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1.
We report on two Rossi X-ray Timing Explorer ( RXTE ) observations of the anomalous X-ray pulsars 1E 1048.1–5937 and 1E 2259+586. Both sources have continued their almost constant spin-down during 1995/96. We carried out a search for orbital Doppler shifts, in their observed spin frequencies, deriving stringent limits on the projected semi-axis. Unless these systems have unlikely small inclinations, main-sequence companions can be excluded. If 1E 1048.1–5937 and 1E 2259+586 are indeed binary systems, their companion stars must be either white dwarfs, or helium-burning stars with M ≲ 0.8 M⊙, possibly underfilling their Roche lobe.  相似文献   

2.
We present the broad-band noise structure of selected anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) in the 2–60 keV energy band. We have analysed Rossi X-Ray Timing Explorer Proportional Counter Array archival light curves for four AXPs and one SGR. We detect that the persistent emission of these sources shows band-limited noise at low frequencies in the range 0.005–0.05 Hz varying from 2.5 to 70 per cent integrated rms in times of prolonged quiescence and following outbursts. We discovered band-limited red noise in 1E 2259+586 only for ∼2 yr after its major 2002 outburst. The system shows no broad-band noise otherwise. Although this rise in noise in 1E 2259+586 occurred following an outburst which included a rotational glitch, the other glitching AXPs showed no obvious change in broad-band noise, thus it does not seem that this noise is correlated with glitches. The only source that showed significant variation in broad-band noise was 1E   1048.1−5937  , where the noise gradually rose for 1.95 yr at a rate of ∼3.6 per cent per year. For this source the increases in broad-band noise was not correlated with the large increases in persistent and pulsed flux, or its two short SGR-like bursts. This rise in noise did commence after a long burst, however, given the sparsity of this event, and the possibility that similar bursts went unnoticed the trigger for the rise is noise in 1E   1048.1−5937  is not as clear as for 1E 2259+586. The other three sources indicate a persistent band-limited noise at low levels in comparison.  相似文献   

3.
Using X-ray data from the Rossi X-ray Timing Explorer , we report the pulse timing results of the accretion-powered, high-mass X-ray binary pulsar 4U 1907+09, covering a time-span of almost two years. We measured three new pulse periods in addition to the previously measured four pulse periods. We are able to connect pulse arrival times in phase for more than a year. The source has been spinning down almost at a constant rate, with a spin-down rate of     for more than 15 yr. Residuals of pulse arrival times yield a very low level of random-walk noise, with a strength of ∼     on a time-scale of 383 d, which is 40 times lower than that of the high-mass X-ray binary pulsar Vela X-1. The noise strength is only a factor of 5 greater than that of the low-mass X-ray binary pulsar 4U 1626−67. The low level of the timing noise and the very stable spin-down rate of 4U 1907+09 make this source unique among the high-mass X-ray binary pulsars, providing another example, in addition to 4U 1626−67, of long-term quiet spin down from an accreting source. These examples show that the extended quiet spin-down episodes observed in the anomalous X-ray pulsars 1RXS J170849.0−400910 and 1E 2259+586 do not necessarily imply that these sources are not accreting pulsars.  相似文献   

4.
We present new X-ray observations of the high-mass X-ray binary (HMXRB) pulsar OAO 1657−415, obtained during one orbital period (10.44 d) with the Rossi X-Ray Timing Explorer ( RXTE ). Using the binary orbital parameters, obtained from Burst and Transient Source Experiment (BATSE) observations, we resolve the fluctuations in the pulse frequency at time-scales on the order of 1 d for the first time. Recent BATSE results by Baykal showed that OAO 1657−415 has spin-up/down trends in its pulse frequency time series, without any correlation with the X-ray luminosity at energies >20 keV. In the present RXTE observations the source is found to be in an extended phase of spin-down. We also find a gradual increase in the X-ray luminosity which is correlated with a marginal spin-up episode. The marginal correlation between the gradual spin-up (or decrease in spin-down rate) and increase in X-ray luminosity suggests that OAO 1657−415 is observed during a stable accretion episode where the prograde accretion disc is formed.  相似文献   

