Radio Observations at 232 MHz and Multifrequency Spectral Studies of SNR HB21

Radio observational results at 232 MHz and multifrequency studies of supernova remnant (SNR) HB21 are presented. Its integrated flux density at 232 MHz is about 390 ± 30 Jy. Both the integrated spectral index and the spatial variations of spectral index of the remnant were calculated by combining the new map at 232 MHz with previously published maps made at 408, 1420, 2695, and 4750 MHz. The SNR has an integrated spectral index of about α = -0.43(S _ν ∝ ν^α) between 232 and 4750 MHz. In general the spectral index varies from –0.5 in southeast and west regions of the remnant to –0.3 in the central region and near the northwest edge. The new data of 232 MHz reveals that there is interaction between the remnant and the surrounding gas along the east edge of the remnant which causes the spectrum flattening at low frequency, while the very good agreement between the structure of X-ray emission and the central flat spectrum area suggests that the existence of thermal emission is the reason of spectrum flattening in the area. This revised version was published online in July 2006 with corrections to the Cover Date.     

Shear-free homogeneous cosmological model with heat flux

We present the condition of vanishing shear in a spatially homogeneous spacetime in terms of the Ricci rotation co-efficients corresponding to an orthonormal tetrad (ν ^α. _A η ^α) (whereν ^α is the unit vector along the time axis and _A η ^α are the three independent reciprocal group vectors). Assuming that the velocity vector can be expanded in the direction ofν ^α and any one of the _A η ^α’s it is shown that shear-free motion is possible only in case of some special Bianchi types, and these cases are studied assuming the velocity vector to be geodetic and that there may be a nonvanishing heat flux term.     

The blazar sequence: validity and predictions

The “blazar sequence” posits that the most powerful BL Lacertae objects and flat-spectrum radio quasars should have relatively small synchrotron peak frequencies, ν _peak, and that the least powerful such objects should have the highest ν _peak values. This would have strong implications for our understanding of jet formation and physics and the possible detection of powerful, moderately high-redshift TeV blazars. I review the validity of the blazar sequence by using the results of very recent surveys and compare its detailed predictions against observational data. I find that the blazar sequence in its simplest form is ruled out. However, powerful flat-spectrum radio quasars appear not to reach the ν _peak typical of BL Lacs. This could indeed be related to some sort of sequence, although it cannot be excluded that it is instead due to a selection effect.      

Magnetic fields and quasi-periodic oscillations of black hole radiation

Various relations are found between the key parameters of black holes and active galactic nuclei. Some have a statistical property, others follow from the theoretical consideration of the evolution of these objects. In this paper we use a recently discovered empirical relation between the characteristic frequency of quasi-periodic oscillations of radiation ν _br of black holes, their masses and matter accretion rates to determine the magnetic field strength B _H at the black hole event horizon. Since the characteristic frequency can be determined from observations, the use of a new relation for the estimations of magnetic field B _H can yield more definite results, since we are decreasing the number of the unknown or poorly-determined parameters of objects (it especially concerns the accretion rate Ṁ). The typical values which we have found are B _H ≃ 10⁸G for the stellar mass black holes, and B _H ≃ 10⁴G for the supermassive black holes. Besides, we demonstrate that if the linear polarization of an object is caused by the radiation of a magnetized accretion disk, then the degree of observable polarization is p ∼ ν _br ^−1/2.     

