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1.
The second known outburst of the WZ Sge type dwarf nova GW Lib was observed in 2007 April. We have obtained unique multiwavelength data of this outburst which lasted ∼26 days. The American Association of Variable Star Observers ( AAVSO ) recorded the outburst in the optical, which was also monitored by Wide Angle Search for Planets , with a peak V magnitude of ∼8. The outburst was followed in the ultraviolet and X-ray wavelengths by the Swift ultraviolet/optical and X-ray telescopes. The X-ray flux at optical maximum was found to be three orders of magnitude above the pre-outburst quiescent level, whereas X-rays are normally suppressed during dwarf nova outbursts. A distinct supersoft X-ray component was also detected at optical maximum, which probably arises from an optically thick boundary layer. Follow-up Swift observations taken 1 and 2 years after the outburst show that the post-outburst quiescent X-ray flux remains an order of magnitude higher than the pre-outburst flux. The long interoutburst time-scale of GW Lib with no observed normal outbursts support the idea that the inner disc in GW Lib is evacuated or the disc viscosity is very low.  相似文献   

2.
We present observations of the 1997 outburst of the X-ray transient GS 1354−64 (BW Cir) at X-ray, optical and, for the first time, radio wavelengths; our results include upper limits to the linear and circular polarization for the radio data. The X-ray outburst was unusual in that the source remained in the low/hard X-ray state throughout; the X-ray peak was also preceded by at least one optical outburst, suggesting that it was an 'outside-in' outburst – similar to those observed in dwarf novae systems, although possibly taking place on a viscous time-scale in this case. It therefore indicates that the optical emission was not dominated by the reprocessing of X-rays, but that instead we see the instability directly. While the radio source was too faint to detect any extended structure, spectral analysis of the radio data and a comparison with other similar systems suggest that mass ejections, probably in the form of a jet, took place and that the emitted synchrotron spectrum may have extended as far as infrared wavelengths. Finally, we compare this 1997 outburst of GS 1354−64 with possible previous outbursts and also with other hard-state objects, both transient and persistent. It appears that a set of characteristics – such as a weak, flat-spectrum radio jet, a mHz QPO increasing in frequency, a surprisingly high optical/X-ray luminosity ratio, and the observed optical peak preceding the X-ray peak – may be common to all hard-state X-ray transients.  相似文献   

3.
We have studied the 1999 soft X-ray transient outburst of XTE J1859+226 at radio and X-ray wavelengths. The event was characterized by strong variability in the disc, corona and jet – in particular, a number of radio flares (ejections) took place and seemed well-correlated with hard X-ray events. Apparently unusual for the canonical 'soft' X-ray transient, there was an initial period of low/hard state behaviour during the rise from quiescence but prior to the peak of the main outburst – we show that not only could this initial low/hard state be a ubiquitous feature of soft X-ray transient outbursts, but also it could be extremely important in our study of outburst mechanisms.  相似文献   

4.
We build a simple model of the optical/ultraviolet (UV) emission from irradiation of the outer disc by the inner disc and coronal emission in black hole binaries. We apply this to the broad-band Swift data from the outburst of the black hole binary XTE J1817−330 to confirm previous results that the optical/UV emission in the soft state is consistent with a reprocessing a constant fraction of the bolometric X-ray luminosity. However, this is very surprising as the disc temperature drops by more than a factor of 3 in the soft state, which should produce a marked change in the reprocessing efficiency. The easiest way to match the observed constant reprocessed fraction is for the disc skin to be highly ionized (as suggested 30 yr ago by van Paradijs), so that the bulk of the disc flux is reflected and only the hardest X-rays heat the disc. The constant reprocessed fraction also favours direct illumination of the disc over a scattering origin as the optical depth/solid angle of any scattering material (wind/corona) over the disc should decrease as the source luminosity declines. By contrast, the reprocessed fraction increases very significantly (by a factor of ∼6) as the source enters the hard state. This dramatic change is not evident from X-ray/UV flux correlations as it is masked by bandpass effects. However, it does not necessarily signal a change in emission, for example, the emergence of the jet dominating the optical/UV flux as the reflection albedo must change with the dramatic change in spectral shape.  相似文献   

