Ionized gas towards galactic centre — Constraints from low-frequency recombination lines

Observations of the H272α recombination line towards the galactic centre show features near V_LSR= 0, −50 and + 36 kms⁻¹. We have combined the parameters of these features with the available H166α measurements to obtain the properties of the ionized gas present along the line of sight and also in the ‘3 kpc arm’. For the line-of-sight ionized gas we get an electron density around 7 cm⁻³ and a pathlength through it ∼ 10–60 pc. The emission measure and the electron temperature are in the range 500–2900 pc cm⁻⁶ and 2000–6000 K. respectively. The ionized gas in the 3 kpc arm has an electron density of 30 cm⁻³ and extends over 9 pc along the line of sight if we assume an electron temperature of 10⁴ K. Using the available upper limit to the intensity of the H351α recombination line, we show that the distributed ionized gas responsible for the dispersion of pulsar signals should have a temperature >4500 K. and a minimum filling factor of 20 per cent. We also show that recombination lines from the ‘warm ionized’ gas proposed by McKee & Ostriker (1977) should be detectable in the frequency range 100–150 MHz towards the galactic centre with the sensitivity available at present.     

A Broadband Solar Radio Spectrometer and Some New Observational Results

A broadband solar radio spectrometer with a bandwidth of about 7 GHz has been developed in China for solar maximum 23. This work is a cooperative project of Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Observatory (YNO), and Nanjing University. The spectrometer of PMO worked in the waveband of 4.5–7.5 GHz, that of BAO in 1–2 GHz, 2.6–3.8 GHz, and 5.2–7.6 GHz, and that of YNO in 0.7–1.5 GHz. The spectrometer of PMO is a multichannel and frequency-agile one with a time resolution of 1–5 ms and a frequency resolution of 10 MHz. It started to operate in August 1999 and since then more than 300 spectral events have been observed, and some type III or type III-like structures have also been found. In this paper, some selected typical events, for example, the events on 25 August 1999 and 27 October 1999, are presented, and some new observed features are also described and discussed.     

Observations of Powerful Type III Bursts in the Frequency Range 10 – 30 MHz

The properties of powerful (flux >10⁻¹⁹ W m⁻² Hz⁻¹) type III bursts observed in July – August 2002 by the radio telescope UTR-2 at frequencies 10 – 30 MHz are analyzed. Most bursts have been registered when the active regions associated to these bursts were located near the central meridian or at 40° – 60° to the East or West from it. All powerful type III bursts drift from high to low frequencies with frequency drift rates 1 – 2.5 MHz s⁻¹. It is important to emphasize that according to our observations the drift rate is linearly increasing with frequency. The duration of the bursts changes mainly from 6 s at frequency 30 MHz up to 12 s at 10 MHz. The instantaneous frequency bandwidth does not depend on the day of observations, i.e. on the disk location of the source active region, and is increasing with frequency.     

Solar Drift Pair Bursts in the Decameter Range

The results of observations of solar decametric drift pair bursts are presented. These observations were carried out during a Type III burst storm on July 11–21, 2002, with the decameter radio telescope UTR-2, equipped with new back-end facilities. High time and frequency resolution of the back-end allowed us to obtain new information about the structure and properties of these bursts. The statistical analysis of more than 700 bursts observed on 13–15 July was performed separately for “forward” and “reverse” drift pair bursts. Such an extensive amount of these kind of bursts has never been processed before. It should be pointed out that “forward” and “reverse” drift pair bursts have a set of similar parameters, such as time delay between the burst elements, duration of an element, and instant bandwidth of an element. Nevertheless some of their parameters are different. So, the absolute average value of frequency drift rate for “forward” bursts is 0.8 MHz s⁻¹, while for “reverse” ones it is 2 MHz s⁻¹. The obtained functional dependencies “drift rate vs. frequency” and “flux density vs. frequency” were found to be different from the current knowledge. We also report about the observation of unusual variants of drift pairs, in particular, of “hook” bursts and bursts with fine time and frequency structure. A possible mechanism of drift pairs generation is proposed, according to which this emission may originate from the interaction of Langmuir waves with the magnetosonic waves having equal phase and group velocities.     

