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1.
The fields of eight X-ray sources in the Magellanic Clouds believed to be Be/X-ray binaries have been searched for possible Be-star counterparts. BVR c and H α CCD imaging was employed to identify early-type emission stars through colour indices and H α fluxes. Spectroscopy of five sources confirms the presence of H α emission in each case. Based on the positional coincidence of emission-line objects with the X-ray sources, we identify Be-star counterparts to the ROSAT sources RX J0032.9-7348, RX J0049.1-7250, RX J0054.9-7226 and RX J0101.0-7206, and to the recently discovered ASCA source AX J0051-722. We confirm the Be star nature of the counterpart to the HEAO1 source H0544-66. In the field of the ROSAT source RX J0051.8-7231 we find that there are three possible counterparts, each showing evidence for H α emission. We find a close double in the error circle of the EXOSAT source EXO 0531.1-6609, each component of which could be a Be star associated with the X-ray source.  相似文献   

2.
We report observations which identify the optical/IR counterpart to the ROSAT X-ray transient RX J0117.6−7330. The counterpart is suggested to be a B1–B2 star (luminosity class III–V) showing an IR excess and strong Balmer emission lines. The distance derived from reddening and systemic velocity measurements is consistent with the distance derived from X-ray measurements and puts the source in the Small Magellanic Cloud (SMC).  相似文献   

3.
We report observations that confirm the identities of the optical counterparts to the transient sources RX J0544.1–7100 and RX J0520.5–6932 . The counterparts are suggested to be B-type stars. Optical data from the observations carried out at ESO and SAAO, together with results from the OGLE data base, are presented. In addition, X-ray data from the RXTE All-Sky Monitor are investigated for long-term periodicities. A strong suggestion for a binary period of 24.4 d is seen in RX J0520.5–6932 from the OGLE data.  相似文献   

4.
We present results from our Chandra and XMM–Newton observations of two low-luminosity X-ray pulsators  SAX J1324.4−6200  and  SAX J1452.8−5949  which have spin periods of 172 and 437 s, respectively. The XMM–Newton spectra for both sources can be fitted well with a simple power-law model of photon index,  Γ∼ 1.0  . A blackbody model can equally well fit the spectra with a temperature,   kT ∼  2 keV, for both sources. During our XMM–Newton observations,  SAX J1324.4−6200  is detected with coherent X-ray pulsations at a period of 172.86 ± 0.02 s while no pulsations with a pulse fraction greater than 18 per cent (at 95 per cent confidence level) in 0.2–12 keV energy band are detected in  SAX J1452.8−5949  . The spin period of  SAX J1324.4−6200  is found to be increasing on a time-scale of     which would suggest that the accretor is a neutron star and not a white dwarf. Using subarcsec spatial resolution of the Chandra telescope, possible counterparts are seen for both sources in the near-infrared images obtained with the son of infrared spectrometer and array camera (SOFI) instrument on the New Technology Telescope. The X-ray and near-infrared properties of  SAX J1324.4−6200  suggest it to be a persistent high-mass accreting X-ray pulsar at a distance  ≤8 kpc  . We identify the near-infrared counterpart of  SAX J1452.8−5949  to be a late-type main-sequence star at a distance ≤10 kpc, thus ruling out  SAX J1452.8−5949  to be a high-mass X-ray binary. However, with the present X-ray and near-infrared observations, we cannot make any further conclusive conclusion about the nature of  SAX J1452.8−5949  .  相似文献   

5.
Hard X-ray selection is the most efficient way to discriminate between accretion-powered sources, such as active galactic nuclei (AGN), and sources dominated by starlight. Hard X-rays are also less affected than other bands by obscuration. We have therefore carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5–10 keV band, finding 180 sources in ∼50 deg2 of sky with flux≳5×10−14 erg cm−2 s−1. After correction for the non-uniform sky coverage this corresponds to resolving about 30 per cent of the hard cosmic X-ray background (XRB). Here we report on a first optical spectroscopic identification campaign, finding 12 AGN out of 14 X-ray error boxes studied. Seven AGN show evidence for obscuration in X-ray and optical bands, a fraction higher than in previous ROSAT or ASCA – ROSAT surveys (at 95–99 and 90 per cent confidence levels respectively), thus supporting the scenario in which a significant fraction of the XRB is created by obscured AGN.  相似文献   