5.
The transient black-hole binary XTE J1118+480 exhibited dramatic rapid variability at all wavelengths which were suitably observed during its 2000 April–July outburst. We examine time-resolved X-ray, ultraviolet, optical and infrared data spanning the plateau phase of the outburst. We find that both X-ray and infrared bands show large amplitude variability. The ultraviolet and optical variability is more subdued, but clearly correlated with that seen in the X-rays. The ultraviolet, at least, appears to be dominated by the continuum, although the lines are also variable. Using the X-ray variations as a reference point, we find that the ultraviolet (UV) variability at long wavelengths occurs later than that at short wavelengths. Uncertainty in the Hubble Space Telescope timing prohibits a determination of the absolute lag with respect to the X-rays, however. The transfer function is clearly not a delta-function, exhibiting significant repeatable structure. For the main signal we can rule out an origin in reprocessing on the companion star – the lack of variation in the lags is not consistent with this, given a relatively high orbital inclination. Weak reprocessing from the disc and/or companion star may be present, but is not required, and another component must dominate the variability. This could be variable synchrotron emission correlated with X-ray variability, consistent with our earlier interpretation of the infrared (IR) flux as due to synchrotron emission rather than thermal disc emission. In fact, the broad-band energy distribution of the variability from IR to X-rays is consistent with expectations of optically thin synchrotron emission. We also follow the evolution of the low-frequency quasi-periodic oscillation in X-rays, UV, and optical. Its properties at all wavelengths are similar, indicating a common origin.  相似文献   

6.
We present Swift observations of the black hole X-ray transient, GRO J1655−40, during the recent outburst. With its multiwavelength capabilities and flexible scheduling, Swift is extremely well suited for monitoring the spectral evolution of such an event. GRO J1655−40 was observed on 20 occasions and data were obtained by all instruments for the majority of epochs. X-ray spectroscopy revealed spectral shapes consistent with the 'canonical' low/hard, high/soft and very high states at various epochs. The soft X-ray source (0.3–10 keV) rose from quiescence and entered the low/hard state, when an iron emission line was detected. The soft X-ray source then softened and decayed, before beginning a slow rebrightening and then spending ∼3 weeks in the very high state. The hard X-rays (14–150 keV) behaved similarly but their peaks preceded those of the soft X-rays by up to a few days; in addition, the average hard X-ray flux remained approximately constant during the slow soft X-ray rebrightening, increasing suddenly as the source entered the very high state. These observations indicate (and confirm previous suggestions) that the low/hard state is key to improving our understanding of the outburst trigger and mechanism. The optical/ultraviolet light curve behaved very differently from that of the X-rays; this might suggest that the soft X-ray light curve is actually a composite of the two known spectral components, one gradually increasing with the optical/ultraviolet emission (accretion disc) and the other following the behaviour of the hard X-rays (jet and/or corona).  相似文献   

7.
We present optical spectroscopy and optical and infrared photometry of the neutron star soft X-ray transient Aql X–1 during its X-ray outburst of 1997 August. By modelling the X-ray, optical and IR light curves, we find a 3-d delay between the IR and X-ray rise times, analogous to the UV–optical delay seen in dwarf novae outbursts and black hole X-ray transients. We interpret this delay as the signature of an 'outside-in' outburst, in which a thermal instability in the outer disc propagates inward. This outburst is the first of this type definitively identified in a neutron star X-ray transient.  相似文献   

8.
We report on a comprehensive and consistent investigation into the X-ray emission from GX 339−4. All public observations in the 11 year RXTE archive were analysed. Three different types of model – single power law, broken power law and a disc + power law – were fitted to investigate the evolution of the disc, along with a fixed Gaussian component at 6.4 keV to investigate any iron line in the spectrum. We show that the relative variation in flux and X-ray colour between the two best sampled outbursts are very similar. The decay of the disc temperature during the outburst is clearly seen in the soft state. The expected decay is   S Disc∝ T 4  ; we measure   T 4.75±0.23  . This implies that the inner disc radius is approximately constant in the soft state. We also show a significant anticorrelation between the iron line equivalent width (EW) and the X-ray flux in the soft state while in the hard state the EW is independent of the flux. This results in hysteresis in the relation between X-ray flux and both line flux and EW. To compare the X-ray binary outburst to the behaviour seen in active galactic nuclei (AGN), we construct a disc fraction luminosity diagram for GX 339−4, the first for an X-ray binary. The shape qualitatively matches that produced for AGN. Linking this with the radio emission from GX 339−4 the change in radio spectrum between the disc and power-law-dominated states is clearly visible.  相似文献   

