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1.
The results of U -filter flare monitoring of the binary flare star FL Vir = Wolf 424 is presented. 57 flares with energies between  2 × 1028  and  2 × 1031 erg  were recorded in 20 h of observation. The properties of flare occurrence and flare time-scales are analysed, and the flare activity level in 1980 April is determined to be   L f( U ) = 8.0 × 1026 erg s−1  . This is larger than previously published results and may indicate a variation in the flare activity level on a time-scale of years. An analysis of existing data indicates that the flare activity level correlates with the relative orbital positions of the stars.  相似文献   

2.
Simultaneous X-ray and extreme ultraviolet (EUV) ( ROSAT XRT and WFC All-Sky Survey) observations of the highly active dMe flare stars YY Gem and AU Mic show that the two stars displayed an unusual type of flaring behaviour. We detect several X-ray and EUV flares superimposed on an enhanced and smoothly varying quiescent background. The two large impulsive-type X-ray flares on YY Gem reach peak X-ray luminosities of     and we estimate that they had similar integrated luminosities (∼6–8×1033 erg). AU Mic also produced several X-ray and EUV flares, with one very impulsive flare producing a 10-fold increase in XRT count rate. This flare was even larger than the YY Gem flares (peak L X of     and integrated L X of    
The     ratio for both stars is at the 'saturation' limit found in rapidly rotating dwarfs and the most active RS CVn stars. We suggest that the gradually varying components are the result of a period of continuous, unresolved flaring activity. Alternatively, they may be the result of the emergence and subsequent decay of a new magnetic active region on the stellar surface of these stars.  相似文献   

3.
We report the serendipitous discovery of a flare star observed with the ROSAT X-ray observatory. From optical spectra, which show strong and variable emission lines of the hydrogen Balmer series and neutral helium, we classify this object as a M3.0Ve star, and estimate a distance of 52 pc from published photometry. Owing to the close proximity of the star (13.6 arcmin) to the calibration source and RS CVn binary AR Lacertae, long-term X-ray coverage is available in the ROSAT archive (∼50 h spanning 6.5 yr). Two large flare events occurred early in the mission (1990 June–July), and the end of a third flare was detected in 1996 June. One flare, observed with the Position Sensitive Proportional Counter (PSPC), had a peak luminosity L X=1.1×1030 erg s−1, an e-folding rise time of 2.2 h and a decay time of 7 h. This decay time is one of the longest detected on a dMe star, providing evidence for the possibility of additional heating during the decay phase. A large High Resolution Imager (HRI) flare (peak L X=2.9×1030 erg s−1) is also studied. The 'background' X-ray emission is also variable – evidence for low-level flaring or microflaring. We find that 59 per cent of the HRI counts and 68 per cent of the PSPC counts are caused by flares. At least 41 per cent of the HRI exposure time and 47 per cent of the PSPC are affected by detectable flare enhancement.  相似文献   

4.
We have investigated multiband optical photometric variability and stability of the Hα line profile of the transient X-ray binary IGR J01583+6713. We set an upper limit of 0.05 mag on photometric variations in the V band over a time-scale of three months. The Hα line is found to consist of non-Gaussian profile and quite stable for a duration of two months. We have identified the spectral type of the companion star to be B2 IVe while the distance to the source is estimated to be ∼4.0 kpc. Along with the optical observations, we have also carried out analysis of X-ray data from three short observations of the source, two with the Swift –XRT and one with the RXTE –PCA. We have detected a variation in the absorption column density, from a value of  22.0 × 1022 cm−2  immediately after the outburst down to  2.6 × 1022 cm−2  four months afterwards. In the quiescent state, the X-ray absorption is consistent with the optical reddening measurement of   E ( B − V ) = 1.46  mag. From one of the Swift observations, during which the X-ray intensity was higher, we have a possible pulse detection with a period of 469.2 s. For a Be X-ray binary, this indicates an orbital period in the range of 216–561 d for this binary system.  相似文献   

