X-ray bright point flares due to magnetic reconnection

Ground-based optical observations coordinated with Yohkoh/SXT X-ray observations of an old, disintegrating bipolar active region AR NOAA 7493 (May 1, 1993) provided a multiwavelength data base to study a flaring active region X-ray bright point (XBP) of about 16 hr lifetime, and the activity related to it in different layers of the solar atmosphere. The XBP appeared to be related to a new minor bipole of about 10²⁰ Mx. Superposed on a global evolution of soft X-ray brightness, the XBP displayed changes of brightness, lasting for 1–10 min. During the brightenings the XBP apparently had a spatial structure, which was (tiny) loop-like rather than point-like. The X-ray brightenings were correlated with chromospheric activity: (i) brightenings of underlying chromospheric faculae, and (ii) appearance of strong turbulent velocities in the arch filament system. We propose that the XBP brightenings were due to reconnection of the magnetic field lines (sketched in 3D) between the new bipole and a pre-existing plage field induced by the motion of one of the new pores (v = 0.2 km s^–1) towards the plage, and that the XBP itself was a reconnected hot loop between them.     

Distribution of lifetimes for coronal soft X-ray bright points

We have measured the lifetimes of all compact emission features visible on three sets of high time resolution soft X-ray images. The spectrum of lifetimes is found to be heavily weighted toward short lifetimes. The number of features present on the disk which live 2–48 hours is at least ten times as great as the number living more than 48 hours. The distribution of lifetimes can be fit in all three cases by a four-parameter function N(t) = N _s exp(-t/ _s ) + N _L exp(-t/ _L ), with _s = 8.7±0.2, _L = 35±4 and N _s 10N _L . Features living two days or less have a very broad latitude distribution (Golub et al., 1974, 1975) whereas nearly all longer-lived features are found within 30° of the equator. The growth rates of long-lived vs short-lived points are the same to within 20%, the major difference being that long-lived points continue to grow and generally reach larger sizes.Harvard College Observatory/Smithsonian Astrophysical Observatory.     

Short-term temporal variations of X-ray bright points

Skylab S-054 data have been used to examine the flux from X-ray bright points with 90 s time resolution. There is evidence of a steady heating input, similar to one reported for active region loops. Also observed are impulsive brightenings of bright points and rapid decays which are consistent with a sudden turn-off of the steady heating.     

The three-dimensional structures of X-ray bright points

Recently, the Converging Flux Model has been proposed for X-ray bright points and cancelling magnetic features. The aim of this peice of work is to try and model theoretically specific X-ray bright points in the framework of the Converging Flux Model. The observational data used includes a magnetogram showing the normal component of the magnetic field at the photosphere and a high-resolution soft X-ray image from NIXT showing the brightenings in the lower solar corona. By approximating the flux concentrations in the magnetograms with poles of the appropriate sign and sense, the overlying three-dimensional potential field structure is calculated. Deduction of plausible motions of the flux sources are made which produce brightenings of the observed shape due to reconnection between neighbouring flux regions. Also the three-dimensional separarix and separator structure and the way the magnetic field lines reconnect in three dimensions is deduced.     

On the lifetime of bright X-ray sources

The lifetime of massive X-ray binaries is (2–5)×10⁵ yr, this time close to the nuclear one. The lifetime of nonmassive X-ray binaries close to thermal one, (0.5–1)×10⁷ yr. Massive systems may be conserved at supernova explosion, the probability of the conservation of nonmassive system is (1–3)×10^–3.     

On the lifetime of bright X-ray sources

The lifetime of massive X-ray binaries is about 2–5×10⁵ yr, close to the nuclear time scale. The lifetime of nonmassive X-ray binaries close to the thermal one is about 0.5–1×10⁷ yr. Massive systems may be conserved at supernova explosion, the probability of the conservation of nonmassive systems being 1–3×10^–3.     

The birthplaces of active regions and X-ray bright points

A comparison of soft X-ray pictures of the Sun (S-054 experiment of Skylab) with K-line spectroheliograms (Mount Wilson) shows that the X-ray bright points tend to emerge randomly throughout the Ca network pattern. However, all those bright points that developed into active regions emerged at the boundaries of network cells. This suggests that the magnetic flux of active regions comes from greater depths in the convection zone than the shallow flux that gives rise to the random emergence of bright points.Also Physics Department, C-011, University of California at San Diego, La Jolla, Calif., U.S.A.     