6.
The excellent spatial resolution of the Chandra observatory offers the unprecedented possibility to measure proper motions at X-ray wavelength with relatively high accuracy using as reference the background of extragalactic or remote galactic X-ray sources. We took advantage of this capability to constrain the proper motion of RX J0806.4-4123 and RX J0420.0-5022, two X-ray bright and radio quiet isolated neutron stars (INSs) discovered by ROSAT and lacking an optical counterpart. In this paper, we present results from a preliminary analysis from which we derive 2σ upper limits of 76 mas/yr and 138 mas/yr on the proper motions of RX J0806.4-4123 and RX J0420.0-5022 respectively. We use these values together with those of other ROSAT discovered INSs to constrain the origin, distance and evolutionary status of this particular group of objects. We find that the tangential velocities of radio quiet ROSAT neutron stars are probably consistent with those of ‘normal’ pulsars. Their distribution on the sky and, for those having accurate proper motion vectors, their possible birth places, all point to a local population, probably created in the part of the Gould Belt nearest to the earth.   相似文献   

7.
It has been proposed that RX J1914.4+2456 is a stellar binary system with an orbital period of 9.5 min. As such it shares many similar properties with RX J0806.3+1527 (5.4 min). However, while the X-ray spectrum of RX J0806.3+1527 can be modelled using a simple absorbed blackbody, the X-ray spectrum of RX J1914.4+2456 has proved difficult to fit using a physically plausible model. In this paper, we re-examine the available X-ray spectra of RX J1914.4+2456 taken using XMM–Newton . We find that the X-ray spectra can be fitted using a simple blackbody and an absorption component which has a significant enhancement of neon compared to the solar value. We propose that the material in the interbinary system is significantly enhanced with neon. This makes its intrinsic X-ray spectrum virtually identical to RX J0806.3+1527. We re-access the X-ray luminosity of RX J1914.4+2456 and the implications of these results.  相似文献   

8.
A total of 235 active galactic nuclei (AGN) from two different soft X-ray surveys [the ROSAT Deep Survey (DRS) and the ROSAT International X-ray Optical Survey (RIXOS)] with redshifts between 0 and 3.5 are used to study the clustering of X-ray selected AGN and its evolution. A 2σ significant detection of clustering of such objects is found on scales < 40–80 h −1 Mpc in the RIXOS sample, while no clustering is detected on any scales in the DRS sample. Assuming a single power-law model for the spatial correlation function (SCF), quantitative limits on the AGN clustering have been obtained: a comoving correlation length 1.5 ≲  r 0 ≲ 3.3  h −1 Mpc is implied for comoving evolution, while 1.9 ≲  r 0 ≲ 4.8 for stable clustering and 2.2 ≲  r 0 ≲ 5.5 for linear evolution. These values are consistent with the correlation lengths and evolutions obtained for galaxy samples, but imply smaller amplitude or faster evolution than recent ultraviolet and optically selected AGN samples. We also constrain the ratio of bias parameters between X-ray selected AGN and IRAS galaxies to be ≲ 1.7 on scales ≲ 10  h −1 Mpc, a somewhat smaller value than is inferred from local large-scale dynamical studies.  相似文献   

9.
During a systematic search for periodic signals in a sample of ROSAT PSPC (0.1–2.4 keV) light curves, we have discovered ∼12-min large-amplitude X-ray pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the Galactic plane. The energy spectrum is well fitted by a power law with a photon index of 0.8, corresponding to an X-ray flux level of ∼ 10−12 erg cm−2 s−1. The source is probably a long-period, low-luminosity X-ray pulsar, similar to X Per, or an intermediate polar.  相似文献   

10.
We present ASCA data on RE J2248−511, extending existing optical and soft X-ray coverage to 10 keV, and monitoring the soft component. These data show that, despite a very strong ultrasoft X-ray excess below 0.3 keV and a soft 0.3–2 keV spectral index in earlier ROSAT data, the hard X-ray spectrum ( α ∼−0.8; 0.6–10 keV) is typical of type 1 active galactic nuclei (AGN), and the soft component has since disappeared. Optical data taken at two different epochs show that the big blue bump is also highly variable. The strength of the ultrasoft X-ray component and the extreme variability in RE J2248−511 are reminiscent of the behaviour observed in many narrow line Seyfert 1s (NLS1s). However, the high-energy end of the ROSAT spectrum, the ASCA spectrum and the Balmer line full widths at half maximum of ∼3000 km s−1 in RE J2248−511 are typical of normal Seyfert 1 AGN.
The change in the soft X-ray spectrum as observed in the ROSAT and ASCA data is consistent with the behaviour of Galactic Black Hole Candidates (GBHCs) as they move from a high to a low state, i.e. a fall in the ultrasoft component and a hardening of the X-ray continuum. This GBHC analogy has also been proposed for NLS1s. Alternatively, the variability may be caused by opacity changes in a hot, optically thin corona which surrounds a cold, dense accretion disc; this was first suggested by Guainazzi et al. for 1H 0419−577, an object which shows remarkably similar properties to RE J2248−511.  相似文献   