5.
We analysed RXTE archival observations of 4U 1907+09 between 1996 February 17 and 2002 March 6. The pulse timing analysis showed that the source stayed at almost constant period around 1998 August and then started to spin-down at a rate of  (−1.887 ∓ 0.042) × 10−14 Hz s−1  which is ∼0.60 times lower than the long-term (∼15 yr) spin-down rate. Our pulse-frequency measurements for the first time resolved significant spin-down rate variations since the discovery of the source. We also presented orbital phase resolved X-ray spectra during two stable spin-down episodes during 1996 November–1997 December and 2001 March–2002 March. The source has been known to have two orbitally locked flares. We found that X-ray flux and spectral parameters except hydrogen column density agreed with each other during the flares. We interpreted the similar values of X-ray fluxes as an indication of the fact that the source accretes not only via transient retrograde accretion disc but also via the stellar wind of the companion, so that the variation of the accretion rate from the disc does not cause significant variation in the observed X-ray flux. Lack of significant change in spectral parameters except hydrogen column density was interpreted as a sign of the fact that the change in the spin-down rate of the source was not accompanied by a significant variation in the accretion geometry.  相似文献   

6.
We present timing and spectral analysis of RXTE -PCA (Proportional Counter Array) observations of the accretion powered pulsar 4U 1907+09 between 2007 June and 2008 August. 4U 1907+09 had been in a spin-down episode with a spin-down rate of  −3.54 × 10−14 Hz s−1  before 1999. From RXTE observations after 2001 March, the source showed a ∼60 per cent decrease in spin-down magnitude, and INTEGRAL observations after 2003 March showed that source started to spin-up. We found that the source recently entered into a new spin-down episode with a spin-down rate of  −3.59 × 10−14 Hz s−1  . This spin-down rate is pretty close to the previous long-term spin-down rate of the source measured before 1999. From the spectral analysis, we showed that hydrogen column density varies with the orbital phase.  相似文献   

7.
LMC X-1 and LMC X-3 are the only known persistent stellar-mass black-hole candidates that have almost always shown spectra that are dominated by a soft, thermal component. We present here results from 170-ks-long Rossi X-ray Timing Explorer ( RXTE ) observations of these objects, taken in 1996 December, where their spectra can be described by a disc blackbody plus an additional soft     high-energy power law (detected up to energies of 50 keV in LMC X-3). These observations, as well as archival Advanced Satellite for Cosmology and Astrophysics ( ASCA ) observations, constrain any narrow Fe line present in the spectra to have an equivalent width ≲90 eV. Stronger, broad lines (≈150 eV EW,     are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately proportional to     between 10−3 and 0.3 Hz with a root-mean-square (rms) variability of ≈7 per cent. At energies >5 keV, the PSD shows evidence of a break at     possibly indicating an outer disc radius of ≲1000  GM c 2 in this likely wind-fed system. Furthermore, the coherence function     a measure of the degree of linear correlation between variability in the >5 keV band and variability in the lower energy bands, is extremely low (≲50 per cent). We discuss the implications of these observations for the mechanisms that might be producing the soft and hard X-rays in these systems.  相似文献   

8.
We report on several pointed Rossi X-ray Timing Explorer observations of the enigmatic low-mass X-ray binary (LMXB) 4U 1957+11 at different X-ray luminosities. The luminosity of the source varied by more than a factor of 4 on time-scales of months to years. The spectrum of the source tends to become harder when its luminosity increases. Only very weak  (1–2  per cent rms amplitude,  0.001–10 Hz  ,  2–60 keV)  rapid X-ray variability was observed during the observations. A comparison of the spectral and temporal behaviour of 4U 1957+11 with other X-ray binary systems, in particular LMC X-3, indicates that 4U 1957+11 is likely to be a persistent LMXB harbouring a black hole and it is persistently in the black hole high state. If confirmed, it would be the only such system known.  相似文献   

9.
We present Chandra , Very Large Array (VLA) and Very Long Baseline Array (VLBA) observations of the nucleus of NGC 4696, a giant elliptical in the Centaurus cluster of galaxies. Like M87 in the Virgo cluster, PKS 1246−410 in the Centaurus cluster is a nearby example of a radio galaxy in a dense cluster environment. In analysing the new X-ray data, we have found a compact X-ray feature coincident with the optical and radio core. While nuclear emission from the X-ray source is expected, its luminosity is low,  <1040 erg s−1  . We estimate the Bondi accretion radius to be 30 pc and the accretion rate to be  0.01 M⊙ yr−1  , which under the canonical radiative efficiency of 10 per cent would overproduce by 3.5 orders of magnitude the radiative luminosity. Much of this energy can be directed into the kinetic energy of the jet, which over time inflates the observed cavities seen in the thermal gas. The VLBA observations reveal a weak nucleus and a broad, one-sided jet extending over 25 pc in position angle −150°. This jet is deflected on the kiloparsec-scale to a more east–west orientation (position angle of −80°).  相似文献   

