Heating rates in X-ray flares: From solar microflares to stellar events

Soft X-ray solar and stellar flares appear in the coronae of solar-like stars due to abrupt release of energy accumulated in magnetic fields. To build a quantitatively correct model of a flare we need to know how much energy is released in flares of different sizes and strengths. Here we estimate and compare the energy release rate in flares as different as microflares occurring over the quiet Sun and strong stellar events in RS CVn systems. We find one simple scaling law which describes flares differing one from another by 10 orders of magnitude in the amount of emission measure.     

Exosat observations of quiescent and flaring emission from active late-type stars

We present the highlights of several Guest Investigator programs carried out with the EXOSAT satellite. We discuss quiescent as well as flaring emission from normal F to M dwarfs, from dMe flare stars and from RS CVn binaries.     

A uvbyβ survey of northern-hemisphere active binaries

A recent detailed calibration of the uvby photometric system for late-type stars has opened the possibility to study photometric effects of stellar activity in the RS CVn grouup of binaries. In this communication, we present preliminary results for uvby Stromgren and Hβ photometry measurements of binary stars showing enhanced chromospheric activity and currently included among the RS CVn-like stars. It is shown that luminosity classes can be separated in terms of the uvby colours and that there is a general tendency for active stars to have smaller m₁ values than expected from standard relations. This δm₁ effect can be related to the degree of activity rather than to the actual metal content of their atmospheres.     

Analysis of activity indicators in two RS CVn systems

In order to contribute to the understanding of the activity phenomena in late-type stars, we have obtained new CaII H and K emission line fluxes and equivalent widths for two RS CVn binaries, namely LX Per and SZ Psc, observed in several orbital phases. Both systems show a large activity and a flare-like phenomenon has been detected in SZ Psc.     

Spectral and temporal studies of various late-type stars

The RS CVn stars Capella and σ² CrB have been measured with EXOSAT in soft and medium X-rays for about 24 hours each and the less active late-type star Procyon for about 6.5 hours. In addition, the RS CVn star γ. And was twice observed about one month apart for a total of about 7 hours, with the ME and the LE in the photometer mode only. All three RS CVn stars were detected with the ME-detector. The star σ² CrB showed a flare both in LE and ME with a rise time of about twelve minutes and a decay time of three hours. The active late-type stars σ² CrB and Capella show in the spectral region between 90 and 140 A lines from Fe XVIII to Fe XXIII, which can be resolved with the moderate resolution (Δγ ≈ 5 A) of the spectrometer. These lines are indicative of the presence of hot (≈ 10 MK) plasma like that in a Solar flare. In contrast, the spectrum of the cooler corona of the star Procyon does not show the hot Fe XXII and Fe XXIII lines but instead a blend at 175 A of Fe IX, X and XI lines that are formed in a typically quiet corona of a temperature around 1.5 MK. From the spectral intensities and the additional results of the simultaneous multi-color photometry coronal temperatures and emission measures are derived. There are indications in the spectra that the emission should be interpreted in terms of differential emission measure distribution models.     

Evolution of activity signatures during the main sequence phase

We review recent work on the decay of magnetic activity signatures, such as chromospheric/transition region/coronal emission as well as mean flare emission, with age for solar and later type stars. Both implicit and explicit evolutionary representations are discussed. In terms of magnetic flux, as measured by excess chromospheric CaII luminosity, we show that a simple dynamo-rotation relation that incorporates both a saturated state with its characteristic critical rotation (or scale factor) as well as an asymptotic linear power law, i.e. a scale free relation, fits the extant data that includes the dMe stars. Introducing the saturated dynamo state, as exemplified by the dMe stars, into activity power-power diagrams, allows us to not only specify the saturated state, but to define evolutionary tracks that represent the decay from the saturated state. Using the quiescent coronal X-ray power (luminosity) as a basic measure of magnetic activity, we find simple monomial relations for both the saturated state (linear) and for the evolutionary tracks governing both quiescent activity and mean flare activity. In particular, the coronal power loss is found to vary quadratically with the chromospheric power loss, hence with magnetic flux.     