X-ray bright points,coronal heating and the solar cycle

Recent Skylab observations about the bright points in the solar X-ray images seem to confirm an essential prediction of a model proposed by this author for the appearance and the disappearance of the photospheric fields during a solar cycle.The segments of the individually rising strands of the fundamental flux-loops proposed in the model may lead to the X-ray bright points with the observed properties.The emergence of such strands may substantially contribute to the coronal heating at different heights.     

A two-component thermal model of X-ray burst sources

Solar Physics - A semi-empirical model of the soft X-ray source associated with solar flares is presented, based on the results of the two temperature analysis of OSO-5 data obtained by Herring and...     

X-ray bright points and the sunspot cycle: Further results and predictions

X-ray images obtained during two rocket flights near the maximum of sunspot cycle 21 now allow the study of the variation of X-ray bright point number over an eleven-year period covering the maxima of the last two cycles. The new data are consistent with the earlier conclusion that the temporal variation of bright point and sunspot number are out of phase. The quantities are related through a power law with a negative exponent of 2/3.     

Continuous energy injection at numerous bright points during soft X-ray flare enhancement

The common assumption that a single volume of plasma produces X-ray emission during solar flares is difficult to reconcile with the very complex structure observed in H spectroheliograms. Nevertheless such a model has been considered reasonably adequate because the rapid rise and slow decay of emission has approximately the form expected when a plasma volume, isolated thermally from its surroundings, is heated quickly in the early stages of the event.Data presented in this paper show that a number of secondary peaks in intensity are usually observed throughout the soft X-ray emission. These can be explained by a model in which the X-ray emission comes from many relatively short-lived volumes of hot plasma. It appears that the lifetime of each such region is about five minutes, while the H spectroheliograms suggest that they are spaced throughout the extended region of activity. The relatively long period of flare activity arises because new volumes of plasma appear as others decay. Since each of these regions has to be energized during its development it is concluded that energy is released throughout the period of the X-ray flare.On leave of absence from the Mullard Space Science Laboratory, University College London WC1E-6BT.     

A cocoon model for thermal X-ray sources and ‘Oscillars’

A gas cocoon surrounding a neutron star can be heated to a high temperature by the low frequency radiation emitted by the neutron star whose rotation axis is inclined to its magnetic axis. This heated gas can emit X-rays and may be identified with thermal X-ray sources. If the neutron star emission shows periodicities larger than the cooling time of the gas, these will be reflected in the emission of X-ray; the recently observed X-ray sources which show oscillations and quasiperiodicities (Oscillars) may be such sources.     

Long term optical variability of bright X-ray point sources in elliptical galaxies

We present long term optical variability studies of bright X-ray sources in four nearby elliptical galaxies with the Chandra Advanced CCD Imaging Spectrometer array(ACIS-S) and observations from the Hubble Space Telescope(HST) Advanced Camera for Surveys.Out of the 46 bright(X-ray counts 60)sources that are in the common field of view of the Chandra and HST observations,34 of them have potential optical counterparts,while the rest of them are optically dark.After taking into account systematic errors,estimated using optical sources in the field as a reference,we find that four of the X-ray sources(three in NGC 1399 and one in NGC 1427) have variable optical counterparts at a high level of significance.The X-ray luminosities of these sources are ~ 10~(38) erg s~(-1) and are also variable on similar time scales.The optical variability implies that the optical emission is associated with the X-ray source itself rather than being the integrated light from a host globular cluster.For one source,the change in optical magnitude is 0.3,which is one of the highest reported for this class of X-ray sources and this suggests that the optical variability is induced by the X-ray activity.However,the optically variable sources in NGC 1399 have been reported to have blue colors(g- z 1).All four sources have been detected in the infrared(IR) by Spitzer as point sources,and their ratios of 5.8 to 3.6 μm flux are 0.63,indicating that their IR spectra are like those of Active Galactic Nuclei(AGNs).While spectroscopic confirmation is required,it is likely that all four sources are background AGNs.We find none of the X-ray sources having optical/IR colors different from AGNs to be optically variable.     