10.
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   

11.
We present Keck II spectroscopy of optical mHz quasi-periodic oscillations (QPOs) in the light curve of the X-ray pulsar binary Hercules X-1. In the power spectrum it appears as 'peaked noise', with a coherency ∼2, a central frequency of 35 mHz and a peak-to-peak amplitude of 5 per cent. However, the dynamic power spectrum shows it to be an intermittent QPO, with a lifetime of ∼100 s, as expected if the lifetime of the orbiting material is equal to the thermal time-scale of the inner disc. We have decomposed the spectral time series into constant and variable components and used blackbody fits to the resulting spectra to characterize the spectrum of the QPO variability and constrain possible production sites. We find that the spectrum of the QPO is best fitted by a small hot region, possibly the inner regions of the accretion disc, where the ballistic accretion stream impacts on to the disc. The lack of any excess power around the QPO frequency in the X-ray power spectrum, created using simultaneous light curves from RXTE , implies that the QPO is not simply reprocessed X-ray variability.  相似文献   

12.
We present a systematic study of GX 339−4 in both its very high and low hard states from simultaneous observations made with XMM–Newton and RXTE in 2002 and 2004. The X-ray spectra of both these extreme states exhibit strong reflection signatures, with a broad, skewed Fe Kα line clearly visible above the continuum. Using a newly developed, self-consistent reflection model which implicitly includes the blackbody radiation of the disc as well as the effect of Comptonization, blurred with a relativistic line function, we were able to infer the spin parameter of GX 339−4 to be  0.935 ± 0.01  (statistical) ±0.01 (systematic) at 90 per cent confidence. We find that both states are consistent with an ionized thin accretion disc extending to the innermost stable circular orbit around the rapidly spinning black hole.  相似文献   

13.
We have monitored the atoll-type neutron star low-mass X-ray binary 4U 1636−53 with the Rossi X-ray Timing Explorer ( RXTE ) for more than 1.5 yr. Our campaign consisted of short (∼2 ks) pointings separated by 2 d, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ∼30–40 d, already seen in the light curves from the RXTE All-Sky Monitor, which corresponded to regular transitions between the hard (island) and soft (banana) states. We detected kilohertz (kHz) quasi-periodic oscillations (QPOs) in about a third of the observations, most of which were in the soft (banana) state. The distribution of the frequencies of the peak identified as the lower kHz QPO is found to be different from that previously observed in an independent data set. This suggests that the kHz QPOs in the system shows no intrinsically preferred frequency.  相似文献   

14.
We analysed Rossi X-ray Timing Explorer Proportional Counter Array observations of a recent outburst of the X-ray pulsar XMMU J054134.7−682550. We calculated the pulse frequency history of the source. We found no sign of a binary companion. The source spins up when the X-ray flux is higher, with a correlation between the spin-up rate and X-ray flux, which may be interpreted as a sign of an accretion disc. On the other hand, the source was found to have an almost constant spin frequency when the X-ray flux is lower without any clear sign of a spin-down episode. The decrease in pulsed fraction with decreasing X-ray flux was interpreted as a sign of accretion geometry change, but we did not find any evidence of a transition from accretor to propeller regimes. The source was found to have variable pulse profiles. Two peaks in pulse profiles were usually observed. We studied the X-ray spectral evolution of the source throughout the observation. Pulse-phase-resolved analysis does not provide any further evidence for a cyclotron line, but may suggest a slight variation of intensity and width of the 6.4 keV iron line with phase.  相似文献   

15.
We present simultaneous X-ray ( RXTE ) and optical (ULTRACAM) narrow-band (Bowen blend/He  ii and nearby continuum) observations of Sco X-1 at 2–10 Hz time resolution. We find that the Bowen/He  ii emission lags the X-ray light curves with a light traveltime of     s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ∼0.5 when Sco X-1 is at the top of the flaring branch (FB). Evidence of echoes is also seen at the bottom of the FB but with time-lags of 5–10 s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.  相似文献   

16.
We use data from the Rossi X-ray Timing Explorer to search for harmonics and sidebands of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in Sco X-1. We do not detect any of these harmonics or sidebands, with 95 per cent confidence upper limits to their power between ∼1 and ∼10 per cent of the power of the upper kHz QPO. The oscillations produced at these frequencies may be attenuated in a scattering corona around the neutron star. We find that upper limits to the unattenuated power of some of the strongest theoretically predicted harmonics and sidebands are as low as ∼2 per cent of the unattenuated power of the high-frequency QPO in Sco X-1.  相似文献   