X-ray diagnostics of globular clusters

The presence of compact X-ray sources in globular clusters allows diagnostic studies of both the X-ray sources themselves and the globular clusters to be carried out. A review of much of this work, primarily based on Einstein X-ray observations and supporting studies of globular clusters at radio through UV wavelengths, is presented. The compact X-ray sources in globular clusters are found to be compact binaries containing neutron stars and - in a separate lower luminosity component of an apparently bimodal luminosity function - possibly white dwarfs. Implications for the formation and evolution of compact binary X-ray sources in globular clusters and in the galactic bulge are discussed. In particular, new evidence is presented that the galactic bulge sources may be compact binaries in the remnants of disrupted globular clusters.     

A statistical study of post-flare-associated CME events

We present a statistical study of post-flare-associated CMEs (PFA-CMEs) during the period from 1996 to 2010. By investigating all CMEs and X-ray flares, respectively, in the LASCO and GOES archives, we found 15875 CMEs of which masses are well measured and 25112 X-ray flares of which positions are determined from their optical counterparts. Under certain temporal and spatial criteria of these CMEs and solar flare events, 291PFA-CMEs events have been selected. Linking the flare fluxes with CME speeds of these paired events, we found that there is a reasonable positive linear relation between the CME linear speed and associated flare flux. The results show also the CME width increases as the flux of its associated solar flare increases. Besides we found that there is a fine positive linear relation between the CME mass and its width. Matching the flare fluxes with CME masses of these paired events, we find the CME mass increases as the flux of its associated solar flare increases. Finally we find the PFA-CME events are in regular more decelerated than the other CMEs.     

Microwave observations of red dwarf flare stars

We discuss some recent observations of red dwarf flare stars. When observed over periods of about 8 hours, each of 4 flare star systems displayed at least one major flare at 20 cm. Quiescent emission at 6 cm was seen from UV Ceti and EQ Peg A, but flares were much less frequent at 6 cm than at 20 cm. We also summarize earlier observations of quiescent emission from UV Ceti. Observations of highly polarized flares with brightness temperatures in excess of 10¹⁰ K appear to be common on red dwarf stars. We have also found narrowband flares which strengthen the argument that a coheren emission mechanism is involved in these flares. One of those narrowband flares allows us to place severe constraints on conditions in the flare source, and if the flare is cyclotron maser emission it seems unlikely that magnetic reconnection is involved in the flare.     

CME–flare association during the 23rd solar cycle

The relation between coronal mass ejections (CMEs) and solar flares are statistically studied. More than 10,000 CME events observed by SOHO/LASCO during the period 1996–2005 have been analyzed. The soft X-ray flux measurements provided by the Geostationary Operational Environmental Satellite (GOES), recorded more than 20,000 flares in the same time period. The data is filtered under certain temporal and spatial conditions to select the CME–flare associated events. The results show that CME–flare associated events are triggered with a lift-off time within the range 0.4–1.0 h. We list a set of 41 CME–flare associated events satisfying the temporal and spatial conditions. The listed events show a good correlation between the CME energy and the X-ray flux of the CME–flare associated events with correlation coefficient of 0.76.     

Statistical study of twist values of transequatorial loops and the relationship with flares

In this paper, the twist values of ‘S’-shape transequatorial loops (TLs) from 1991 to 2001 are calculated, GOES soft X-ray flares dataset of the active regions connected by these TLs are investigated. The result shows the twist value of the TLs has a weak relation with the flare flux. There is no clear correlation between the twist value and the distance between the footpoint of TLs and location of flare in the corresponding active regions.     

