Spectral, polarization and time-lag properties of GRS 1915+105 radio oscillations

We report high-sensitivity dual-frequency observations of radio oscillations from GRS 1915+105 following the decay of a major flare event in 2000 July. The oscillations are clearly observed at both frequencies, and the time-resolved spectral index traces the events between optically thin and thick states. While previously anticipated from sparse observations and simple theory, this is the first time a quasi-periodic signal has been seen in the radio spectrum, and is a clear demonstration that flat radio spectra can arise from the combination of emission from optically thick and thin regions. In addition, we measure the linear polarization of the oscillations, at both frequencies, at a level of about  1–2  per cent, with a flat spectrum. Cross-correlating the two light curves we find a mean delay, in the sense that the emission at 8640 MHz leads that at 4800 MHz, of around 600 s. Comparison with frequency-dependent time-delays reported in the literature reveals that this delay is variable between epochs. We briefly discuss possible origins for a varying time-delay, and suggest possible consequences.     

Analysis of spike emission data at 2545/2645 MHz in the peak year of the 22nd solar cycle

We briefly discuss the observed features including the high flux density, short duration, narrow emission band, fast frequency drift, quasi-periodic oscillation and fast variation of polarized components, of 51 spike emission events observed at 2545/2645 MHz in the solar activity peak year, 1991 January–December, and carry out correlation analysis between these events and optical flares, magnetic field intensity and configuration of flare regions, and sunspot evolution types of active regions. In view of the fact that the observed and statistical characteristics of the spike emissions are very different from those of known types of solar radio burst and known solar radio components, we think that the spike emission in the peak years is probably a new type of radio burst excited by electron cyclotron maser instability under wave-particle resonance, or a new solar radio component.     

On the Possible Connection between Photospheric 5-Min Oscillation and Solar Flare Microwave Emission

Dynamic spectra of low-frequency modulation of microwave emission from solar flares are obtained. Data of 15 bursts observed in 1989–2000 with Metsähovi radio telescope at 37 GHz have been used. During 13 bursts a 5-min modulation of the microwave emission intensity was detected with the frequency of ν _I = 3.2± 0.24 (1σ) mHz. Five bursts revealed a 5-min wave superimposed on a ～1 Hz, linear frequency modulated signal generated, presumably, by coronal magnetic loop, this wave frequency is ν_fm = 3.38± 0.37 (1σ) mHz. Both intensity and frequency modulations detected are in good agreement with the data on 5-min global oscillations of photosphere and with the data on the umbral velocity oscillations observed in the vicinity of sunspots. Possible role of p-mode photospheric oscillations in modulation of microwave burst emission is discussed.     

Dual pulsations in solar radio bursts at short centimeter wavelengths

It is well known that the oscillating MHD waves drive periodic variations in the magnetic field. But how the MHD waves can be triggered in the flaring loops is not yet well known. It seems to us that this problem should be connected with the physical processes occurring in the flare loop during a solar flare. A peculiar solar flare event at 04:00–04:51 UT on May 23, 1990 was observed simultaneously with time resolutions 1 s and 10 ms by Nanjing University Observatory and Beijing Normal University Observatory, which are about 1000 km apart, at 3.2 cm and 2 cm wavelengths, respectively. Two kinds of pulsations with quasi-periods 1.5 s and 40 s were found in radio bursts at the two short centimeter waves; however, the shorter quasi-periodic pulsations were superimposed upon the longer ones. From the data analysis of the above-mentioned quasi-periodic pulsations and associated phenomena in radio and soft X-ray emissions during this flare event published in Solar Geophysical Data (SGD), the authors suggest that the sudden increase in plasma pressure inside (or underlying) the flare kernel due to the upward moving chromospheric evaporated gas, which is caused by the explosive collision heating of strong non-thermal electrons injected downwards from the microwave burst source, plays the important role of triggering agents for MHD oscillations (fast magneto-acoustic mode and Alfvén mode) of the flare loop. These physical processes occurring in the flare loop during the impulsive phase of the solar flare may be used to account for the origin and observational characteristics of quasi-periodic pulsations in solar radio bursts at the two short centimeter wavelengths during the flare event of 1990 May 23. In addition, the average physical parameters N, T, B inside or underlying the flare kernel can be also evaluated.     