Measurement of the orbital period of the X-ray burster GS 1826-238 based on observations of its optical brightness variations

The variability of the optical and X-ray fluxes from the binary GS 1826-238 is investigated. An epoch-folding analysis of the optical data obtained with the RTT-150 telescope in 2003–2004 has revealed periodic brightness variations in the source with a period P _orb = 2.24940 ± 0.00015 h with a high statistical significance. When estimating the detection significance of the periodic signal, we have specially taken into account the presence of a powerful aperiodic component (“red noise”) in the source’s brightness variability. The source’s power density spectra in the frequency range ∼10⁻⁵–0.01 Hz have been obtained. We have detected a statistically significant break in the power density spectrum of GS 1826-238 at a frequency ν _br ≈ (8.48 ± 0.14) × 10⁻⁵ Hz in both optical and X-ray energy bands. We have estimated the orbital period of the binary GS 1826-238 using the correlation between the break frequency in the power density spectrum and the orbital period of binaries, P _orb ∝ 1/ν _br, found by Gilfanov and Arefiev (2005): P _orb = 3.7 ± 0.8 h and P _orb = 11.3 ± 5.9 h when using Sco X-1 and 1H 16267-273, respectively, as reference sources. It seems to us that the method for estimating the orbital periods of low-mass X-ray binaries using the correlation P _orb ∝ 1/ν _br may turn out to be very promising, especially for persistent low-luminosity X-ray binaries.     

Two new LBV candidates in the M 33 galaxy

We present two new luminous blue variable (LBV) candidate stars discovered in the M33 galaxy. We identified these stars as massive star candidates at the final stages of evolution, presumably with a notable interstellar extinction. The candidates were selected from the Massey et al. catalog based on the following criteria: emission in H _α , V<18^./m 5 and 0_.^m 35 < (B - V) < 1_.^m 2. The spectra of both stars reveal a broad and strong H _α emission with extended wings (770 and 1000 kms⁻¹). Based on the spectra we estimated the main parameters of the stars. Object N45901 has a bolometric luminosity log(L/L_⊙) = 6.0–6.2 with the value of interstellar extinction A _V = 2.3 ± 0.1. The temperature of the star’s photosphere is estimated as T⋆ ∼ 13000–15000 K, its probable mass on the Zero Age Main Sequence is M∼ 60–80 M_⊙. The infrared excess in N 45901 corresponds to the emission of warm dust with the temperature Twarm ∼ 1000 K, and amounts to 0.1%of the bolometric luminosity. A comparison of stellar magnitude estimates from different catalogs points to the probable variability of the object N45901. Bolometric luminosity of the second object, N125093, is log(L/L_⊙) = 6.3 − 6.6, the value of interstellar extinction is A _V = 2.75 ± 0.15. We estimate its photosphere’s temperature as T⋆∼ 13000–16000K, the initial mass as M ∼ 90–120 M_⊙. The infrared excess in N125093 amounts to 5–6% of the bolometric luminosity. Its spectral energy distribution reveals two thermal components with the temperatures T_warm ∼ 1000K and T_cold ∼ 480 K. The [Ca II] λλ7291, 7323 lines, observed in LBV-like stars Var A and N93351 in M33 are also present in the spectrum of N 125093. These lines indicate relatively recent gas eruptions and dust activity linked with them. High bolometric luminosity of these stars and broad H α emissions allow classifying the studied objects as LBV candidates.     

The Infrared Spectrum of Comet Hele–Bopp as Seen by the Infrared Space Observatory

