0.9 is a particularly intriguing plerionic SNR in which the central powering engine is not yet detected. Early CHANDRA observations revealed a faint extended X-ray halo which was suggested to be associated with the SNR shell; however its spectrum was non-thermal, unlike what is expected from an SNR shell. On the other hand, a plerionic origin to the halo is problematic since the X-ray plerion would be larger than the radio plerion. We present here our analysis of an integrated 245 ks of archival CHANDRA data acquired with the High-Resolution Camera (HRC) and 520 ks acquired with the Advanced CCD Imaging Spectrometer (ACIS). This study provides the deepest and highest resolution images obtained to date. The resulting images reveal for the first time: (1) a limb-brightened morphology in the eastern section of the halo, and (2) a rich structure in the inner (40″-radius) bright plerion including wisps and a double-lobed morphology with an axis of symmetry running in the northwest–southeast direction. Our spatially resolved spectroscopic study of the ACIS-I data indicates that the photon index steepens with increasing distance from the central point source out to a radius of 40″ then becomes constant at ∼2.4 in the X-ray halo (for a column density N_H = 2.2 × 10²² cm⁻²). No line emission was found from the eastern limb; however marginal evidence for line emission in the halo’s northern knots was found. This study illustrates the need for deep CHANDRA observations to reveal the missing SNR material in Crab-like plerions.     

Numerical modeling of the pulsar wind interaction with ISM

Time dependent numerical simulation of relativistic wind interaction with interstellar medium was performed. The winds are ejected from magnetosphere of rotation powered pulsars. The particle flux in the winds is assumed to be isotropic. The energy flux is taken as strongly anisotropic in accordance with prediction of the MHD theory of the relativistic winds. The modeling has been performed for the wind magnetization in the range 3 × 10⁻³–10⁻¹. The numerical solutions reproduce the most spectacular features observed in the central part of plerions: toroidal structure and jet-like features. Increase of the wind’s magnetization results in decrease of the size of the synchrotron nebula.     

High energy emission from pulsars: Outer gap scenario

I will give a brief review of the recent development in the emission models of isolated, rapidly rotating neutron stars, focusing on the γ-ray radiation mechanism in their outer magnetospheres. By examining the Poisson equation for the electrostatic potential, I show that an active particle accelerator must extend from the vicinity of the neutron star surface to the vicinity of light cylinder. Furthermore, combining the Poisson equation with the Boltzmann equations for electrons/positrons and γ-rays, and assuming that the gap trans-field thickness is large compared to the longitudinal width, I demonstrate that the energy distribution of ultra-relativistic particles cannot be described by a power-law but by a quasi-monoenergetic distribution at the terminal Lorentz factor. The particles are accelerated in the gap and escape from it with large Lorentz factors. Is is shown that such energetic particles migrating outside of the gap contribute significantly to the γ-ray luminosity and reproduce the observed soft γ-ray spectrum between 100 MeV and 3 GeV for the Vela pulsar.     

A search for radio emission from the young 16-ms X-ray pulsar PSR J0537−6910

PSR J0537−6910 is a young, energetic, rotation-powered X-ray pulsar with a spin period of 16 ms located in the Large Magellanic Cloud. We have searched for previously undetected radio pulsations (both giant and standard) from this pulsar in a 12-h observation taken at 1400 MHz with the Parkes 64-m radio telescope. The very large value of the magnetic field at the light cylinder radius suggests that this pulsar might be emitting giant radio pulses like those seen in other pulsars with similar field strengths. No radio emission of either kind was detected from the pulsar, and we have established an upper limit of ∼25 mJy kpc² for the average 1400-MHz radio luminosity of PSR J0537−6910. The 5σ single-pulse detection threshold was ∼750 mJy for a single 80-μs sample. These limits are likely to be the best obtainable until searches with greatly improved sensitivity can be made with next-generation radio instruments.     

Hard X-ray component in Sco X-1 spectrum: Synchrotron emission from a nano quasar

Sco X-1 is a low mass X-ray binary system and with the recent observations of a resolved radio jet, the source has been included in the list of galactic microquasars. The observed spectral data in the 2–20 keV energy band fits a thermal emission. Above 20 keV, a hard tail has been reported on occasions. During our continuing balloon borne X-ray survey in the 20–200 keV region using high sensitivity Large Area Scintillation counter Experiment, Sco X-1 was observed on two different occasions. Even though the total X-ray luminosity of the source was different, the spectral nature of the source did not show any variation. The presence of hard X-ray flux is unmistakable. We present the spectral and temporal data in the hard X-ray band and discuss the results in terms of geometrical characteristics of X-ray source and its observed temporal properties. We note that the jet activity is similar to the microquasars, however, the absence of the large magnitude abrupt changes in X-ray light curve compared to GRS1915 + 105 suggest that the quasar-like behaviour is at a nano scale.     

Study of >100 GeV electrons with BETS detector using a long duration balloon flight in Antarctica

We have observed cosmic-ray electrons from 10 to 1000 GeV by a long duration balloon flight using Polar Patrol Balloon (PPB) in Antarctica. The observation was carried out for 13 days at an altitude of 35 km in January 2004. The detector is an imaging calorimeter composed of scintillating-fiber belts and plastic scintillators inserted between lead plates. The geometrical factor of detector is about 600 cm²sr and the total thickness of lead absorber is 9 radiation lengths. The performance of the detector has been confirmed by the CERN-SPS beam test and also investigated by Monte-Carlo simulations. New telemetry system using a commercial satellite of iridium, power supply by solar batteries, and automatic level control using CPU have successfully been developed and operated during the flight. We have collected 5.7 × 10³ events over 100 GeV including nearly 100 candidates of primary electrons.     

Heliospheric implications of structure in the interstellar medium

Equilibrium models of diffuse interstellar material (ISM) near the Sun show a range of cloud densities, ionization, and temperatures which are consistent with data, although the local ISM must be inhomogeneous over ∼2 pc scales. The ISM close to the Sun has properties that are consistent with the sheetlike warm neutral (and partially ionized) gas detected in the Arecibo Millennium Survey. Local interstellar magnetic fields are poorly understood, but data showing weak polarization for nearby stars indicate dust may be trapped in fields or currents in the heliosheath nose region. Implications of this dust capture are widespread, and may impact the interpretation of the cosmic microwave background data. Observations of interstellar H⁰ inside of the solar system between 1975 and 2000 do not suggest any variation in the properties or structure of local interstellar H⁰ over distance scales of ∼750 AU to within the uncertainties.