Gamma-ray observations of explosive nucleosynthesis products

In this review I discuss the various γ-ray emission lines that can be expected and, in some cases have been observed, from radioactive explosive nucleosynthesis products. The most important γ-ray lines result from the decay chains of ⁵⁶Ni, ⁵⁷Ni, and ⁴⁴Ti. ⁵⁶Ni is the prime explosive nucleosynthesis product of Type Ia supernovae, and its decay determines to a large extent the Type Ia light curves. ⁵⁶Ni is also a product of core-collapse supernovae, and in fact, γ-ray line emission from its daughter product, ⁵⁶Co, has been detected from SN1987A by several instruments. The early occurrence of this emission was surprising and indicates that some fraction of ⁵⁶Ni, which is synthesized in the innermost supernova layers, must have mixed with the outermost supernova ejecta.Special attention is given to the γ-ray line emission of the decay chain of ⁴⁴Ti (⁴⁴Ti → ⁴⁴Sc → ⁴⁴Ca), which is accompanied by line emission at 68, 78, and 1157 keV. As the decay time of ⁴⁴Ti is ∼86 yr, one expects this line emission from young supernova remnants. Although the ⁴⁴Ti yield (typically 10⁻⁵–10⁻⁴M_⊙) is not very high, its production is very sensitive to the energetics and asymmetries of the supernova explosion, and to the mass cut, which defines the mass of the stellar remnant. This makes ⁴⁴Ti an ideal tool to study the inner layers of the supernova explosion. This is of particular interest in light of observational evidence for asymmetric supernova explosions.The γ-ray line emission from ⁴⁴Ti has so far only been detected from the supernova remnant Cas A. I discuss these detections, which were made by COMPTEL (the 1157 keV line) and BeppoSAX (the 68 and 78 keV lines), which, combined, give a flux of (2.6 ± 0.4 ± 0.5) × 10⁻⁵ ph cm⁻² s⁻¹ per line, suggesting a ⁴⁴Ti yield of (1.5 ± 1.0) × 10⁻⁴M_⊙. Moreover, I present some preliminary results of Cas A observations by INTEGRAL, which so far has yielded a 3σ detection of the 68 keV line with the ISGRI instrument with a flux that is consistent with the BeppoSAX detections. Future observations by INTEGRAL-ISGRI will be able to constrain the continuum flux above 90 keV, as the uncertainty about the continuum shape, is the main source of systematic error for the 68 and 78 keV line flux measurements. Moreover, with the INTEGRAL-SPI instrument it will be possible to measure or constrain the line broadening of the 1157 keV line. A preliminary analysis of the available data indicates that narrow line emission (i.e., Δv < 1000 km s⁻¹) can be almost excluded at the 2σ level, for an assumed line flux of 1.9 × 10⁻⁵ ph cm⁻² s⁻¹.     

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.     

Hard X-ray footpoint motions in solar flares: Comparing magnetic reconnection models with observations

Hard X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) of the October 29, 2003 GOES X10 two-ribbon flare are used together with magnetic field observations from the Michelson Doppler Imager (MDI) onboard SoHO to compare footpoint motions with predictions from magnetic reconnection models. The temporal variations of the velocity v of the hard X-ray footpoint motions and the photospheric magnetic field strength B in footpoints are investigated. The underlying photospheric magnetic field strength is generally higher (B ∼ 700–1200 G) in the slower moving (v ∼ 20–50 km s⁻¹) western footpoint than in the faster (v ∼ 20–100 km s⁻¹) moving eastern source (∼100–600 G). Furthermore, a rough temporal correlation between the HXR flux and the product vB² is observed.     

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.     

Detection of thermal X-ray emission in the halo of the plerionic supernova remnant G21.5-0.9

The detection of a soft thermal X-ray component in the spectrum of a bright knot in the halo of the plerion G21.5-0.9 is reported. Using a collisional ionization equilibrium model for an hot optically thin plasma, a temperature of kT 0.12–0.24 keV, a mass of 0.3–1.0 M_⊙ and a density of 1.6–6 cm⁻³ is derived. The spectral analysis suggests a possible overabundance of Silicon with respect to the solar value in the knot; if this will be confirmed this object may be a clump of shocked ejecta.     