Low-Frequency Quasi-Periodic Oscillations in the X-ray Nova MAXI J1535-571 at the Initial Stage of Its 2017 Outburst

We report the discovery of low-frequency quasi-periodic oscillations (QPOs) in the power spectrum of the X-ray nova MAXI J1535-571 at the initial stage of its outburst in September 2017. Based on data from the SWIFT and INTEGRAL instruments, we have traced the evolution of the QPO parameters (primarily their frequency) with time and their correlation with changes in the X-ray spectrum of the source (changes in the emission flux and hardness). We place constraints on the theoretical QPO generation models.     

Seismic Emissions from a Highly Impulsive M6.7 Solar Flare

On 10 March 2001 the active region NOAA 9368 produced an unusually impulsive solar flare in close proximity to the solar limb. This flare has previously been studied in great detail, with observations classifying it as a type 1 white-light flare with a very hard spectrum in hard X-rays. The flare was also associated with a type II radio burst and coronal mass ejection. The flare emission characteristics appeared to closely correspond to previous instances of seismic emission from acoustically active flares. Using standard local helioseismic methods, we identified the seismic signatures produced by the flare that, to date, is the least energetic (in soft X-rays) of the flares known to have generated a detectable acoustic transient. Holographic analysis of the flare shows a compact acoustic source strongly correlated with the impulsive hard X-rays, visible continuum, and radio emission. Time?–?distance diagrams of the seismic waves emanating from the flare region also show faint signatures, mainly in the eastern sector of the active region. The strong spatial coincidence between the seismic source and the impulsive visible continuum emission reinforces the theory that a substantial component of the seismic emission seen is a result of sudden heating of the low photosphere associated with the observed visible continuum emission. Furthermore, the low-altitude magnetic loop structure inferred from potential-field extrapolations in the flaring region suggests that there is a significant anti-correlation between the seismicity of a flare and the height of the magnetic loops that conduct the particle beams from the corona.     

Stellar dynamic spectroscopy

The dynamic spectrum, a three dimensional record of the radio intensity as a function both of time and frequency, has long been used as a probe of plasma processes in the solar corona. Beginning with the work of Wild and McCready (1950) dynamic spectroscopy has been used to distinguish between the multitude of radio wave emitting phenomena which occur in the solar corona and to infer the physical mechanisms responsible.

Stellar dynamic spectroscopy has always been a tantalizing prospect. The vast body of experience with solar dynamic spectroscopy would prove invaluable in interpreting stellar dynamic spectra. Further, the new parameter regimes presented by stellar coronas would allow further insight to be gained in the physical processes at work in stellar coronas.

Recently, Bastian and Bookbinder (1987) used the Very Large Array in spectral line mode at 1.4 GHz with a bandwidth of 50 MHz to obtain the first dynamic spectra of nearby flare stars. The spectral resolution was 3.125 MHz and the temporal resolution was 5 s. While the relative bandwidth was less than ideal (δν/ν ∼ 5%), the spectra so obtained were sufficient to show the presence of narrowband structure in a radio outburst from the well-known dMe flare star UV Ceti.

Several efforts are now underway to obtain stellar dynamic spectra, of both RS CVn binaries and dMe flare stars, with higher degrees of spectral and temporal resolution. Among these are use of a 1024 channel correlator with the 1000' telescope at Arecibo and use of the Berkeley Fast Pulsar Search Machine (Kulkarni et al. 1984) with the Green Bank 140' telescope.

     

Long term spectral variability in the soft gamma-ray repeater SGR 1900+14

We present a systematic analysis of all the BeppoSAX data of SGR1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ～20 days after the August 1998 giant flare and during the 10⁵?s long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term spectral evolution only with a change of the temperature of the blackbody component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR1900+14 persistent emission was significantly harder and detected also above 10 keV with the PDS instrument. In the last BeppoSAX observation (April 2002) the flux was at least a factor 1.2 below the historical level, suggesting that the source was entering a quiescent period.     