Time – Frequency Analysis of GALLEX and GNO Solar Neutrino Data

Time – frequency analysis of data from the GALLEX and GNO solar neutrino experiments shows that some features in power-spectrum analyses of those datasets are due to aliasing (a result of the fact that run durations tend to be small multiples of one week). Displays formed from the published GALLEX data show a sharp discontinuity that we attribute to some systematic effect. We therefore normalize data for each of the four experiments in the GALLEX series and concatenate the resulting normalized data. This step effectively removes the presumed systematic effect. To help understand the effect of aliasing, we form time – frequency displays of the two principal modulations found in the data, at 11.87 year⁻¹ and at 13.63 year⁻¹. We also form time – frequency displays of datasets formed by subtracting these modulations from the actual (normalized) data. The results suggest that the true principal modulation is that at 11.87 year⁻¹. Comparison with helioseismology data suggests that modulation may be occurring in the core, perhaps resulting from inhomogeneities and fluctuations in the nuclear-burning process, and that the sidereal rotation rate of the core is 12.87 year⁻¹, or 408 nHz.     

The radio source Z0254+43: z = 4.067

In January 2005 spectral observations of the radio source Z0254+43 were made on the BTA at the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences (RAN) and its red shift was found to be z=4.057. The BVRI magnitudes were found to be 22.68, 21.19, 19.94, and 19.23, respectively. Photometric observations in December 2005 on the Zeiss-1000 at the SAO revealed no significant variation in the optical emission from this object over that year. We can discuss its variability on an hourly time scale with some caution. The variability of the flux from Z0254+43 was observed from 1990–2005 on the RATAN-600 over a wide range of frequencies. It turns out that the amplitude of the variability is minimal at a frequency of ∼8 GHz. A model for the variability has been constructed which yields an estimate of ∼28° for the orientation of the jet of Z0254+43 to the line of sight. The luminosity of Z0254+43 in the optical range is ∼2·10²⁶ W/Hz and in the radio frequency range, ∼2·1027 W/Hz. __________ Translated from Astrofizika, Vol. 49, No. 2, pp. 209–220 (May 2006).     

Positively Drifting Structures During the 18 March 2003 Solar Flare

We analyze the high-frequency drift radio structures observed by the spectrometer at Purple Mountain Observatory (PMO) over the frequency range of 4.5 – 7.5 GHz during the 18 March 2003 solar flare. The drifting structures take place before the soft X-ray maximum, almost at the maximum of hard X-ray flux at 25 – 50 keV. For the first time, the positive drift in this kind of radio structures is detected in such a high frequency range. Their global drifting rate is roughly estimated as 3.6 GHz s⁻¹. They appear in four groups, lasting in total for less than 6 s, and have a broad bandwidth of more than 2 GHz but a smaller ratio of the bandwidth of the drifting structures to mean frequency than that of the lower frequency range. The lifetime of each individual burst in this event can be derived by using the high temporal resolution of the spectrometer at PMO and has an average value of 36.3 ms. Since the negative drifting structures observed in the 0.6 – 4.5 GHz frequency range were interpreted to be a radio signature of a plasmoid ejected upward (moving out of the Sun), the present observation may imply that it is possible for a plasmoid to move downward during a solar flare. However, for a confirmation of this suggestion direct radio imaging observation would be needed.     

DEVELOPMENT OF AN ADVANCED DUST TELESCOPE

There are different types of dust particles in interplanetary space, such as dust from comets and asteroids, and interstellar grains traversing the solar system. Based on experience with current space dust instruments, a novel dust telescope is being developed. A dust telescope is a combination of a dust trajectory sensor for the identification and an analyzer for the elemental composition of the dust. Dust particles’ trajectories are determined by the measurement of the electric signals that are induced when a charged grain flies through a position-sensitive electrode system. The objective of the trajectory sensor is to measure dust charges in the range 10⁻¹⁶–10⁻¹³ C and dust speeds in the range 6–100 km/s. First tests with a laboratory setup have been performed. The chemical analyzer will have an impact area of 0.1 m². It consists of a target with an acceleration grid and a single-stage reflectron for energy focusing, and a central ion detector. Results from SIMION simulations show that a mass resolution of M/ΔM>150 can be obtained.     

Temperature of Polar Corona of the Sun According to Kislovodsk Observations During 1957–2002

We continued a study of the long-term variations of temperature in the solar corona at all latitudes (Makarov, Tlatov, and Callebaut, 2002a). The series of the green (Fe xiv 530.3 nm; KI₅₃₀₃) and red (Fe x 637.4 nm; KI₆₃₇₄) coronal intensities for 1957–2002 has been obtained using the coronal observations at the Kislovodsk Solar Station. The mean monthly coronal intensities have been calculated at all latitudes (0–90^˚) and in the high latitude (45–90^˚) zones. It was found that the value of KI₆₃₇₄/KI₅₃₀₃increased about 2.0 times at the high latitudes during the last 45 years. This corresponds to a decrease of the average temperature by 0.1 ×10⁶K of the polar corona. We suppose that a polar decrease of coronal temperature is connected with an increase of the area of polar zones A _PZoccupied by unipolar magnetic fields (Makarov et al., 2002) and, probably, with an increase of the area of polar coronal holes. The maximum ratio KI₆₃₇₄/KI₅₃₀₃is observed during the minimum sunspot activity.     