Spectra of comet C/1995 O1 (Hale-Bopp) were obtained with the Infrared Space Observatory (ISO) at medium resolution with the grating spectrometer in the photometer (PHT-S) and/or at high resolution with the short wavelength spectrometer (SWS) and long wavelength spectrometer (LWS) in April 1996 (Crovisier et al., 1996), September–October 1996 (Crovisier et al., 1997a, b) and December 1997, at distances from the Sun of 4.6, 2.9 and 3.9 AU, respectively. For the first time, high-resolution spectra of a comet covering the entire 2.4 to 200 μm spectral range were obtained. The vibrational bands of H₂O, CO₂ and CO are detected in emission with PHT-S. Relative production rates of 100:22:70 are derived for H₂O:CO₂:CO at 3 AU pre-perihelion. H₂O is observed at high spectral resolution in the ν₃ group of bands around 2.7 μm and the ν₂ group around 6 μm with SWS, and in several rotational lines in the 100–180 μm region with LWS. The high signal-to-noise ratio of the ν₃ band observed on September–October 1996 allows accurate determinations of the water rotational temperature (28 K) and of its ortho-to-para ratio(2.45 ± 0.10, which significantly differs from the high temperature limit and corresponds to a spin temperature of 25 K). Longward of 6 μm the spectrum is dominated by dust thermal continuum emission, upon which broad emission features are superimposed. The wavelengths of the emission peaks correspond to those of Mg-rich crystalline olivine (forsterite). In the September–October 1996 spectra, emission features at 45 and 65 μm and possible absorption at 2.9–3.2 μm suggest that grains of water ice were present at 3 AU from the Sun. The observations made post-perihelion in late December 1997 led to the detections of H₂O, CO₂ and CO at 3.9 AU from the Sun (Figures 1 and 2). The production rates were ≈3.0 × 10²⁸,3.5 × 10²⁸ and ≈1.5 × 10²⁹ s^-1, respectively. This corresponds to H₂O:CO₂:CO = 100:110:500 and confirms that at such distances from the Sun, cometary activity is dominated by sublimation of CO and CO₂ rather than by H₂O. This revised version was published online in July 2006 with corrections to the Cover Date.     

Relation between the Spatial Distribution and Spectral Index of Superthermal Electron Distribution in Solar cm-Radio Sources

A series of solar cm-radio bursts are analyzed by a new inverse method estimating spatial changes of the superthermal electron distribution in solar cm-radio burst sources. It is found that the measure of the spatial change of superthermal electrons in the radio source ν _n is always greater than that for the magnetic field ν _B and it is linearly dependent on the spectral index of the electrons δ as ν _n ≈0.5δ. This relation is explained in the simplified flare-loop model integrating the analytical solutions of the Fokker – Planck equation. The mean value of ν _B is found to be 0.36±0.04, which is very close to the value of ν _B =0.38±0.02 derived from the dependence of the magnetic field strength on the height in the active region measured by RATAN-600.     

Multi-Wavelength Analysis of High-Energy Electrons in Solar Flares: A Case Study of the August 20, 2002 Flare

A multi-wavelength spatial and temporal analysis of solar high-energy electrons is conducted using the August 20, 2002 flare of an unusually flat (γ₁ = 1.8) hard X-ray spectrum. The flare is studied using RHESSI, Hα, radio, TRACE, and MDI observations with advanced methods and techniques never previously applied in the solar flare context. A new method to account for X-ray Compton backscattering in the photosphere (photospheric albedo) has been used to deduce the primary X-ray flare spectra. The mean electron flux distribution has been analysed using both forward fitting and model-independent inversion methods of spectral analysis. We show that the contribution of the photospheric albedo to the photon spectrum modifies the calculated mean electron flux distribution, mainly at energies below ∼100 keV. The positions of the Hα emission and hard X-ray sources with respect to the current-free extrapolation of the MDI photospheric magnetic field and the characteristics of the radio emission provide evidence of the closed geometry of the magnetic field structure and the flare process in low altitude magnetic loops. In agreement with the predictions of some solar flare models, the hard X-ray sources are located on the external edges of the Hα emission and show chromospheric plasma heated by the non-thermal electrons. The fast changes of Hα intensities are located not only inside the hard X-ray sources, as expected if they are the signatures of the chromospheric response to the electron bombardment, but also away from them.     

The X-ray jet and lobes of PKS 1354+195 (=4C 19.44)

We present a Chandra image of the quasar, jet, and lobes of PKS 1354+195 (=4C 19.44). The radio jet is 18 arcsec long, and appears to be very straight. The length gives many independent spatial resolution elements in the Chandra image while the straightness implies that the geometrical factors are constant along the jet although their values are uncertain. We also have 4 frequency radio images with half to one arcsecond angular resolution, and use HST and Spitzer data to study the broad band spectral energy distributions. The X-ray and radio spectra are both consistent with a spectrum f _ν ∝ ν ^−0.7 for the integrated jet. Using that spectral index, the model of inverse Compton scattering of electrons on the cosmic microwave background (IC/CMB) gives magnetic field strengths and Doppler factors that are relatively constant along the jet. Extended X-ray emission is evident in the direction of the otherwise unseen counter-jet. X-ray emission continues past the radio jet to the South, and is detected within both the southern and northern radio lobes.     