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.     

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.     

Features of the 11-year variation of galactic cosmic rays in different periods of solar magnetic cycles

Variations of galactic cosmic ray intensity have been studied based on the neutron monitors and interplanetary magnetic field experimental data for different ascending and descending epochs of solar activity. The dependence of the diffusion coefficient on the cosmic ray particles rigidity R is stronger in the maxima epoch than in the minima epoch of solar activity. For the period of 1977–1981 (qA > 0) the diffusion coefficient for the minimum epoch is, χ_min ∝ R^{0.7 ± 0.04} and for the maximum χ_max ∝ R^{1.3 ± 0.05}; for the period of 1987–1990 (qA < 0), χ_min ∝ R^{0.8 ± 0.05} and χ_max ∝ R^{1.1 ± 0.04}. The exponents ν_y and ν_z of the power spectral density of the B_y and B_z components of the IMF in the region of the frequencies (10⁻⁶– 4 × 10⁻⁶) Hz are larger for the minimum epoch of 1987 (ν_y ≈ 2.0 and ν_z ≈ 1.93) than for the maximum epoch of 1990 (ν_y ≈ 1.43 and ν_z ≈ 1.27).     

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.     

On the quality of mutations in mammalian cells induced by high LET radiations

The deleterious effects of accelerated heavy ions as component of the space radiation environment on living cells are of increasing importance for long duration human space flight activities. The most important aspect of such densely ionizing particle radiation is attributed to the type and quality of biological damage induced by them. This issue is addressed by investigating cell inactivation and mutation induction at the Hprt locus (coding for hypoxanthine-guanine-phosphoribosyl-transferase) of cultured V79 Chinese hamster cells exposed to densely ionizing radiation (accelerated heavy ions with different LETs from oxygen to gold, specific energies ranging from 1.9 to 69.7 MeV/u, corresponding LET values range from 62 to 13,223 keV/μm) and to sparsely ionizing radiation (200 kV X-rays). 30 spontaneous, 40 X-ray induced and 196 heavy ion induced 6-thioguanine resistant Hprt mutant colonies were characterized by Southern technique using the restriction enzymes EcoRI, PstI and BglII and a full length Hprt cDNA probe isolated from the plasmid pHPT12. Restriction patterns of the spontaneous Hprt mutants were indistinguishable from the wild type pattern, as these mutants probably contain only small deletions or even point mutations in the Hprt locus. In contrast, the overall spectrum of heavy ion induced mutations revealed a majority of partial or total deletions of the Hprt gene. With constant particle fluence (3 × 10⁶ particles/cm²) the quality of heavy ion induced mutations in the Hprt locus depends on physical parameters of the beam (atomic number, specific energy, LET). This finding suggests a relationship between the type of DNA damage and track structure. The fraction of mutants with severe deletions in the Hprt locus after exposure to oxygen ions increases from 65% at 60 keV/μm up to a maximum (100%) at 300 keV/μm and declines with higher LET values to 75% at 750 keV/μm. With heavier ions (Ca- and Au-ions) and even higher LET-values this mutant fraction decreases to 58% at 13,200 keV/μm. Heavy ion induced DNA break points in the Hprt locus are not randomly distributed.     