Solar cycle model for oscillating transient X-ray sources

The oscillations observed recently in the transient X-ray source A1118-61 are attributed to variations in the magnetic activity of a rotating white dwarf similar to the 11-year solar activity cycle. The transient nature, in analogy with the sun, is attributed to the variation in the strength of the cycle.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Electrodynamics of the EUV/X-ray bright point and filamentary flux loop complexes

The electrodynamic model of generation of electric currents in the solar atmosphere, by means of twisting of emerging magnetic flux loops, is investigated with emphasis on the small-scaled EUV/X-ray bright points. It is found that the corresponding power input from such conversion of kinetic energy of the turbulent photospheric plasma into magnetic energy could amount to about 25% of the total energy flux of the solar wind and solar radiation. However, if similar filamentary structures containing colder material are formed in abundance, the total energy budget would be correspondingly larger and the resulting mass injection phenomena may be related to the so-called coronal bullets observed in UV. These energetic features suggest that the coronal dynamics and heating could be dictated by plasma structures with angular sizes <0.1–1. The Solar and Heliospheric Observatory (SOHO) mission will be essential in addressing these issues basic to solar corona and solar wind acceleration.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Rapid flux variations in compact radio sources: A shell model

In this paper, we discuss the flux variations of a shell whose expansion and bulk-motion are both at relativistic speeds. It is shown that superluminal flux variations can be produced even if the Lorentz factors of expansion and bulk-motion take moderate values (2–3), and that this kind of shell model suggested here could be significant for interpreting very rapid flux variations at centimeter wavelengths (time scale of 1–10 days) in some radio variable sources.     

The eccentric-orbit binary model for the transient X-ray sources

Eccentric-orbit binary models for transient X-ray sources are investigated. In these models, a compact star is in an eccentric orbit around a more massive star. As the compact star accretes mass from the stellar wind of the massive star, the accretion rate becomes time-dependent. The accretion rate is determined by Bondi's accretion radius, which depends on both the relative velocity of the stellar wind to the compact star and the sound velocity through the stellar wind. With reasonable sets of the eccentricity, the semi-major axis, the stellar wind velocity and the sound velocity, we obtain the variations of the light curves compatible with observations for the transient X-ray sources. It is likely that many transient X-ray sources are explainable by eccentric-orbit binary models.     

A study of coronal bright points at 20 cm wavelength

We present the results of a study of coronal bright points observed at 20 cm with the VLA on a day when the Sun was exceptionally quiet. Microwave maps of bright points were obtained using data for the entire observing period of 5 hours, as well as for shorter periods of a few minutes. Most bright points, especially those appearing in the full-period maps appear to be associated with small bipolar structures on the photospheric magnetogram. Overlays of bright point (B.P.) maps on the Ca⁺K picture, show that the brightest part of a B.P. tends to lie on the boundary of a supergranulation network. In general, the bright points exhibit rapid variations in intensity. There is no systematic correlation between the size of a B.P. and its intensity; the apparently slow variation of B.P. emission may have rapid fluctuations superimposed on it.     

Two X-ray sources model of Cyg X-3

A model of Cyg X-3, as a binary cocooned star system with two sources of X-rays, one above the polar caps of the neutron star — the usual pulsar radiation — and the other around the equatorial plane of the magneto-bounding surface formed due to the interaction of the infalling plasma and the magnetic field of the neutron star, is made. The X-ray, -ray, and IR radiation light curves are considered from the shadow effect. An upper limit on the mass of the neutron star is estimated from the consideration of periodic derivative purely due to mass loss. A comparison is made with the results of Elsneret al. (1980) and Ghoshet al. (1981), which they derived from the consideration of period derivative purely from apsidal motion.     

A theory for gamma ray and X-ray burst sources

A common mechanism for both X-ray and-ray bursters is proposed on the basis that a window can be created transiently in the polar cap of a degenerate star, a white dwarf for X-ray bursts and a neutron star for-ray bursts. The window exposes transiently a hot degenerate sub-layer of the star at shallow depth, from which escapes blackbody flux for a source at temperature 3 kev with window radius 10 km in the case of X-ray bursts and for a source at temperature 300 kev with window radius 0.5 km in the case of-ray bursts.