恒星射电耀发6cm波段偏振观测试验

利用新疆天文台南山基地25m射电望远镜在6cm波段对恒星V772 Her和βPer进行了偏振观测试验.通过数据处理和校准得到恒星的射电光变曲线.探测到V772 Her的射电耀发现象,耀发时的线偏振度约达30%,偏振位置角约4°;探测到βPer的缓变成份及叠加其上的快速耀发,βPer耀发时线偏振很弱.     

A synchrotron self-Compton scenario for the very high energyγ-ray emission of the intermediate BL Lacertae object W Comae

W Comae has significant variability in multi-wavelengthes, from radio to gamma-ray bands. A bright outburst in optical and X-ray bands was observed in 1998, and most recently, a strong TeV flare was detected by VERITAS in 2008. It is the first TeV intermediate-frequency-peaked BL Lacertae source. I find that both the broadband spectral energy distributions (SEDs) which were quasi-simultaneously obtained during the TeV flare and during the optical/X-ray outburst are well fit by using a single-zone synchrotron + synchrotron-self-Compton model. The satisfactory fitting requires a large beaming factor, i.e., δ～25 and δ～20 for the TeV flare and the optical/X-ray outburst, respectively, suggesting that both the optical/X-ray outburst and the TeV flare are from a relativistic jet. The size of the emission region of the TeV flare is three times larger than that of the optical/X-ray outburst, and the strength of the magnetic field for the TeV flare is～14 times smaller than that of the X-ray/optical outburst, likely indicating that the region of the TeV flare is more distant from the core than that of the X-ray/optical outburst. The inverse Compton component of the TeV flare peaks around 1.3 GeV, but it is around 0 MeV for the X-ray/optical outburst, lower than that for the TeV flare by two orders of magnitude. The model predicts that the optical/X-ray outburst might be accompanied by a strong MeV/GeV emission, but the TeV flare may be not associated with the X-ray/optical outburst. The GeV emission is critical for characterizing the SEDs of the optical/X-ray outburst and the TeV flare. The predicted GeV flux is above the sensitivity of Fermi/LAT, and it could be verified with the observations by Fermi/LAT in the near future.     

A Highly Circularly Polarized Solar Radio Emission Component Observed at Hectometric Wavelengths

We report here the observation of a rare solar radio event at hectometric wavelengths that was characterized by essentially 100% circularly polarized radiation and that was observed continuously for about six days, from May 17 to 23, 2002. This was the first time that a solar source with significantly polarized radiation was detected by the WAVES experiment on the Wind spacecraft. From May 19 to 22, the intense polarized radio emissions were characterized by quasi-periodic intensity variations with periods from one to two hours and with superposed drifting, narrowband, fine structures. The bandwidth of this radiation extended from about 400 kHz to 7 MHz, and the peak frequency of the frequency spectrum slowly decreased from 2 MHz to about 0.8 MHz over the course of four days. The radio source, at each frequency, was observed to slowly drift from east to west about the Sun, as viewed from the Earth and was estimated to lie between 26 and 82R_⊙ (R_⊙ = 696 000 km). We speculate that this unusual event may represent an interplanetary manifestation of a moving type IV burst and discuss possible radio emission mechanisms. The ISEE-3 spacecraft may possibly have detected a similar event some 26 years ago.     

Radio emission from flare stars

Observations of radio emission from flare stars are reviewed, including surveys of flare stars in the solar neighborhood and in stellar associations, studies of quiescent emission, and continuum and spectral studies of radio burst emission. The radio observations are placed in an observational context provided by soft X-ray, UV, and optical observations. It is stressed that, as is the case for the latter wavelength regimes, observations of rado bursts on flare stars are qualitatively similar to those on the Sun, albeit in a dramatically scaled-up fashion.     

Crystal spectrometer studies of the Sun employing a rotation modulation collimator

The use of rotating modulation collimators in high resolution solar X-ray spectroscopy is discussed with reference to the recent flight of a sounding rocket payload. This rocket carried an experiment which combined a modulation collimator and a Bragg crystal spectrometer to make moderately high resolution spectral and spatial measurements simultaneously. The response of the instrument to extended sources is described. It is shown that the technique is particularly suited to long term observations of active region emission, but that useful measurements can be made even during a short rocket flight.Our observations were made in 1972 October 26 between 0432 UT and 0436 UT. During the flight a scan of the solar X-ray spectrum was made which covered the wavelength range 1.45–1.71 nm. A small flare commenced at about 0432 UT in McMath calcium plage region 12094; the emission from this provided the major contribution to the observed spectrum. X-ray emission from a group of active regions in the SW and the small McMath region 12090 has also been detected.Spectral and spatial observations have been combined to investigate the conditions in both active region and flare plasmas.     