5.
We examine a binary merger model for the formation of the mysterious triple-ring nebula surrounding Supernova 1987A, which still has not been convincingly explained in detailed hydrodynamical calculations. During the merger of 15 and  5 M⊙  binary systems, mass is ejected primarily at mid-latitudes for a sufficiently evolved primary, as demonstrated by Morris & Podsiadlowski. This material is swept up by the fast wind of the central star during its post-merger blue supergiant phase, leading to a density contrast of ∼150 in the outer rings at the time of the supernova. The equatorial ring probably formed later when the star contracted to become a blue supergiant. The asymmetry between the northern and southern outer rings can be explained by a 10 per cent asymmetry during the merger, perhaps due to a pulsational instability in the common envelope.
We present a parameter study from which we determine a mass-loss rate in the blue supergiant wind in the range  1.5–3 × 10−7 M⊙ yr−1  in agreement with previous estimates. The morphology of the best model is consistent with the well-known Hubble Space Telescope image at better than 5 per cent and is also in broad agreement with light-echo observations. The circumstellar environment on larger scales (up to 3 pc) is also investigated. We conclude with a brief discussion of the bipolar nebulae surrounding the Galactic stars, Sheridan 25, HD 168625 and η Carinae.  相似文献   

6.
We present high- and medium-resolution spectroscopic observations of the cataclysmic variable BF Eridani (BF Eri) during its low and bright states. The orbital period of this system was found to be 0.270881(3) d. The secondary star is clearly visible in the spectra through the absorption lines of the neutral metals Mg  i , Fe  i and Ca  i . Its spectral type was found to be K3±0.5. A radial velocity study of the secondary yielded a semi-amplitude of   K 2= 182.5 ± 0.9 km s−1  . The radial velocity semi-amplitude of the white dwarf was found to be   K 1= 74 ± 3 km s−1  from the motion of the wings of the Hα and Hβ emission lines. From these parameters, we have obtained that the secondary in BF Eri is an evolved star with a mass of  0.50–0.59 M⊙  , whose size is about 30 per cent larger than a zero-age main-sequence single star of the same mass. We also show that BF Eri contains a massive white dwarf  ( M 1≥ 1.2 M⊙)  , which allows us to consider the system as a Type Ia supernova progenitor. BF Eri also shows a high γ-velocity  (γ=−94 km s−1)  and substantial proper motion. With our estimation of the distance to the system  ( d ≈ 700 ± 200 pc)  , this corresponds to a space velocity of ∼350 km s−1 with respect to the dynamical local standard of rest. The cumulative effect of repeated nova eruptions with asymmetric envelope ejection might explain the high space velocity of the system. We analyse the outburst behaviour of BF Eri and question the current classification of the system as a dwarf nova. We propose that BF Eri might be an old nova exhibiting 'stunted' outbursts.  相似文献   

7.
We have observed a number of minor radio flares in Cyg X-3 using the MERLIN array. Photometric observations show the system to be highly active with multiple flares on hourly time-scales over the one month observing programme. Analysis of the power spectrum of the source show no persistent periodicities in these data, and no evidence of the 4.8-h orbital period. An upper limit of 15 mJy can be placed on the amplitude of any sinusoidal variation of source flux at the orbital period. The brightness temperature of a flare is typically T b≥109–1010 K , with a number of small flares of 5-min duration having brightness temperatures of T b≥ few×1011 K . For such a change in flux to occur within a typical 10-min time-scale, the radiation must originate from plasmons with a size ≤1.22 au. This emission is unlikely to originate close to the centre of the system as both the jets and compact object are buried deep within an optically thick stellar wind. Assuming a spherically symmetric wind, plasmons would become visible at distances ∼13 au from the core.  相似文献   

8.
The binary companion to the peculiar F supergiant HD 172481 is shown to be a Mira variable with a pulsation period of 312 d. Its characteristics are within the normal range found for solitary Miras of that period, although its pulsation amplitude and mass-loss rate ̇ ∼3×10−6 M⊙ yr−1 are higher than average. Reasons are given for suspecting that the F supergiant, which has L ∼104 L⊙, is a white dwarf burning hydrogen accreted from its companion.  相似文献   