On the envelope soliton below the H + gyrofrequency in low auroral region

Two envelope soliton events below the H ⁺ gyrofrequency with localized density depletion were discovered in low auroral region (∼ 1760 km)by Freja satellite. These events were correlated in time with the observations of the ratio of oxygen ion density to hydrogen ion density sharp increase and the electrons energization. These envelope solitons have a characteristic frequency at ∼ 180–190 Hz, which are obviously different from the electron-ion lower hybrid wave frequency and the helium ion gyrofrequency in low auroral plasma, but it is close to the resonancefrequency of hydrogen ion-oxygen ion hybrid wave. A modulational instability model of an ion-ion hybrid wave has been discussed here. It is found that the envelope soliton below the H ⁺ gyrofrequency in low auroral region may be generated by this modulational instability on condition that the local oxygen ion density is larger than the local hydrogen ion density. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conversion Gain and Interpixel Capacitance of Cmos Hybrid Focal Plane Arrays

The conversion gain of optical and infrared focal plane CMOS hybrid arrays is a fundamental parameter, whose value computes into the derivation of other parameters characterizing the performance of a detector. The widespread “noise squared versus signal” method used to obtain the conversion gain can overestimate the nodal capacitance of the detector pixel by more than 20% for infrared arrays and by more than 100% for Si-PIN diode arrays. This is because this method does not take account of the capacitive coupling between neighboring pixels. A simple technique has been developed to measure the nodal capacitance directly by comparing the voltage change of an external calibrated capacitor with the voltage change on the nodal capacitor of the detector pixel. The method is elaborated in detail and has been verified with a Si-PIN diode array hybridized to a Hawaii-2RG multiplexer using an Fe ⁵⁵ X-ray source. It is also in good agreement with a stochastic method based on 2D autocorrelation.     

Broadband Radio Bursts and Fine Structures during the Great Solar Event on 14 July 2000

25 MHz–7.6 GHz global and detailed (fine structure – FS) radio spectra are presented, which were observed in the NOAA 9077 active region for the Bastille Day (14 July 2000) flare at 10:10–11:00 UT. Besides broadband radio bursts, high-resolution dynamic spectra reveal metric type II burst, decimetric type IV burst and various decimetric and microwave FSs, such as type III bursts, type U bursts, reverse-slope (RS)-drifting burst, fiber bursts, patch and drifting pulsation structure (DPS). The peak-flux-density spectrum of the radio bursts over the range 1.0–7.6 GHz globally appears as a U-shaped signature. Analyzing the features of backbone and herringbones of the type II burst, the speeds of shock and relevant energetic electron beams were estimated to be 1100 km s⁻¹ and 58 500 km s⁻¹, respectively. Also the time sequence of the radio emission is analyzed by comparing with the hard X-rays (HXRs) and the soft X-rays (SXRs) in this flare. After the maxima of the X-rays, the radio emission in the range 1.0–7.6 GHz reached maxima first at the higher frequency, then drifted to the lower frequency. This comparison suggested that the flare included three successive processes: firstly the X-rays rose and reached maxima at 10:10–10:23 UT, accompanied by fine structures only in the range 2.6–7.6 GHz; secondly the microwave radio emission reached maxima accompanied by many fine structures over the range 1.0–7.6 GHz at 10:23–10:34 UT; then a decimetric type IV burst and its associated FSs (fibers) in the range 1.0–2.0 GHz appeared after 10:40 UT.     

Electronic Multi-beam Radio Astronomy Concept: Embrace a Demonstrator for the European SKA Program

ASTRON has demonstrated the capabilities of a 4 m², dense phased array antenna (Bij de Vaate et al., 2002) for radio astronomy, as part of the Thousand Element Array project (ThEA). Although it proved the principle, a definitive answer related to the viability of the dense phased array approach for the SKA could not be given, due to the limited collecting area of the array considered. A larger demonstrator has therefore been defined, known as “Electronic Multi-Beam Radio Astronomy Concept”, EMBRACE, which will have an area of 625 m², operate in the band 0.4–1.550 GHz and have at least two independent and steerable beams. With this collecting area EMBRACE can function as a radio astronomy instrument whose sensitivity is comparable to that of a 25-m diameter dish. The collecting area also represents a significant percentage area (∼10%) of an individual SKA “station.” This paper presents the plans for the realisation of the EMBRACE demonstrator.     