11.
A search for high-redshift ( z  > 0.5) ultrasoft X-ray AGN in pointed ROSAT PSPC observations is made, using the wgacat catalogue. Evidence for a further three such objects is found, adding to three identified previously (E1346+266, EXO 1346.4+2637 and RX J0947.0+4721). The flux of one new object [1WGA J1342.4+2720; L (0.1–2 keV)∼ 2 × 1045 erg s−1)] was found to be variable (a factor of 1.7 over 2 d), and this object has relatively narrow permitted line emission. These properties, also seen in E1346+266 and EXO 1346.4+2637, are typical of ultrasoft AGN at all redshifts, in particular the narrow-line Seyfert 1s (NLS1s). The rarity of high- z ultrasoft AGN places limits on the 'temperature' of the soft X-ray component in the rest frame of AGN in general. It also provides an opportunity to investigate models for the soft component in the high-temperature extreme, e.g., for accretion disc models, testing the physics at the inner edge of the disc, and the effects of Comptonization in a hot, optically thin corona.  相似文献   

12.
We present a sample of eight extended X-ray sources detected in the wide-field (∼2.3 deg2), bright (2–10 ks) XMM–Newton /2dF survey, reaching a flux limit of  ∼2 × 10−14 erg s−1 cm−2  . Of these, seven are identified as secure X-ray clusters in the soft 0.3–2 keV band using a standard wavelet algorithm on either the PN or the MOS images. Spectroscopic or photometric redshifts are available for five clusters, spanning a range between 0.12 and 0.68. The X-ray spectral fittings show temperatures between 1 and 4.6 keV, characteristic of poor clusters and groups of galaxies. We derive for the first time the XMM–Newton cluster number count  log  N –log  S   distribution albeit with poor statistics. Both the  log  N –log  S   and the luminosity–temperature relation are in good agreement with previous ROSAT results.  相似文献   

13.
We investigate the X-ray and optical properties of a sample of X-ray bright sources from the Small Magellanic Cloud (SMC) Wing Survey. We have detected two new pulsars with pulse periods of 65.8 s (CXOU J010712.6−723533) and 700 s (CXOU J010206.6−714115), and present observations of two previously known pulsars RX J0057.3−7325 (SXP101) and SAX J0103.2−7209 (SXP348). Our analysis has led to three new optical identifications for the detected pulsars. We find long-term optical periods for two of the pulsars, CXOU J010206.6−714115 and SXP101, of 267 and 21.9 d, respectively. Spectral analysis of a subset of the sample shows that the pulsars have harder spectra than the other sources detected. By employing a quantile-based colour–colour analysis we are able to separate the detected pulsars from the rest of the sample. Using archival catalogues we have been able to identify counterparts for the majority of the sources in our sample. Combining this with our results from the temporal analysis of the Chandra data and archival optical data, the X-ray spectral analysis, and by determining the X-ray to optical flux ratios we present preliminary classifications for the sources. In addition to the four detected pulsars, our sample includes two candidate foreground stars, 12 probable active galactic nuclei, and five unclassified sources.  相似文献   

14.
We present a measurement of the cluster X-ray luminosity–temperature ( L – T ) relation out to high redshift ( z ∼0.8). Combined ROSAT PSPC spectra of 91 galaxy clusters detected in the Wide Angle ROSAT Pointed Survey (WARPS) are simultaneously fitted in redshift and luminosity bins. The resulting temperature and luminosity measurements of these bins, which occupy a region of the high-redshift L – T relation not previously sampled, are compared with existing measurements at low redshift in order to constrain the evolution of the L – T relation. We find the best fit to low-redshift ( z <0.2) cluster data, at T >1 keV, to be L ∝ T 3.15±0.06. Our data are consistent with no evolution in the normalization of the L – T relation up to z ∼0.8. Combining our results with ASCA measurements taken from the literature, we find η =0.19±0.38 (for Ω0=1, with 1 σ errors) where L Bol∝(1+ z ) η T 3.15, or η =0.60±0.38 for Ω0=0.3. This lack of evolution is considered in terms of the entropy-driven evolution of clusters. Further implications for cosmological constraints are also discussed.  相似文献   