Superfine Temporal Structure of the Microwave Burst on 21 April 2002: What Can We Learn about the Emission Mechanism?

Zebra pattern is observed as a number of almost parallel bright and dark stripes in the dynamic spectrum of solar radio emission. Recent observations show that zebra patterns in the microwave range often have superfine temporal structure, when the zebra stripes consist of individual short pulses similar to millisecond spikes. In this article, the burst on 21 April 2002 is investigated. The burst with a distinct superfine structure was detected at the Huairou Station (China) in 2.6?–?3.8 GHz frequency range. It is found that the emission pulses are quasi-periodic, the pulse period is about 25?–?40 ms and decreases with an increase of the emission frequency. The degree of circular polarization of zebra pattern increases with an increase of the emission frequency, it varies from moderate (about 20%) to relatively high (>60%) values. The temporal delay between the signals with left- and right-handed polarization is not found. The conclusion is made that the emission is generated by plasma mechanism at the fundamental plasma frequency in a relatively weak magnetic field. The observed polarization of the emission is formed during its propagation due to depolarization effects. A model is proposed in which the superfine temporal structure is formed due to modulation of the emission mechanism by downward propagating MHD oscillations; this model allows us to explain the observed variation of the pulse period with the emission frequency.     

High Spectral Resolution Observation of Decimetric Radio Spikes Emitted by Solar Flares – First Results of the Phoenix-3 Spectrometer

A new multichannel spectrometer, Phoenix-3, is in operation having capabilities to observe solar flare radio emissions in the 0.1?–?5 GHz range at an unprecedented spectral resolution of 61.0 kHz with high sensitivity. The present setup for routine observations allows measuring circular polarization, but requires a data compression to 4096 frequency channels in the 1?–?5 GHz range and to a temporal resolution of 200 ms. First results are presented by means of a well observed event that included narrowband spikes at 350?–?850 MHz. Spike bandwidths are found to have a power?–?law distribution, dropping off below a value of 2 MHz for full width at half maximum (FWHM). The narrowest spikes have a FWHM bandwidth less than 0.3 MHz or 0.04% of the central frequency. The smallest half-power increase occurs within 0.104 MHz at 443.5 MHz, which is close to the predicted natural width of maser emission. The spectrum of spikes is found to be asymmetric, having an enhanced low-frequency tail. The distribution of the total spike flux is approximately an exponential.     

X-ray variability of the pulsar Vela X-1 as observed with ART-P/Granat

Observations of the X-ray pulsar Vela X-1 with the ART-P telescope onboard the Granat Observatory are presented. Variability on a time scale of several thousand seconds was detected; intensity variations are shown to be accompanied by changes in the source’s spectrum. The hardness was also found to be highly variable on a scale of one pulsation period. The source’s spectrum exhibits an absorption feature at energy ～7 keV, which is apparently attributable to cyclotron scattering/absorption in the neutron-star magnetic field. Weak persistent emission was detected during an X-ray eclipse, which probably resulted from the scattering of pulsar emission in the stellar wind from an optical star.     

VHF double-peaked spectra at negative Doppler shifts in the morning sector of radio aurora

This paper presents a new Doppler spectral type of VHP (42 MHz) radio auroral backscatter. This spectrum, which has a double-peaked structure, was observed repeatedly during the morning sector of an exceptionally strong event (A_p = 48) and is due to irregularities moving northwards with quite different velocities. The stronger spectral component, which has a smaller Doppler shift, is centred in frequency at ～?130 Hz, corresponding to the ion-acoustic velocity range in the medium; the weak component, which has a greater frequency shift, usually is centred at about ?300 Hz (～ 1050 m s^?1). Evidence based on spectral analysis of sequential short time sequences shows that the spectral power alternates in time between the two distinct frequency bands where the peaks are located, suggesting that the double-peaked spectrum may result from two competing processes which cannot operate simultaneously. The possibility exists that the theoretical model proposed by Sato (1977), which predicts two different quasi-linear stabilization mechanisms for the two-stream instability, could explain the observed double-peaked spectral type.     