The X-ray evolution of galaxies: implications for future X-ray observatories

The X-ray evolution of the luminosity of normal galaxies is primarily driven by the evolution of their X-ray binary populations. The imprints left by a cosmological evolution of the star formation rate (SFR) will cause the average X-ray luminosity of galaxies to appear higher in the redshift range 1–3. As reported by White and Ghosh [ApJ, 504 (1998) L31] the profile of X-ray luminosity with redshift can both serve as a diagnostic probe of the SFR profile and constrain evolutionary models for X-ray binaries. In order to observe the high redshift (z>3) universe in the X-ray band, it is necessary to avoid confusion from foreground field galaxies. We report on the predictions of these models of the X-ray flux expected from galaxies and the implications for the telescope parameters of future deep universe X-ray observatories.     

XMM-Newton observations of low luminosity Be/X-ray candidates

A small number of early Be stars exhibit X-ray luminosities intermediate between those typical of early type stars and those radiated by Be/X-ray binaries in the quiescent state. We report on XMM-Newton observations of two such Be stars, HD 161103 and SAO 49725 which were originally discovered in a systematic cross-correlation between the ROSAT all-sky survey and SIMBAD. The new observations confirm the X-ray luminosity detected by ROSAT (L_X  10³² erg s⁻¹) and the hardness of their X-ray spectra (thin thermal with kT  8–10 keV or power law with photon index of 1.7) which are both unusual for normal early type stars. We discuss the possible origin of this excess X-ray emission in the light of the models proposed for γ-Cas, magnetic disc-star interaction or accretion onto a compact companion object, neutron star or white dwarf, and compare the properties of these two sources with those of the new massive systems discovered in the XMM- Newton/SSC survey of the Galactic plane.     

应用高频多普勒方法对耀斑期间电离层TEC变化的估算

通过分析前人对耀斑期间电离层各区域电子密度的变化情况,总结了一个应用耀斑期间的高频多普勒扰动记录估算出层总电子含量变化的方法,并应用这一方法计算了１９９０年３－６月几次耀斑引起的低电离层总电子含量的变化,同时还与各耀斑对应的最大流量密度进行了比较,并对两者之间的相关情况进行了分析。     

LMXB luminosity function and connection to globular cluster in early type galaxies

We derive bias-corrected X-ray luminosity functions (XLFs) of LMXBs detected in 14 E and S0 galaxies observed with Chandra. After correcting for incompleteness, the individual XLFs are statistically consistent with a single power-law. A break at or near L_X,Eddington , as previously reported, is not required in any individual case. The combined XLF with a reduced error, however, suggests a possible break at L_X = 5 × 10³⁸ erg s⁻¹, which may be consistent with the Eddington luminosity of neutron stars with the largest possible mass (3 M), or of He-enriched neutron star binaries. We confirm that the total X-ray luminosity of LMXBs is correlated with the the near-IR luminosities, but the scatter exceeds that expected from measurement errors. The scatter in L_X(LMXB)/L_K appears to be correlated with the specific frequency of globular clusters, as reported earlier.

We cross-correlate X-ray binaries with globular clusters determined by ground-based optical and HST observations in 6 giant elliptical galaxies. With the largest sample reported so far (300 GC LMXBs with a 5:2 ratio between red and blue GCs), we compare their X-ray properties, such as X-ray hardness, XLF and L_X/L_B and find no statistically significance difference between different groups of LMXBs. Regardless of their association with GCs, both GC and field LMXBs appear to follow the radial profile of the optical halo light, rather than that of more extended GCs. This suggests that while metallicity is a primary factor in the formation of LMXBs in GCs, there may be a secondary factor (e.g., encounter rate) playing a non-negligible role.     

Einstein observations of cool stars

Observations of cool stars with the $\text{Einstein}$ Observatory (HEAO-2) have brought about a fundamental change in our knowledge and understanding of stellar coronae. The existence of X-ray emission from stars throughout the H-R diagram, the wide range of X-ray luminosity within a given spectral and luminosity class, and the strong correlation of X-ray luminosity with stellar age and rotation are among the more significant $\text{Einstein}$ results. These results are strong evidence for the influence of stellar dynamo action on the formation and heating of stellar coronae. A discussion of relevant consortium and guest observations will be given. The Hyades cluster, in particular, will serve as an example to demonstrate the usefulness of X-ray observations in the study of stellar activity and coronal evolution.     