15.
I use ASCA data to investigate the 2–10 keV X-ray emission of active galactic nuclei (AGN) taken from the ROSAT International X-ray Optical Survey (RIXOS). I find that the integrated spectrum of these faint, soft X-ray-selected AGN in the 2–10 keV band is harder (best-fitting α = 0.8 ± 0.1) than the slope measured with ROSAT between 0.1 and 2 keV, but softer than the 2–10 keV X-ray background, and consistent with the average 2–10 keV spectrum of bright, nearby Seyfert galaxies. With this spectral slope and using measurements of the AGN contribution to the 1–2 keV X-ray background, I estimate that the AGN percentage contribution to the 2–10 keV background is 0.60 +0.19−0.14 times the AGN percentage contribution to the 1–2 keV background. Hence AGN produce between 12 and 32 per cent of the 2–10 keV X-ray background. This is only the contribution from the types of AGN which are found in soft X-ray surveys; a population of absorbed AGN could represent an additional component of the 2–10 keV X-ray background.  相似文献   

16.
We report on a ROSAT PSPC observation of the highly luminous z =4.72 radio-loud quasar GB 1428+4217 obtained between 1998 December 11 and 17, the final days of the ROSAT satellite. The low-energy sensitivity of the PSPC detector was employed to constrain the intrinsic X-ray absorption of the currently most distant X-ray detected object. Here we present the detection of significant soft X-ray absorption towards GB 1428+4217, making the absorbing material the most distant matter yet probed with X-ray spectroscopy. X-ray variability by 25±8 per cent is detected on a time-scale of 6500 s in the rest frame. The X-ray variation requires an unusually high radiative efficiency of at least 4.2, further supporting the blazar nature of the source.  相似文献   

17.
We report on multi-epoch X-ray observations of the Type IIn (narrow emission line) Supernova (SN) 1995N with the ROSAT and ASCA satellites. The 1998 January ASCA X-ray spectrum is well fitted by a thermal bremsstrahlung         or power-law         model. The X-ray light curve shows evidence for significant flux evolution between 1996 August and 1998 January: the count rate from the source decreased by 30 per cent between our 1996 August and 1997 August ROSAT observations, and the X-ray luminosity most likely increased by a factor of ∼2 between our 1997 August ROSAT and 1998 January ASCA observations, although evolution of the spectral shape over this interval is not ruled out. The high X-ray luminosity,     places SN 1995N in a small group of Type IIn supernovae with strong circumstellar interaction, and the evolving X-ray luminosity suggests that the circumstellar medium is distributed inhomogeneously.  相似文献   

18.
We present a catalogue of 147 serendipitous X-ray sources selected to have hard spectra ( α <0.5) from a survey of 188 ROSAT fields. Such sources must be the dominant contributors to the X-ray background at faint fluxes. We have used Monte Carlo simulations to verify that our technique is very efficient at selecting hard sources: the survey has 10 times as much effective area for hard sources as it has for soft sources above a 0.5–2 keV flux level of 10−14 erg cm−2 s−1. The distribution of best-fitting spectral slopes of the hard sources suggests that a typical ROSAT hard source in our survey has a spectral slope α ∼0. The hard sources have a steep number flux relation (d N /d S ∝ S − γ with a best-fitting value of γ =2.72±0.12) and make up about 15 per cent of all 0.5–2 keV sources with S >10−14 erg cm−2 s−1. If their N ( S ) continues to fainter fluxes, the hard sources will comprise ∼40 per cent of sources with 5×10−15< S <10−14 erg cm−2 s−1. The population of hard sources can therefore account for the harder average spectra of ROSAT sources with S <10−14 erg cm−2 s−1. They probably make a strong contribution to the X-ray background at faint fluxes and could be the solution to the X-ray background spectral paradox.  相似文献   

19.
We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensitive to X-ray sources down to a luminosity of approximately 1038 erg s−1. The galaxies are selected from a complete, volume-limited sample of 46 galaxies with     for which we have extensive multiwavelength data. For an almost complete subsample with     (29/31 objects) we have HRI images. Contour maps and source lists are presented within the central region of each galaxy, together with nuclear upper limits where no nuclear source was detected. Nuclear X-ray sources are found to be very common, occurring in ∼35 per cent of the sample. Nuclear X-ray luminosity is statistically connected to host galaxy luminosity – there is not a tight correlation, but the probability of a nuclear source being detected increases strongly with galaxy luminosity, and the distribution of nuclear luminosities seems to show an upper envelope that is roughly proportional to galaxy luminosity. While these sources do seem to be a genuinely nuclear phenomenon rather than nuclear examples of the general X-ray source population, it is far from obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and H  ii region nuclei are detected just as often as LINERs. Finally, we also note the presence of fairly common superluminous X-ray sources in the off-nuclear population – out of 29 galaxies we find nine sources with a luminosity greater than 1039 erg s−1. These show no particular preference for more luminous galaxies. One is already known to be a multiple SNR system, but most have no obvious optical counterpart and their nature remains a mystery.  相似文献   

20.
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   

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