9.
We report on new X-ray outbursts observed with Swift from three Supergiant Fast X-ray Transients (SFXTs): XTE J1739−302, IGR J17544−2619 and IGR J08408−4503. XTE J1739−302 underwent a new outburst on 2008 August 13, IGR J17544−2619 on 2008 September 4 and IGR J08408−4503 on 2008 September 21. While the XTE J1739−302 and IGR J08408−4503 bright emission triggered the Swift /Burst Alert Telescope, IGR J17544−2619 did not, thus we could perform a spectral investigation only of the spectrum below 10 keV. The broad-band spectra from XTE J1739−302 and IGR J08408−4503 were compatible with the X-ray spectral shape displayed during the previous flares. A variable absorbing column density during the flare was observed in XTE J1739−302 for the first time. The broad-band spectrum of IGR J08408−4503 requires the presence of two distinct photon populations, a cold one (∼0.3 keV) most likely from a thermal halo around the neutron star and a hotter one (1.4–1.8 keV) from the accreting column. The outburst from XTE J1739−302 could be monitored with a very good sampling, thus revealing a shape which can be explained with a second wind component in this SFXT, in analogy to what we have suggested in the periodic SFXT IGR J11215−5952. The outburst recurrence time-scale in IGR J17544−2619 during our monitoring campaign with Swift suggests a long orbital period of ∼150 d (in a highly eccentric orbit), compatible with what previously observed with INTEGRAL .  相似文献   

10.
Using the MegaCam imager on the Canada–France–Hawaii Telescope, we have resolved individual stars in the outskirts of the nearby large spiral galaxy M81 (NGC 3031) well below the tip of the red giant branch of metal-poor stellar populations over  ∼60 × 58 kpc2  . In this paper, we report the discovery of new young stellar systems in the outskirts of M81. The most prominent feature is a chain of clumps of young stars distributed along the extended southern H  i tidal arm connecting M 81 and NGC 3077. The colour–magnitude diagrams of these stellar systems show plumes of bright main sequence stars and red supergiant stars, indicating extended events of star formation. The main sequence turn-offs of the youngest stars in the systems are consistent with ages of ∼40 Myr. The newly reported stellar systems show strong similarities with other known young stellar systems in the debris field around M81, with their properties best explained by these systems being of tidal origin.  相似文献   

11.
We present near-infrared polarimetric images of the dusty circumstellar envelope (CSE) of IRAS 19306+1407, acquired at the United Kingdom Infrared Telescope (UKIRT) using the UKIRT 1–5 μm Imager Spectrometer (UIST) in conjunction with the half-waveplate module IRPOL2. We present additional 450- and 850-μm photometry data obtained with the Submillimetre Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), as well as archived Hubble Space Telescope ( HST ) F606W - and F814W -filter images. The CSE structure in polarized flux at J and K bands shows an elongation north of north-east and south of south-west with two bright scattering shoulders north-west and south-east. These features are not perpendicular to each other and could signify a recent 'twist' in the outflow axis. We model the CSE using an axisymmetric light scattering ( als ) code to investigate the polarization produced by the CSE, and an axisymmetric radiation transport ( dart ) code to fit the spectral energy distribution. A good fit was achieved with the als and dart models using silicate grains, 0.1–0.4 μm with a power-law size distribution of a −3.5, and an axisymmetric shell geometry with an equator-to-pole ratio of 7:1. The spectral type of the central star is determined to be B1 i supporting previous suggestions that the object is an early planetary nebula. We have constrained the CSE and interstellar extinction as 2.0 and 4.2 mag, respectively, and have estimated a distance of 2.7 kpc. At this distance, the stellar luminosity is ∼4500 L⊙ and the mass of the CSE is ∼0.2 M⊙. We also determine that the mass loss lasted for ∼5300 yr with a mass-loss rate of ∼3.4 × 10−5 M⊙ yr−1.  相似文献   