Ionized gas in the Irr galaxy IC 10: The emission spectrum and ionization sources

We present the observations of the Irr galaxy IC 10 at the 6-m SAO telescope with the panoramic Multi-Pupil Fiber Spectrograph (MPFS). Based on the results of these observations and our long-slit spectroscopy performed previously, we have investigated the ionized-gas emission spectrum in the region of intense star formation and refined the gas metallicity estimates. We show that the “diagnostic diagrams” constructed from our observations agree best with the new improved ionization models by Martin-Manjon et al. Using these models, we have determined the electron density and gas ionization parameter and ionizing-cluster characteristics, the age and mass, from the spectra of the investigated HII regions. The cluster ages and masses are shown to be within the ranges 2.5–5 Myr and (0.2–1) × 10⁵ M _⊗, respectively.     

The Shape of the Blue/UV Continuum of B3-VLA Radio Quasars

We present some results based on optical photometry of an unbiased subsample of 64 sources, from a sample of radio quasars 80 per cent complete. The quasars (z=0.4-2.8) show approximately a similar spectral energy distribution (SED). Only two quasars appear clearly differenciated, exhibiting redder colours than the rest, and they have low or moderate redshifts (z=0.50 and 1.12). Composite broad-band spectra for the remaining 62 quasars reveal the presence of a break in the SED at ∼ 3000 Å, where the average slope α (defined by S_ν ∝ ν^α) changes from α_blue=0.11±0.16 at λ>3000 Å to α_UV=-0.66±0.15 at λ<3000 Å. Composite spectra from a low and a high redshift subsample, with the redshift separation at the median value z=1.2, yields α_UV=-0.87±0.20, for z<1.2 and α_UV=-0.48±0.12 for z>1.2 and the difference is 98 per cent significant. This trend could be intrinsic or could arise from a dependence between α_UV and the blue/UV luminosity, since the three quantities α_UV - L_blue/UV - z are correlated. Finally, an intrinsic correlation is found between blue/UV and radio luminosity. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new sample of gigahertz-peaked-spectrum, extra-galactic radio sources

A new sample of extra-Galactic radio sources having a peak in the continuous spectrum near 1 GHz has been compiled, using data available in the literature on their radio flux densities, and some characteristic parameters of radio sources in this sample have been calculated. The median value of the calculated spectral indices of radio sources in the sample is -0.95, the median flux density at the peak (S_m) is 465 mJy, and almost half the radio sources (14 out of 30) have ultrasteep spectra, for which the spectral indices in the high-frequency range are α <-1.0 (S ∝ v^α). Translated from Astrofizika, Vol. 41, No. 3, pp. 377–388, July–September, 1998.     

Search for HH Objects and Emission Stars in Star Formation Regions. III. PMS Stars in NGC 7129

Slitless spectroscopy is used to discover 22 emission stars in the central and northwest regions of the cluster located in the NGC 7129 nebula. 16 of them are found for the first time. This sample is essentially complete up to V 20.0. The emission stars are distributed nonuniformly over the field of the cluster and are concentrated toward its center. Photometry in the V, R, and I bands is conducted on more than a hundred stars in the cluster. This yields an average absorption coefficient A _V=1.7±0.27 for this region. Based on their positions in color diagrams for the optical and near IR ranges, most of the emission stars can be regarded as T Tau objects.     