9.
An intense outburst of hard radiation (with a peak flux of ～50 mCrab) was detected from the X-ray transient AX J1749.1-2733 by the IBIS/ISGRI gamma-ray telescope onboard the INTEGRAL observatory when the Galactic center field was monitored on September 8–10, 2003. Previously, this source had never been observed in a bright X-ray state. During the outburst, the source’s radiation spectrum was gently sloping and hard (extended to ～100 keV), followed a power law in the standard X-ray energy range, and had an exponential cutoff above 40–50 keV. The spectral hardness decreased with increasing flux. These and other properties described here and the shortness of the outburst (<2 days) allow the source to be attributed to the group of fast X-ray transients many representatives of which have an early O-B supergiant as their optical counterpart. Possible causes of the outbursts of fast transients are discussed. We show that accretion from the supergiant’s stellar wind should have led to intense persistent radiation from transients. The absence of radiation can be explained by the ejection of accreting matter from the system (propeller effect) during its contact with the magnetosphere of a rapidly rotating neutron star. Transient outbursts could originate in sources of this type if the spin period of their neutron star P s differed only slightly from the critical period P s * ? 3 s at which the propeller effect is still possible. The outburst is triggered by an insignificant rise in the local stellar wind density, by a factor of (P s * /P s)7/3. The entire outburst profile cannot be explained by an individual inhomogeneity in the wind, but is the reflection of a long-term (～2 days for AX J1749.1-2733) change in the rate of wind outflow from the supergiant’s surface facing the compact source. The rate of wind outflow could be enhanced through X-ray heating of the supergiant’s surface.  相似文献   

10.
We report the discovery of a new hysteresis effect in black hole X-ray binary state transitions, that of the near-infrared (NIR) flux (which most likely originates in the jets) versus X-ray flux. We find, looking at existing data sets, that the IR emission of black hole X-ray transients appears to be weaker in the low/hard state rise of an outburst than the low/hard state decline of an outburst at a given X-ray luminosity. We discuss how this effect may be caused by a shift in the radiative efficiency of the inflowing or outflowing matter, or variations in the disc viscosity or the spectrum/power of the jet. In addition we show that there is a correlation (in slope but not in normalization) between IR and X-ray luminosities on the rise and decline, for all three low-mass black hole X-ray binaries with well-sampled IR and X-ray coverage:   L NIR∝ L 0.5–0.7X  . In the high/soft state this slope is much shallower;   L NIR∝ L 0.1–0.2X  , and we find that the NIR emission in this state is most likely dominated by the viscously heated (as opposed to X-ray heated) accretion disc in all three sources.  相似文献   

11.
The optical counterpart of the transient, millisecond X-ray pulsar SAX J1808.4–3658 was observed in four colours ( BVRI ) for five weeks during the 2005 June–July outburst. The optical fluxes declined by ∼2 mag during the first 16d and then commenced quasi-periodic secondary outbursts, with time-scales of several days, similar to those seen in 2000 and 2002. The broad-band spectra derived from these measurements were generally consistent with emission from an X-ray heated accretion disc. During the first 16d decline in intensity the spectrum became redder. We suggest that the primary outburst was initiated by a viscosity change driven instability in the inner disc and note the contrast with another accreting millisecond pulsar, XTE J0929−314, for which the spectrum becomes bluer during the decline. On the night of 2005 June 5 (HJD 245 3527) the I -band flux was ∼0.45-mag brighter than on the preceding or following nights whereas the BV and R bands showed no obvious enhancement. A type I X-ray burst was detected by the Rossi X-ray Timing Explorer spacecraft during this I -band integration. It seems unlikely that reprocessed radiation from the burst was sufficient to explain the observed increase. We suggest that a major part of the I -band excess was due to synchrotron emission triggered by the X-ray burst. Several other significant short duration changes in V − I were detected. One occurred at about HJD 245 3546 in the early phase of the first secondary outburst and may be due to mass-transfer instability or to another synchrotron emission event.  相似文献   

