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M. J. Church

Publications and source records attributed to M. J. Church.

At least 37 records · Page 2Linked to original sources
arXiv2003

The orbital period of the dipping, bursting, globular cluster X-ray source XB 1746-371 from Rossi X-ray Timing Explorer observations

We present results from two long observations of XB 1746-371 by the Rossi X-ray Timing Explorer (RXTE) in 2002 January and May, lasting 4 and 5 days respectively. Dips are observed in the X-ray light curves with a depth of 25 per cent, largely independent of energy within the usable band of the PCA instrument of 2.1 - 16.0 keV. X-ray bursting and flaring activity are also evident. The dips define the orbital period of the system, and using a power spectral analysis and a cycle counting technique we derive an accurate period of P_orb = 5.16 +/- 0.01 hr. The previously-reported candidate period of 5.73$\pm$0.15 hr, obtained using Ginga data, is inconsistent with our determination, perhaps due to the weakness of the dipping and the variability of the source during that observation. The dips in the RXTE observations presented here do not align with the Ginga period, however our improved period is consistent with a wide range of archival data.

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arXiv2003

Physical changes during Z-track movement in Sco X-1 on the flaring branch

We present results of a detailed study of X-ray flaring in the Z-track source Sco X-1 in a highly super-Eddington state made using high quality Rossi-XTE data from the PCA and HEXTE instruments. The emission model successfully used to explain the dipping LMXB, and other classes of LMXB in recent years, was applied to study the physical evolution along the Z-track which remains a major problem. This model consists of blackbody emission from the neutron star plus Comptonized emission from an extended accretion disk corona. As found in earlier work, major changes take place in the neutron star blackbody emission with kT increasing in flaring, and the blackbody radius R_BB increasing substantially to a maximum value of 9.4 +/- 0.6 km, consistent with the radius of the neutron star, after which R_BB decreases. Thus this result is a measurement of neutron star radius. The behaviour of Sco X-1 in flaring is compared with our previous results for the strong flaring that takes place in the bright dipping, flaring LMXB X 1624-490. Remarkably, during movement along the Normal Branch towards the apex with the Flaring Branch, the luminosities of both spectral components decrease, suggesting the possibility that Mdot may decrease on the Normal Branch, contrary to the widely-held view that Mdot increases monotonically along the Z-track. During flaring, we detect for the first time an increase of the Comptonization cut-off energy which may suggest heating of the ADC plasma by the neutron star flare. The energy of a broad Gaussian line at 6.4 keV does not change, but the intensity of the line increases in flaring suggesting either an increase in ADC size in flaring or the effects of irradiation by the neutron star.

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arXiv2003

Ultra-Luminous X-ray Source Populations in Normal Galaxies: a Preliminary Survey with Chandra

We present results of a Chandra survey of the ultra-luminous X-ray sources (ULX) in 13 normal galaxies, in which we combine source detection with X-ray flux measurement. 22 ULX were detected, i.e. with L_x > 1 x 10^{39} erg s^{-1} (L_10), and 39 other sources were detected with L_x > 5 x 10^{38} erg s^{-1} (L_5). We also use radial intensity profiles to remove extended sources from the sample. The majority of sources are not extended, which for a typical distance constrains the emission region size to less than 50 pc. X-ray colour-colour diagrams and spectral fitting results were examined for indicators of the ULX nature. In the case of the brighter sources, spectral fitting generally requires two-component models. In only a few cases do colour-colour diagrams or spectral fitting provide evidence of black hole nature. We find no evidence of a correlation with stellar mass, however there is a strong correlation with star formation as indicated by the 60 micron flux as found in previous studies.

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arXiv2002

A comparison of neutron star blackbody luminosities in LMXB with the theory of accretion flow spreading on the stellar surface

We present a comparison of the results of the ASCA survey of LMXB with the Inogamov and Sunyaev theory of accretion flow spreading on the surface of neutron stars. The ASCA survey of LMXB of Church and Balucinska-Church (2001) revealed a systematic variation of the luminosity of blackbody emission from the neutron star spanning 3 decades in total X-ray luminosity suggesting that the level of blackbody emission is controlled by the physics of the inner disk/stellar interface, which we can hope to understand. Two types of explanation exist: firstly that there is radial flow between the inner disk and star at all vertical positions above the orbital plane so that the height of the disk directly determines the area of star emitting. Secondly, the height of the emitting region on the star is not directly related to the disk properties but depends on the mass accretion rate as suggested by Inogamov and Sunyaev (1999) in their theory of accretion flow spreading on the stellar surface. We find that the survey results for the emitting area agree with this theory at the lowest luminosities. However, for higher luminosities, the blackbody emission is stronger than predicted by spreading theory suggesting that the emitting area is controlled by radial flow between disk and star.