The Effect of Abnormal Granulation on Acoustic Wave Travel Times and Mode Frequencies

Observations indicate that in plage areas (i.e. in active regions outside sunspots) acoustic waves travel faster than in the quiet Sun, leading to shortened travel times and higher p-mode frequencies. Coupled with the 11-year variation of solar activity, this may also explain the solar cycle variation of oscillation frequencies. While it is clear that the ultimate cause of any difference between the quiet Sun and plage is the presence of magnetic fields of order 100 G in the latter, the mechanism by which the magnetic field exerts its influence has not yet been conclusively identified. One possible such mechanism is suggested by the observation that granular motions in plage areas tend to be slightly “abnormal”, dampened compared to the quiet Sun. In this paper we consider the effect that abnormal granulation observed in active regions should have on the propagation of acoustic waves. Any such effect is found to be limited to a shallow surface layer where sound waves propagate nearly vertically. The magnetically suppressed turbulence implies higher sound speeds, leading to shorter travel times. This time shift Δ τ is independent of the travel distance, while it shows a characteristic dependence on the assumed plage field strength. As a consequence of the variation of the acoustic cutoff with height, Δ τ is expected to be significantly higher for higher frequency waves within the observed regime of 3 – 5 mHz. The lower group velocity near the upper reflection point further leads to an increased envelope time shift, as compared to the phase shift. p-mode frequencies in plage areas are increased by a corresponding amount, Δ ν/ν=ν Δ τ. These characteristics of the time and frequency shifts are in accordance with observations. The calculated overall amplitudes of the time and frequency shifts are comparable to, but still significantly less than (by a factor of 2 to 5), those suggested by measurements.     

Panoramic spectroscopy of galaxies with star-formation regions. a study of SBS 1202?+?583

The methods of panoramic (3D) spectroscopy are used by us in a detailed study of galaxies with ongoing star formation chosen from among objects in seven selected fields of the Second Byurakan Survey (SBS). This article deals with the irregular galaxy SBS 1202 + 583, which our classification scheme identifies as being in a continuous phase of star formation. Observations were made with the panoramic spectrographs MPFS at the 6-m telescope of the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences and VAGR at the 2.6-m telescope of the Byurakan Astrophysical Observatory (BAO) in Armenia. The data are used to construct maps of the radiative fluxes in the continuum and various emission lines. Special attention is devoted to analyzing the emission in the Hα hydrogen recombination line and in the forbidden low-ionization doublets of nitrogen [NII] λλ6548, 6583 and sulfur [SII] λλ6716, 6731, and the ratios of the intensities of the forbidden lines to Hα. The observable characteristics (size, Hα fluxes, etc.) of nine HII regions are studied. The estimated current rates of star formation in the individual HII regions based on the Hα fluxes lie within the range of 0.3-1.2_⨀ M /year. The dependence of the ratio of the intensities of the emission in these above mentioned forbidden doublets on the rate of star formation in the HII regions is found.     

A diffraction limit on the gravitational lens effect

The focussing of gravitational radiation by the interior and exterior gravitational field of a Newtonian gravitational lens is considered. A graphical method for determining the caustic structure of a Newtonian gravitational lens is presented and the caustic structure of a solar type gravitational lens is discussed. Estimates of the amplitude magnification in the caustic region indicate that waves with frequencies less than a critical cutoff frequency ω _c are not amplified significantly. For a lens of massM this cutoff frequency is ω _c ≈(10^-1πM)^-1; for the Sun ω _c ≈10⁴s^-1. Work supported in part by National Science Foundation Grant PHY78-05368.     