12.
  相似文献   

13.
We report the first extensive set of optical photometric observations of the counterpart to SAX J1808.4−3658 (V4580 Sagittarii) in quiescence. The source was detected at V ∼21 , 5 mag fainter than at the peak of its 1998 outburst. However, a comparable ∼6 per cent semi-amplitude 2-h modulation of its flux is revealed. This has the same phasing and approximately sinusoidal modulation as seen during outburst, and with photometric minimum when the pulsar is behind the companion. The lack of a double-humped morphology rules out an ellipsoidal origin, implying that the bulk of the optical flux does not arise from the companion. Moreover, applying crude modelling to the disc and X-ray irradiated face of the donor shows that the internal energy release of a remnant disc (with mass transfer driven by gravitational radiation) is sufficient to explain most of the optical emission, and with the modulation because of the varying contribution of the heated face of the star. We note that this model is also consistent with the much lower X-ray to optical flux ratio in quiescence versus outburst, and with the phasing of the optical modulation.  相似文献   

14.
We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated.
The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve.
X-ray and extreme-ultraviolet luminosities imply accretion rates of  3 × 1015 g s−1  in quiescence,  1 × 1016 g s−1  when the boundary layer becomes optically thick, and  ∼1018 g s−1  at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state.  相似文献   

15.
We present the results from a monitoring campaign of the Narrow-Line Seyfert 1 galaxy PG 1211+143. The object was monitored with ground-based facilities ( UBVRI photometry; from 2007 February to July) and with Swift [X-ray photometry/spectroscopy and ultraviolet (UV)/optical photometry; between 2007 March and May]. We found PG 1211+143 in a historical low X-ray flux state at the beginning of the Swift monitoring campaign in 2007 March. It is seen from the light curves that while violently variable in X-rays, the quasar shows little variations in optical/UV bands. The X-ray spectrum in the low state is similar to other narrow-line Seyfert 1 galaxies during their low states and can be explained by a strong partial covering absorber or by X-ray reflection on to the disc. With the current data set, however, it is not possible to distinguish between both scenarios. The interband cross-correlation functions indicate a possible reprocessing of the X-rays into the longer wavelengths, consistent with the idea of a thin accretion disc, powering the quasar. The time lags between the X-ray and the optical/UV light curves, ranging from ∼2 to ∼18 d for the different wavebands, scale approximately as  ∼λ4/3  , but appear to be somewhat larger than expected for this object, taking into account its accretion disc parameters. Possible implications for the location of the X-ray irradiating source are discussed.  相似文献   

16.
We present the results of the analysis of Rossi X-ray Timing Explorer ( RXTE ) observations of the new X-ray transient, SWIFT J1753.5−0127, during its outburst in 2005 July. The source was caught at the peak of the burst with a flux of 7.19e-09 erg s−1cm−2 in the 3–25 keV energy range and observed until it decreased by about a factor of 10. The photon index of the power-law component, which is dominant during the entire outburst, decreases from ∼1.76 to 1.6. However, towards the end of the observations the photon index is found to increase, indicating a softening of the spectra. The presence of an ultrasoft thermal component, during the bright phases of the burst, is clear from the fits to the data. The temperature associated with this thermal component is 0.4 keV. We believe that this thermal component could be due to the presence of an accretion disc. Assuming a distance of 8.5 kpc,   L X/ L Edd≃ 0.05  at the peak of the burst, for a black hole of mass  10 M⊙  . The source is found to be locked in the low/hard state during the entire outburst and likely falls in the category of the X-ray transients that are observed in the low/hard state throughout the outburst. We discuss the physical scenario of the low/hard state outburst for this source.  相似文献   