A Gemini observation of the anomalous X-ray pulsar 1RXS J170849-400910

The anomalous X-ray pulsars (AXPs) represent a growing class of neutron stars discovered at X-ray energies. While the nature of their multi-wavelength emission mechanism is still under debate, evidence has been recently accumulating in favor of their magnetar nature. Their study in the optical and infrared (IR) wavelengths has recently opened a new window to constrain the proposed models. We here present a brief overview of AXPs and our Gemini-South observation of 1RXS J170849-400910, which is a relatively bright AXP discovered with ROSAT and later found to be an 11 s X-ray pulsar by ASCA. The observation was taken with the near-IR imager Flamingos in J (1.25 μm), H (1.65 μm), and K_s (2.15 μm). We confirm the recent detection by (ApJ, 589, L93–L96) of a source coincident with the CHANDRA source (candidate ‘A’). Our derived magnitudes of J = 20.6 (0.2), H = 18.6 (0.2), and K_s = 17.1 (0.2) are consistent with those derived by (ApJ, 589, L93–L96), and indicate that if this source is indeed the IR counterpart to 1RXS J170849-400910, then there is no evidence of variability from this AXP. However, given the lack of IR variability and the relatively high IR to X-ray flux of this source when compared to the other AXPs, we conclude that this source is unlikely the counterpart of the AXP, and that the other source (candidate ‘B’) within the CHANDRA error circle should not be ruled out as the counterpart. Further monitoring of these sources and a deep observation of this complex field are needed to confirm the nature of these sources and their association with the AXP.     

Scaling and universality of the thermal conductivity of liquid 4He near the superfluid transition and in restricted geometries

We present measurements of the thermal conductivity λ(t, P, L) = l/ρ(t, P, L) near the superfluid transition of ⁴He at saturated vapor pressure and confined in cylindrical geometries with radii L = 0.5 and 1.0 μm (t ≡ T/T_λ(P) − 1). For L = 1.0 μm measurements at six pressures P are presented. At and above T_λ the data are consistent with a universal scaling function F(X) = (L/ξ_o)^x/ν(ρ/ρ₀), X = (L/ξ_o)^1/νt valid for all P (ρ₀ and x are the pressure-dependent amplitude and effective exponent of the bulk resistivity ρ(t, P, ∞) = ρ₀t^x and ξ = ξ₀t^−ν is the correlation length). Indications of breakdown of scaling and universality are observed below T_λ.     

Detection of meteors and sub-relativistic dust grains by the fluorescence detectors of ultra high energy cosmic rays

Fluorescence detectors of ultra high energy cosmic rays (UHECR) allow to record not only the extensive air showers, initiated by the UHECR particles, but also to detect light, produced by meteors and by the fast dust grains. It is shown that the fluorescence detector operated at the mountain site can register signals from meteors with kinetic energy threshold of about 25 J (meteor mass ∼ 5 × 10⁻⁶ g, velocity ∼ 3 × 10⁶ cm/s). The same detector might be used for recording of the dust grains of smaller mass (of about 10⁻¹⁰ g) but with velocity 10⁹ cm/s, close to the light velocity (sub-relativistic dust grains). The light signal from a sub-relativistic dust grain is expected in much shorter time scale (∼0.001 s), in comparison with the meteor signal (∼0.1–1 s), and much longer than duration of the UHECR signals (tens of μs). The fluorescence detector capable to register various phenomena: from meteors to UHECR – should have a variable pixel and selecting system integration time. A study of the new phenomenon of sub-relativistic grains will help to understand the mechanism of particle and dust grain acceleration in astrophysical objects (in SN explosions, for example).     

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.     

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.     

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.     

OPTIS – A Satellite test of Special and General Relativity

OPTIS has been proposed as a small satellite platform in a high elliptical orbit (apogee 40,000 km, perigee 10,000 km) and is designed for high precision tests of foundations of Special and General Relativity. The experimental set-up consists of two ultrastable Nd:YAG lasers, three crossed optical resonators (monolithic cavities), an ensemble of atomic clocks, an optical comb generator, laser tracking devices and a drag-free control system. OPTIS enables improved tests of (1) the isotropy and (2) constancy of the speed of light, (3) special relativistic time dilation, (4) the universality of the gravitational redshift by comparison of various clocks, can measure (5) the absolute value of the gravitational redshift, (6) the Lense–Thirring effect and (7) the perigee advance and (8) can make a test of a hypothetical Yukawa part in the gravitational potential. To avoid any influence from atmospheric drag, solar radiation, or Earth albedo, the satellite needs drag-free control to depress the residual acceleration down to 10⁻¹⁴ m/s² in the frequency range between 10⁻² and 10⁻³ Hz. Precise thermal control must be used to stabilize the cavity temperature to within one part in 10⁷ at time scales of 100 s and to one part in 10⁵ on the orbit time scale.     