12.
We used the detected pulsation modes and adiabatic pulsation models to do seismology of the class of ZZ Ceti stars and measure the H layer mass for 83 stars. We found the surface hydrogen layer to be within the range  10−9.5≥ M H/ M *≥ 10−4  , with an average of   M H/ M *= 10−6.3  , which is thinner than the predicted value of   M H/ M *= 10−4  , indicating that the stars lose more mass during their evolution than previously expected. These results are preliminary and do not include the possible effects of realistic C/O profiles on the fits.  相似文献   

13.
The timing properties of the 4.45 s pulsar in the Be X-ray binary system GRO J1750−27 are examined using hard X-ray data from INTEGRAL and Swift during a type II outburst observed during 2008. The orbital parameters of the system are measured and agree well with those found during the last known outburst of the system in 1995. Correcting the effects of the Doppler shifting of the period, due to the orbital motion of the pulsar, leads to the detection of an intrinsic spin-up that is well described by a simple model including     and     terms of  −7.5 × 10−10 s s−1  and  1 × 10−16 s s−2  , respectively. The model is then used to compare the time-resolved variation of the X-ray flux and intrinsic spin-up against the accretion torque model of Ghosh & Lamb; this finds that GRO J1750−27 is likely located 12–22 kpc distant and that the surface magnetic field of the neutron star is  ∼2 × 1012  G. The shape of the pulse and the pulsed fraction shows different behaviour above and below 20 keV, indicating that the observed pulsations are the convolution of many complex components.  相似文献   

14.
New optical spectroscopy of the high-mass X-ray binary microquasar LS I +61 303 is presented. Eccentric orbital fits to our radial velocity measurements yield updated orbital parameters in good agreement with previous work. Our orbital solution indicates that the periastron passage occurs at radio phase 0.23 and the X-ray/radio outbursts are triggered 2.5–4 d after the compact star passage. The spectrum of the optical star is consistent with a B0 V spectral type and contributes ∼65 per cent of the total light, the remainder being the result of emission by a circumstellar disc. We also measure the projected rotational velocity to be   v sin  i ≃ 113 km s−1  .  相似文献   

15.
Two nights of phase-resolved medium-resolution Very Large Telescope spectroscopy of the extra-galactic low-mass X-ray binary LMC X−2 have revealed a 0.32 ± 0.02 d spectroscopic period in the radial velocity curve of the He  ii λ4686 emission line that we interpret as the orbital period. However, similar to previous findings, this radial velocity curve shows a longer term variation that is most likely due to the presence of a precessing accretion disc in LMC X−2. This is strengthened by He  ii λ4686 Doppler maps that show a bright spot that is moving from night to night. Furthermore, we detect narrow emission lines in the Bowen region of LMC X−2, with a velocity of   K em= 351 ± 28 km s−1  , that we tentatively interpret as coming from the irradiated side of the donor star. Since K em must be smaller than K 2, this leads to the first upper limit on the mass function of LMC X−2 of   f ( M 1) ≥ 0.86  M⊙  (95 per cent confidence), and the first constraints on its system parameters.  相似文献   

16.
We have used the RXTE and INTEGRAL satellites simultaneously to observe the high-mass X-ray binary (HMXB) IGR J19140+0951. The spectra obtained in the 3–80 keV range have allowed us to perform a precise spectral analysis of the system along its binary orbit. The spectral evolution confirms the supergiant nature of the companion star and the neutron star nature of the compact object. Using a simple stellar wind model to describe the evolution of the photoelectric absorption, we were able to restrict the orbital inclination angle in the range 38°–75°. This analysis leads to a wind mass-loss rate from the companion star of  ∼5 × 10−8 M⊙ yr−1  , consistent with an OB I spectral type. We have detected a soft excess in at least four observations, for the first time for this source. Such soft excesses have been reported in several HMXBs in the past. We discuss the possible origin of this excess, and suggest, based on its spectral properties and occurrences around the superior conjunction, that it may be explained as the reprocessing of the X-ray emission originating from the neutron star by the surrounding ionized gas.  相似文献   