17.
We reanalyse archival Ginga data of the soft X-ray transient source GS 2023+338 covering the beginning of its 1989 May outburst. The source showed a number of rather unusual features: very high and apparently saturated luminosity, dramatic flux and spectral variability (often on ∼1 s time-scale), and generally very hard spectrum, with no obvious soft thermal component characteristic for soft/high state.
We describe the spectrum obtained at the maximum of flux and we demonstrate that it is very different from spectra of other soft X-ray transients at similar luminosity. We confirm previous suggestions that the dramatic variability was the result of heavy and strongly variable photoelectric absorption. We also demonstrate that for a short time the spectrum of the source did look like a typical soft/high state spectrum but that this coincided with very heavy absorption.  相似文献   

18.
We have undertaken an extensive study of X-ray data from the accreting millisecond pulsar XTE J1751 − 305 observed by RXTE and XMM–Newton during its 2002 outburst. In all aspects this source is similar to the prototypical millisecond pulsar SAX J1808.4 − 3658, except for the higher peak luminosity of 13 per cent of Eddington, and the optical depth of the hard X-ray source, which is larger by a factor ∼2. Its broad-band X-ray spectrum can be modelled by three components. We interpret the two soft components as thermal emission from a colder  ( kT ∼ 0.6 keV)  accretion disc and a hotter (∼1 keV) spot on the neutron star surface. We interpret the hard component as thermal Comptonization in plasma of temperature ∼40 keV and optical depth ∼1.5 in a slab geometry. The plasma is heated by the accretion shock as the material collimated by the magnetic field impacts on to the surface. The seed photons for Comptonization are provided by the hotspot, not by the disc. The Compton reflection is weak and the disc is probably truncated into an optically thin flow above the magnetospheric radius. Rotation of the emission region with the star creates an almost sinusoidal pulse profile with an rms amplitude of 3.3 per cent. The energy-dependent soft phase lags can be modelled by two pulsating components shifted in phase, which is naturally explained by a different character of emission of the optically thick spot and optically thin shock combined with the action of the Doppler boosting. The observed variability amplitude constrains the hotspot to lie within 3°–4° of the rotational pole. We estimate the inner radius of the optically thick accreting disc to be about 40 km. In that case, the absence of emission from the antipodal spot, which can be blocked by the accretion disc, gives the inclination of the system as ≳70°.  相似文献   

19.
I propose that the properties of the two outbursts observed in the X‐ray transient XTEJ1118+480 in 2000 are akin to superoutbursts of SU UMa stars. In these systems a ‘normal’ outburst immediately precedes a 5–10 times longer (‘super’) outburst. The optical light curve of the outbursts of XTEJ1118+480 is remarkably similar to that seen in some SU UMa stars, such as UVPer and TLeo, where the precursor outburst is distinct from the superoutburst, but the time scales are a factor of ∼15 different. The first outburst of XTEJ1118+480 was relatively short (∼1 month) while the second outburst was ∼5 times longer. During the second outburst superhumps were seen, a feature characteristic for superoutbursts. The gap of about a month between the two outbursts is longer in X‐rays with respect to the optical, a feature not previously recognized for X‐ray transients. Also in SU UMa stars the precursor outburst becomes more distinct at shorter wavelengths. Finally, I show that the time of appearance of the superhumps in XTEJ1118+480 is consistent with the expected superhump growth time, if the superhump mechanism was triggered during the first outburst. I conclude that the similarity in outburst behaviour in the two types of systems provides further support that a common mechanism is at work to start the long (‘super’) outbursts.  相似文献   

20.
XTE J1748−288 is a black hole X-ray transient which went into outburst in 1998 June. The X-ray light curves showed canonical morphologies, with minor variations on the 'fast rise exponential decay' profile. The radio source, however, reached an unusually high flux density of over 600 mJy. This high radio flux was accompanied by an exceptional  (>20  per cent)  fractional linear polarization, the variability of which was anticorrelated with the flux density. We use this variability to discuss possible depolarization mechanisms and to predict the underlying behaviour of the (unresolved) core/jet components.  相似文献   

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