17.
The X-ray binary system GX 301−2 consists of a neutron star in an eccentric orbit accreting from the massive early-type star Wray 977. It has previously been shown that the X-ray orbital light curve is consistent with the existence of a gas stream flowing out from Wray 977 in addition to its strong stellar wind. Here, X-ray monitoring observations by the Rossi X-ray Timing Explorer ( RXTE )/All-Sky Monitor and pointed observations by the RXTE /Proportional Counter Array over the past decade are analysed. We analyse both the flux and column density dependence on orbital phase. The wind and stream dynamics are calculated for various system inclinations, companion rotation rates and wind velocities, as well as parametrized by the stream width and density. These calculations are used as inputs to determine both the expected accretion luminosity and the column density along the line-of-sight to the neutron star. The model luminosity and column density are compared to observed flux and column density versus orbital phase, to constrain the properties of the stellar wind and the gas stream. We find that the change between bright and medium intensity levels is primarily due to decreased mass loss in the stellar wind, but the change between medium and dim intensity levels is primarily due to decreased stream density. The mass-loss rate in the stream exceeds that in the stellar wind by a factor of ∼2.5. The quality of the model fits is better for lower inclinations, favouring a higher mass for Wray 977 in its allowed range of  40–60 M⊙  .  相似文献   

18.
In this short paper, we suggest that the missing boundary layer luminosity of dwarf novae in quiescence is released mainly in the ultraviolet (UV) as the second component commonly identified in the far-UV as the 'accretion belt'. We present the well-studied SU UMa-type system VW Hyi in detail as a prototype for such a scenario. We consider detailed multiwavelength observations and in particular the recent Far Ultraviolet Spectroscopic Explorer ( FUSE ) observations of VW Hyi in quiescence, which confirm the presence of a second component (the 'accretion belt') in the far-UV spectrum. The temperature  (≈ 50 000 K)  and rotational velocity  (≈ 3000 km s −1)  of this second component are entirely consistent with the optically thick region  (τ≈ 1)  located just at the outer edge of optically thin boundary layer in the simulations of Popham. This second component contributes about 20 per cent of the far-UV flux. Using geometrical assumptions and taking into account the X-ray luminosity, we find that the total boundary layer luminosity sums up to   L BL= (0.53 ± 0.25) L disc  , while the theory (Kluźniak) predicts, for the rotation rate of VW Hyi's white dwarf,   L BL≈ (0.76 ± 0.03) L disc  . About one-fifth of the boundary layer energy is emitted in the X-ray and the remaining is emitted in the UV. This scenario is consistent with the recent simultaneous X-ray and UV observations of VW Hyi by Pandel, Córdova & Howell, from which we deduce here that the viscosity in the boundary layer region must be of the order of  ν≈ 1013–1014 cm2 s −1  , depending on the white dwarf mass and the size of the boundary layer.  相似文献   

19.
We report the discovery of type I X-ray bursts from the low-mass X-ray binary  4U 1708 − 40  during the 100-ks observation performed by BeppoSAX on 1999 August 15–16. Six X-ray bursts have been observed. The unabsorbed 2–10 keV fluxes of the bursts range from ∼3 to  9 × 10−10 erg cm−2 s−1  . A correlation between peak flux and fluence of the bursts is found, in agreement with the behaviour observed in other similar sources. There is a trend of the burst flux to increase with the time interval from the previous burst. From the value of the persistent flux we infer a mass accretion rate     , which may correspond to the mixed hydrogen/helium burning regime triggered by thermally unstable hydrogen. We have also analysed a BeppoSAX observation performed on 2001 August 22 and previous RXTE observations of  4U 1708 − 40  , where no bursts have been observed; we find persistent fluxes of more than a factor of 7 higher than the persistent flux observed during the BeppoSAX observation showing X-ray bursts.  相似文献   

20.
We carried out I , R , V and B photometric observations of the neutron star X-ray binary RXTE J2123−058 shortly after the end of the X-ray outburst in mid-1998. We adopt the low-mass binary model to interpret our observations. After folding our data on the 0.24 821‐d orbital period, and correcting for the steady brightness decline following the outburst, we observed sinusoidal oscillations with hints of ellipsoidal modulations which became progressively more evident. Our data also show that the decline in brightness was faster in the V band than in the R and I bands. This suggests both the cooling of an irradiation-heated secondary star and the fading of an accretion disc over the nights of our observations.  相似文献   

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