Excitations of nonmigrating diurnal tides in the mesosphere and lower thermosphere simulated by the Kyushu-GCM

Excitation mechanisms of nonmigrating diurnal tides in the MLT region simulated by the Kyushu-GCM are examined. It is shown that the westward propagating diurnal tide with zonal wavenumber s = 2 is mainly excited by nonlinear interactions between the migrating diurnal tide and the stationary planetary wave with zonal wavenumber s = 1, while the nonlinear excitation of the standing diurnal tide with zonal wavenumber s = 0 is less important than the excitation by tropospheric heating. Nonlinear interactions between the migrating diurnal tide and the stationary planetary wave with zonal wavenumber s = 2 are not dominant to excite the westward propagating diurnal tide with zonal wavenumber s = 3, and it is shown that the excitation by tropospheric heating is comparable to the nonlinear excitation. It is also shown that other nonmigrating diurnal tides are excited by tropospheric heating.     

Propagation of secondary antiprotons and cosmic rays in the Galaxy

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratios (e.g., B/C) produce too few antiprotons. In the present paper, we discuss one possibility to overcome these difficulties. Using the measured antiproton flux and B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local “unprocessed” component at low energies perhaps associated with the Local Bubble, thus decreasing the measured secondary to primary nuclei ratio. The independent evidence for SN activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni (Z ⩽ 28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three “radioactive clock” isotopes in CR, ¹⁰Be, ²⁶Al, ³⁶Cl, are all consistent and indicate a halo size z_h ∼ 4 kpc based on the most accurate data taken by the ACE spacecraft.     

Impacts on Hubble Space Telescope solar arrays: Discrimination between natural and man-made particles

A Post-Flight Investigation was initiated by the European Space Agency to analyze impacts on solar arrays of the Hubble Space Telescope (HST), exposed to space for 8.25 years at approximately 600 km altitude. The solar cells deployed during the first Service Mission (SM-1 in December 1993) were retrieved in March 2002 as part of Service Mission 3B (SM-3B). A sub-panel of 2 m² was cut from the arrays for subsequent selection and removal of individual solar cells for analysis. Six cells (4.8 × 10⁻³ m²) were surveyed for flux of all craters of sizes greater than 5 microns. Analytical scanning electron microscopy was used to analyse residues in 111 features of 3–4000 micron conchoidal detachment diameter (D_co), examined on 23 solar cells. Eighty three show identifiable residue: 38 are Space Debris impacts and 45 Micrometeoroid impacts. Of the remaining 28, 2 contain residue of ambiguous origin, 1 is probably a minor manufacturing flaw, 1 is obscured by contamination, and 24 are unresolved, lacking recognizable residue.The majority of space debris impacts on the SM-3B cells are less than 80 microns D_co, dominated by Al-rich residue, probably of solid rocket motor origin, although three may be due to sodium metal droplet impacts. Three larger features include paint pigment and binder, ferrous alloy, and possible carbon-fibre composite material debris.Micrometeoroid residues are found across the entire crater size range and dominate features of between 100 and 1000 microns, their residues are similar to those found in earlier SM-1 surveys. Fe- and Mg-rich silicates dominate; Fe sulphides are common and there are occasional vesicular Ni- and S-bearing mafic silicates of hydrous phyllosilicate origin. A single sodium aluminosilicate residue and one Fe Ni metal residue were found; as well as enigmatic Mg- and S-bearing residues, all considered as probably of micrometeoroid origin. A few Fe-, O- and C-bearing residues were classified as of ambiguous origin.