Observations of magnetic flux compression in jet impact experiments

The astrophysical jet experiment at Caltech generates a T=2–5 eV, n=10²¹–10²² m⁻³ plasma jet using coplanar disk electrodes linked by a poloidal magnetic field. A 100 kA current generates a toroidal magnetic field; the toroidal field pressure inflates the poloidal flux surface, magnetically driving the jet. The jet travels at up to 50 km/s for ∼20–25 cm before colliding with a cloud of initially neutral gas. We study the interaction of the jet and the cloud in analogy to an astrophysical jet impacting a molecular cloud. Diagnostics include magnetic probe arrays, a 12-channel spectroscopic system and a fast camera with optical filters. When a hydrogen plasma jet collides with an argon target cloud, magnetic measurements show the magnetic flux compressing as the plasma jet deforms. As the plasma jet front slows and the plasma piles up, the density of the frozen-in magnetic flux increases.     

XEUS: the physics of the hot evolving universe

This paper describes the next generation X-ray observatory XEUS which has been submitted to the European Space Agency in the framework of the Cosmic Vision 2015–2025 competition and has been selected for an assessment study. The paper summarizes the scientific goals and instrumental concepts of the proposed X-ray telescope with 5 m² effective area and angular resolution better than 5 arc sec.     

Relationship between the Parameters of Coronal Holes and High-Speed Solar Wind Streams over an Activity Cycle

The comparison of the brightness and area of coronal holes (CH) to the solar wind speed, which was started by Obridko et al. (Solar Phys. 260, 191, 2009a) has been continued. While the previous work was dealing with a relatively short time interval 2000 – 2006, here we have analyzed the data on coronal holes observed in the Sun throughout activity Cycle 23. A catalog of equatorial coronal holes has been compiled, and their brightness and area variations during the cycle have been analyzed. It is shown that CH is not merely an undisturbed zone between the active regions. The corona heating mechanism in CH seems to be essentially the same as in the regions of higher activity. The reduced brightness is the result of a specific structure with the magnetic field being quasi-radial at as low an altitude as 1.1R _⊙ or a bit higher. The plasma outflow decreases the measure of emission from CH. With an adequate choice of the photometric boundaries, the CH area and brightness indices display a fairly high correlation (0.6 – 0.8) with the solar wind velocity throughout the cycle, except for two years, which deviate dramatically – 2001 and 2007, i.e., the maximum and the minimum of the cycle. The mean brightness of the darkest part of CH, where the field lines are nearly radial at low altitudes, is of the order of 18 – 20% of the solar brightness, while the brightness of the other parts of the CH is 30 – 40%. The solar wind streams originate at the base of the coronal hole, which acts as an ejecting nozzle. The solar wind parameters in CH are determined at the level where the field lines are radial.     

Low-Energy Cutoffs in Electron Spectra of Solar Flares: Statistical Survey

The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) X-ray data base (February 2002 – May 2006) has been searched to find solar flares with weak thermal components and flat photon spectra. Using a regularized inversion technique, we determine the mean electron flux distribution from count spectra for a selection of events with flat photon spectra in the 15 – 20 keV energy range. Such spectral behavior is expected for photon spectra either affected by photospheric albedo or produced by electron spectra with an absence of electrons in a given energy range (e.g., a low-energy cutoff in the mean electron spectra of nonthemal particles). We have found 18 cases that exhibit a statistically significant local minimum (a dip) in the range of 13 – 19 keV. The positions and spectral indices of events with low-energy cutoff indicate that such features are likely to be the result of photospheric albedo. It is shown that if the isotropic albedo correction is applied, all low-energy cutoffs in the mean electron spectrum are removed, and hence the low-energy cutoffs in the mean electron spectrum of solar flares above ∼ 12 keV cannot be viewed as real features. If low-energy cutoffs exist in the mean electron spectra, their energies should be less than ∼ 12 keV.     