A review of stellar flares and their characteristics

We review the flaring activity of stars across the HR-diagram. Brightenings have been reported along the entire Main Sequence and in many stars off the Main Sequence. Some stars are decidedly young, others are in advanced stages of stellar evolution. Flares are common on stars with outer convection zones and outbursts have been reported also on other types of stars, although confirmations are needed for some of them.Analyses of flare occurrence sometimes find flares to be randomly distributed in time, and sometimes indicate a tendency for flares to come in groups. Preferred active longitudes have been suggested. Recent solar results, where the occurrence rate for flares is found to exhibit a periodicity of 152 days, suggest that stellar flare data should be reanalyzed over long time baselines to see if the present confusing situation can be resolved.The radiation from stellar flares is dominated by continuum emission and about equal amounts of energy have been recorded in the optical, UV, and X-ray regions of the spectrum. In solar flares strong continuum emission is rarely recorded and a large collection of bright emission lines takes prominence. Small flares occur more frequently than large ones and the latter have longer time-scales. Flare energies can exceed 10³⁷ erg. The most productive flare stars are those where the convective envelopes occupy large volumes. Slow stellar rotation rates are believed to reduce the level when the star has been braked significantly from its young rotation rate.     

Plasma interactions of exoplanets with their parent star and associated radio emissions

The relatively high contrast between planetary and solar low-frequency radio emissions suggests that the low-frequency radio range may be well adapted to the direct detection of exoplanets. We review the most significant properties of planetary radio emissions (auroral as well as satellite induced) and show that their primary engine is the interaction of a plasma flow with an obstacle in the presence of a strong magnetic field (of the flow or of the obstacle). Scaling laws have been derived from solar system planetary radio emissions that relate the emitted radio power to the power dissipated in the various corresponding flow–obstacle interactions. We generalize these scaling laws into a “radio-magnetic” scaling law that seems to relate output radio power to the magnetic energy flux convected on the obstacle, this obstacle being magnetized or unmagnetized. Extrapolating this scaling law to the case of exoplanets, we find that hot Jupiters may produce very intense radio emissions due to either magnetospheric interaction with a strong stellar wind or to unipolar interaction between the planet and a magnetic star (or strongly magnetized regions of the stellar surface). In the former case, similar to the magnetosphere–solar wind interactions in our solar system or to the Ganymede–Jupiter interaction, a hecto-decameter emission is expected in the vicinity of the planet with an intensity possibly 10³–10⁵ times that of Jupiter's low frequency radio emissions. In the latter case, which is a giant analogy of the Io–Jupiter system, emission in the decameter-to-meter wavelength range near the footprints of the star's magnetic field lines interacting with the planet may reach 10⁶ times that of Jupiter (unless some “saturation” mechanism occurs). The system of HD179949, where a hot spot has been tentatively detected in visible light near the sub-planetary point, is discussed in some details. Radio detectability is addressed with present and future low-frequency radiotelescopes. Finally, we discuss the interests of direct radio detection, among which access to exoplanetary magnetic field measurements and comparative magnetospheric physics.     

Long-term INTEGRAL and RXTE observations of the X-ray pulsar LMC X-4

We analyze the observations of the X-ray pulsar LMCX-4 performed by the INTEGRAL observatory and the All-Sky Monitor (ASM) of the RXTE observatory over a wide energy range. The observed hard X-ray flux from the source is shown to change by more than a factor of 50 (from ～70 mCrab in the high state to ～1.3 mCrab in the low state) on the time scale of the accretion-disk precession period, whose mean value for 1996–2004 was determined with a high accuracy, P_prec = 30.275 ± 0.004 days. In the low state, a flare about 10 h in duration was detected from the source; the flux from the source increased by more than a factor of 4 during this flare. The shape of the pulsar’s broadband spectrum is essentially invariable with its intensity; no statistically significant features associated with the possible resonance cyclotron absorption line were found in the spectrum of the source.