Stellar activities and their relation to dynamo mechanism

Stellar X-ray and radio activities are discussed in connection with the magnetic field and the dynamo mechanism. It is claimed that activities of stars of different species can be quite different from those of the sun-like stars, and in some cases, they are not related to the solar-like dynamo process betraying a simple-minded guess. The cases of very young stars and of early type stars are discussed as examples of these. In contrast, the case of RS CVn stars is discussed as a case in which the action of the solar-like dynamo and the activities related to it may be drastically amplified due to the enhanced differential rotation between the synchronously rotating envelopes and the more-or-less inertially rotating cores in both of the component stars. It is argued that the relaxation and dissipation of the energy built up in the form of the twist of the magnetic ropes created in the sheared layers between the cores and envelopes may be the source of energy of their activities. The magnetic twist in the loops emerged in the active longitude belt relaxes dynamically through magnetic reconnections into the large scale loops interconnecting two stars, and hot loops of the binary scale are formed as the result of the shock heating as well as of the dynamical dissipation of the magnetic twist.     

A statistical study of CME-associated flare during the solar cycle 24

We investigate on the relationship between flares and coronal mass ejections (CMEs) in which a flare started before and after the CME events which differ in their physical properties, indicating potentially different initiation mechanisms. The physical properties of two types flare-correlated CME remain an interesting and important question in space weather. We study the relationship between flares and CMEs using a different approach requiring both temporal and spatial constraints during the period from December 1, 2008 to April 30, 2017 in which the CMEs data were acquired by SOHO/LASCO (Solar and Heliospheric Observatory/Large Angle Spectrometric Coronagraph) over the solar cycle 24. The soft X-ray flare flux data, such as flare class, location, onset time and integrated flux, are collected from Geostationary Environmental satellite (GOES) and XRT Flare catalogs. We selected 307 CMEs-flares pairs applying simultaneously temporal and spatial constraints in all events for the distinguish between two associated CME-flare types. We study the correlated properties of coincident flares and CMEs during this period, specifically separating the sample into two types: flares that precede a CME and flares that follow a CME. We found an opposite correlation relationship between the acceleration and velocity of CMEs in the After- and Before-CMEs events. We found a log-log relation between the width and mass of CMEs in the two associated types. The CMEs and flares properties show that there were significant differences in all physical parameters such as (mass, angular width, kinetic energy, speed and acceleration) between two flare-associated CME types.     

Evolution of X-ray binaries: Achievements and advances

Three recent developments in the field of formation and evolution of neutron stars and black holes in binaries are addressed:

• The finding that there is a class of neutron stars, formed in interacting binaries, that do not receive kick velocities in their birth events. This finding is particularly important for our understanding of the formation – and formation rates – of double neutron stars. It is argued that these low-kick neutron stars, which tend to have low masses, are formed by a different physical mechanism than the neutron stars that receive large kick velocities at birth.

• The occurrence of velocity kicks in the formation events of stellar black holes.

• The nature of the companions of millisecond X-ray pulsars.

Solar and late-type dwarfs

The Einstein Observatory and the IUE satellite have provided the observational basis for a major restructuring in theories of coronal formation for late-type stars. For the first time, coronal and transition region emission from a large sample of low mass (1 M_o) dwarf stars has been directly observed, with the unexpected result that essentially all such stars are x-ray emitters. The Sun, which was previously assumed to be typical, is now known to be at the low end of the x-ray luminosity function for solar-type stars. K- and M-dwarfs are observed to have nearly the same luminosity distributions as G-dwarfs and all of these spectral types have a large spread in x-ray luminosity.Observationally, there is a strong correlation between the strength of coronal emission in stars with outer convective zones and the rotation rates of these stars. At the present time we have only the beginnings of a satisfactory theoretical explanation for this correlation; although we are beginning to understand the connection between coronal emission strength and the magnetic field, we do not yet understand the stellar dynamo which generates the magnetic field. Studies of the coronal emission of stars may lead to a better understanding of stellar dynamos.