Analysis of Ca+ K plage area for short period variation

We have analysed the observations of Solar Ca⁺K daily plage area for the period 1951-1977 to find evidence for the existence of short period (around 12–13 days) variation in the data. We divided the data in three groups—two corresponding to 10–20‡N and 10–20‡S latitude belts, and one corresponding to the total plage area—and used the power spectrum and autocorrelation techniques for the analysis. Both the techniques clearly show the 27-day periodicity due to solar rotation modulation in all the sets. A 12–13 day periodicity is seen in only 3, out of a total of 57 data sets when autocorrelation technique is used. A generally weak peak around 12–13 days is, however, seen in the power spectrum of all the data sets. The relative power in the 12–13 day peak is found to be significantly higher in those three data sets where the autocorrelation also shows this periodicity. On these two epochs the sunspot area distribution showed the existence of two distinct active longitudes separated by about 140–170 degrees. This seems to be the cause for the existence of a periodicity around 12–13 days in the autocorrelation and enhancement in the relative power of the 12–13 days peak in the power spectrum of these two epochs     

GRI: focusing on the evolving violent universe

The gamma-ray imager (GRI) is a novel mission concept that will provide an unprecedented sensitivity leap in the soft gamma-ray domain by using for the first time a focusing lens built of Laue diffracting crystals. The lens will cover an energy band from 200–1,300 keV with an effective area reaching 600 cm². It will be complemented by a single reflection multilayer coated mirror, extending the GRI energy band into the hard X-ray regime, down to ∼10 keV. The concentrated photons will be collected by a position sensitive pixelised CZT stack detector. We estimate continuum sensitivities of better than 10^− 7 ph cm^− 2s^− 1keV^− 1 for a 100 ks exposure; the narrow line sensitivity will be better than 3 × 10^− 6 ph cm^− 2s^− 1 for the same integration time. As focusing instrument, GRI will have an angular resolution of better than 30 arcsec within a field of view of roughly 5 arcmin—an unprecedented achievement in the gamma-ray domain. Owing to the large focal length of 100 m of the lens and the mirror, the optics and detector will be placed on two separate spacecrafts flying in formation in a high elliptical orbit. R&D work to enable the lens focusing technology and to develop the required focal plane detector is currently underway, financed by ASI, CNES, ESA, and the Spanish Ministery of Education and Science. The GRI mission has been proposed as class M mission for ESAs Cosmic Vision 2015–2025 program. GRI will allow studies of particle acceleration processes and explosion physics in unprecedented detail, providing essential clues on the innermost nature of the most violent and most energetic processes in the universe. All authors are on behalf of a large international collaboration The GRI mission has been proposed as an international collaboration between (in alphabetical order) Belgium (CSR), China (IHEP, Tsinghua Univ.), Denmark (DNSC, Southern Univ.), France (CESR, APC, ILL, CSNSM, IAP, LAM), Germany (MPE), Ireland (UCD School of Physics), Italy (INAF/IASF Rome, Bologna, Milano, Palermo; INAF/OA Brera, Roma; UNIFE, CNR/IMEM), Poland (NCAC), Portugal (Combra Univ., Evora Univ.), Russia (SINP, MSU, Ioffe Inst.), Spain (IEEC-CSIC-IFAE, CNM-IMB), the Netherlands (SRON, Utrecht Univ.), Turkey (Sabanci Univ.), United Kingdom (Univ. of Southampton, MSSL, RAL, Edinburgh Univ.), and the United States of America (SSL UC Berkeley, Argonne National Lab., MSFC, GSFC, US NRL).     

Searches for proton decay and superheavy Magnetic Monopoles

Conclusion In conclusion, as of 1984 January, it may be stated that the existence of super-heavy monopoles and the phenomenon of nucleon decay, both of which are extremely important from the point of view of grand unification theories, are still very open questions. While there has been just one magnetic monopole candidate so far, there have been several as far as nucleon decay is concerned. The first candidates for nucleon decay came from the fine-grain calorimeters of KGF, and NUSEX; recently there have been candidates from the water Cerenkov experiments as well. The experimental situation regarding the other important phenomena of relevance to grand unification which we have not discussed in this article — like the finite mass of neutrinos, neutrino oscillations, and neutron oscillations — continues to be indefinite though many dedicated experiments are in progress. With the continued operation of the nucleon decay experiments already collecting data and the commissioning of the new generation of experiments over the next few years, the stage is set for a resolution of this problem in a time scale of 5–10 years. The present indication that the dominant decay mode for the proton (even if it decays) is notp → e⁺π⁰ and that the lower limit to the lifetime of the nucleon is 10³¹ yr, does not favour the simple SU(5) type models. The remarkable discoveries of W^± and Z⁰ with mass values exactly as predicted, have given a boost to the unification based on the gauge theoretical approaches. Whether grand unification can be extended to super-unification, experiment alone can tell. This will be the challenge for the remaining years of this century.