17.
We present CCD photometry in the Johnson U , B and V and Kron–Cousins I passbands for the open cluster NGC 2587. The sample consists of 4406 stars reaching down to   V ∼ 21.0  . We developed a new method to clean statistically the colour–magnitude diagrams. NGC 2587 appears to be a sparse, relatively bright open cluster, with a few tens of members projected on to a populous star field. The comparatively bright F7/8 II type star HD 70927, located close to the cluster centre, seems not to be a member. Our analysis suggests that NGC 2587 is slightly younger than the Hyades and probably of solar metallicity. A cluster radius of roughly 8 arcmin was estimated from the radial stellar density profile. From 18 probable cluster members with measured proper motions, we derive the following mean values for NGC 2587:  μα=−4.3 ± 3.6 mas yr−1  and  μδ=−2.5 ± 3.4 mas yr−1  . Adopting the theoretical metal content   Z = 0.02  , which provides the best global fit, we derive a cluster age of  500+60−50  . Simultaneously, colour excesses   E ( B − V ) = 0.10  and   E ( V − I ) = 0.15  and an apparent distance modulus of   V − MV = 12.50  are obtained. The interstellar extinction in the cluster direction is found to follow the normal law. NGC 2587 is located at a distance of (2.70 ± 0.70) kpc from the Sun and ∼9.8 kpc from the Galactic centre.  相似文献   

18.
We present Chandra and XMM–Newton observations of 12 bright  [ f (2–10 keV) > 10−13 erg cm−2 s−1]  sources from the ASCA search for the High Energy Extragalactic Population (SHEEP) survey. Most of these have been either not observed or not detected previously with the ROSAT mission, and therefore they constitute a sample biased towards hard sources. The Chandra observations are important in locating the optical counterpart of the X-ray sources with accuracy. Optical spectroscopic observations show that our sample is associated with both narrow-line (NL) (six objects) and broad-line (BL) active galactic nuclei (AGN) (five objects), with one source remaining unidentified. Our sources cover the redshift range 0.04–1.29, spanning luminosities from 1042 to  1045 erg s−1  (2–10 keV). The NL sources have preferentially lower redshift (and luminosity) compared to the BL ones. This can be most easily explained in a model where the NL AGN are intrinsically less luminous than the BL ones in line with the results of Steffen et al. The X-ray spectral fittings show a roughly equal number of obscured  ( N H > 1022 cm−2)  and unobscured  ( N H < 1022 cm−2)  sources. There is a clear tendency for obscured sources to be associated with NL AGN and unobscured sources with BL ones. However, there is a marked exception with the highest obscuring column observed at a BL AGN at a redshift of z = 0.5.  相似文献   

19.
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1. The highest energy lines require a column density of   N H∼ 5 × 1023 cm−2  , at an ionization parameter of  log ξ∼ 3.4  . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143.  相似文献   

20.
There is strong evidence for some kind of massive dark object in the centres of many galaxy bulges. The detection of flares from tidally disrupted stars could confirm that these objects are black holes (BHs). Here we present calculations of the stellar disruption rates in detailed dynamical models of real galaxies, taking into account the refilling of the loss cone of stars on disruptable orbits by two-body relaxation and tidal forces in non-spherical galaxies. The highest disruption rates (one star per 104 yr) occur in faint ( L ≲1010 L⊙) galaxies, which have steep central density cusps. More luminous galaxies are less dense and have much longer relaxation times and more massive BHs. Dwarf stars in such galaxies are swallowed whole by the BH and hence do not emit flares; giant stars could produce flares as often as every 105 yr, although the rate depends sensitively on the shape of the stellar distribution function. We discuss the possibility of detecting disruption flares in current supernova searches. The total mass of stars consumed over the lifetime of the galaxy is of the order of 106 M⊙, independent of galaxy luminosity; thus, disrupted stars may contribute significantly to the present BH mass in galaxies fainter than ∼109 L⊙.  相似文献   

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