HI fluctuations at large redshifts: I-visibility correlation

We investigate the possibility of probing the large scale structure in the universe at large redshifts by studying fluctuations in the redshifted 1420 MHz emission from the neutral hydrogen (HI) at early epochs. The neutral hydrogen content of the universe is known from absorption studies forz ≲ 4.5. TheHI distribution is expected to be inhomogeneous in the gravitational instability picture and this inhomogeneity leads to anisotropy in the redshifted HI emission. The best hope of detecting this anisotropy is by using a large low-frequency interferometric instrument like the Giant Meter-Wave Radio Telescope (GMRT). We calculate the visibility correlation function 〈V_v(U) V_v′(U)〉 at two frequenciesi andv′ of the redshiftedHI emission for an interferometric observation. In particular we give numerical results for the two GMRT channels centered aroundν = 325 MHz andν = 610 MHz from density inhomogeneity and peculiar velocity of the HI distribution. The visibility correlation is- 10^-10-10^-9 Jy². We calculate the signal-to-noise for detecting the correlation signal in the presence of system noise and show that the GMRT might detect the signal for integration times - 100 hrs. We argue that the measurement of visibility correlation allows optimal use of the uncorrelated nature of the system noise across baselines and frequency channels. On leave from Harish-Chandra Research Institute, Chhatnag Road, Jhusi, Allahabad 211 019, India.     

Comparative Analysis of Super-Kamiokande and SNO Solar-Neutrino Data and the Photospheric Magnetic Field

We carry out a comparative analysis of Super-Kamiokande, SNO, and photospheric magnetic-field data for the interval in which these datasets overlap. This proves to be the interval of operation of the D₂O phase of the SNO experiment. Concerning solar-rotational modulation, we find that the magnetic-field power spectrum shows the strongest peaks at the second and sixth harmonics of the solar synodic rotation frequency [i.e., at 3ν _rot and 7ν _rot]. We find that the restricted Super-Kamiokande dataset has a strong modulation at the second harmonic, as we found to be the case for the complete Super-Kamiokande dataset. The SNO D₂O dataset exhibits weak modulation at that frequency, but shows strong modulation in the band corresponding to the sixth harmonic (too high a frequency to be detectable in the Super-Kamiokande dataset, which is available only in five-day bins, whereas SNO data is available in one-day bins). We estimate the significance level of the correspondence of the Super-Kamiokande second-harmonic peak with the corresponding magnetic-field peak to be 0.0004, and the significance level of the correspondence of the SNO D₂O sixth-harmonic peak with the corresponding magnetic-field peak to be 0.009. By estimating the amplitude of the modulation of the solar-neutrino flux at the second harmonic from the restricted Super-Kamiokande dataset, we find that the weak power at that frequency in the SNO D₂O power spectrum is not particularly surprising. We also examine power spectra in the neighborhood of 9.43 year⁻¹, which is the frequency of a particularly strong modulation in the entire Super-Kamiokande dataset. There is no peak at this frequency in the power spectrum formed from the restricted Super-Kamiokande dataset. It is therefore not surprising that we find (in agreement with the recent analysis by the SNO collaboration) that this peak does not show up in the SNO D₂O dataset either.     

The Influence of Fluctuations of the Solar Emission Line Profileon the Doppler Shift of Interplanetary HLyα Lines Observed by the Hubble–Space–Telescope

In an earlier research the employment of a radiation transport model with angle-dependent partial frequency redistribution, self-absorption by interplanetary hydrogen, realistic solar H_Lyαemission profile, and a time dependent `hot' hydrogen model to analyze 5 interplanetary H_Lyα glow spectra obtained with theHubble–Space–Telescope–GHRS spectrometer, has not resulted in unequivocal determination of a set of thermodynamical parameters of the interstellar hydrogen The residual discrepancies between the model and the data concern the observations performed within an interval of 1 year close to the solar minimum from very similar lines of sight. In this paper we investigate by calculating interplanetary H_Lyα lines with the use of a one hydrogen distribution and several solar H_Lyα line profiles whether this residual may be caused by possible variations in time of the shape of the solar H_Lyα emission line profile which cause variable illuminations of the interplanetary gas. These variations of illuminations cause variations in Doppler shift of the resonant interplanetary H_Lyα line that can amount to ≃ 4 km s^-1in the line peak. Consequently we conclude that without adequate knowledge of the solar H_Lyα emission line profile during spectral observations of the interplanetary hydrogen gas it is impossible to obtain an agreement between models and observations better than by this value. This revised version was published online in July 2006 with corrections to the Cover Date.