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arXiv2001

A study of the spectral evolution during dipping in XB 1323-619 with Rossi-XTE and BeppoSAX

We report results from analysis of the observations of the dipping low mass X-ray binary XB 1323-619 made with BeppoSAX} and Rossi-XTE. The dust-scattered halo contributes significantly in this source, and the observation made with BeppoSAX on 1997 August was used to provide MECS radial intensity profiles at several energies. From these, the halo fractions were obtained and thus an optical depth to dust scattering of 1.8 +/- 0.4 derived. In the Rossi-XTE observation of April 25-28, 1997, seven X-rays dips were observed together with 7 bursts repeating approximately periodically. Non-dip and dip PCA spectra can be well-described by assuming the emission consists of point-like blackbody emission identified with the neutron star, plus Comptonized emission from an extended ADC. The blackbody temperature is 1.79 +/- 0.21 keV and the cut-off power law photon index 1.61 +/- 0.04. Spectral evolution in dipping is well described by progressive covering of the extended Comptonizing region by absorber plus more rapid removal of the point-like blackbody. The effects of dust scattering and of the X-ray pulsar 1SAX J1324.4-6200 also in the field of view are included in the fitting. We detect an iron line at ~6.4 keV and its probable origin in the ADC is discussed.

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arXiv2001

Neutron star blackbody contraction during flaring in X1624-490

We present results of an investigation of the physical changes taking place in the emission regions of the LMXB X1624-490 during strong flaring in RXTE observations. Based on the detailed light curve, we propose that the flaring consists of a superposition of X-ray bursts. It is shown that major changes take place in the blackbody emission component, the temperature kT_BB increasing to ~2.2 keV in flaring. Remarkably, the blackbody area decreases by a factor of ~5 in flaring. During flare evolution, the blackbody luminosity remains approximately constant, constituting a previously unknown Eddington limiting effect which we propose is due to radiation pressure of the blackbody as kT_BB increases affecting the inner disk or accretion flow resulting in a decreased emitting area on the star. We argue that the large decrease in area cannot be explained in terms of modification of the blackbody spectrum by electron scattering in the atmosphere of the neutron star. The height of the emitting region on the non-flaring neutron star is shown to agree with the height of the inner radiatively-supported disk as found for sources in the ASCA survey of LMXB of Church & Balucinska-Church (2001). The decrease in height during flaring is discussed in terms of possible models, including radial accretion flow onto the stellar surface and the theory of accretion flow spreading on the neutron star surface of Inogamov & Sunyaev (1999). We demonstrate that the intensity of the broad iron line at 6.4 keV is strongly correlated with the luminosity of the blackbody emission from the neutron star, and discuss the probable origin of this line in the ADC. Finally, possible reasons for non-detection of a reflection component in this source, and LMXB in general, are discussed.

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arXiv2001

Results of a LMXB survey: variation in the height of the neutron star blackbody emission region

We present results of a survey of the spectra of Low Mass X-ray Binaries using ASCA. It is shown that all sources in the survey are well-fitted by the same two-component emission model that we have previously shown is able to describe both the non-dip and dip spectra of the dipping class of LMXB. This model consists of point-like blackbody emission from the neutron star plus Comptonized emission from a disk-like accretion disk corona of radius typically 50,000 km. Additional data from results published elsewhere by us from BeppoSAX and ASCA are added to the survey. The large variation in blackbody luminosity of survey sources is shown to be due primarily to major changes in blackbody emitting area. Fitting a multi-temperature disk blackbody plus Comptonization model to the survey spectra requires values of inner disk radius substantially less than the neutron star radius in many cases, making disk origin of the blackbody highly unlikely. Assuming that the emission is from an equatorial strip on the neutron star, it is shown that the half-height of the strip h agrees well with the half-height H of the radiatively-supported inner accretion disk, this agreement spanning three orders of magnitude in each parameter. Possible mechanisms for the agreement are discussed, including radial accretion flow between inner disk and star, and accretion flow ``creep' on the surface of the neutron star.

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arXiv2000

The emission regions in X-ray binaries: dipping as a diagnostic

X-ray dipping in the black hole binary Cygnus X-1, the Galactic jet source GRO J1655-40 and in low mass X-ray binaries is discussed. It is shown that spectral analysis strongly constrains emission models. Measurement of dip ingress/egress times allows the sizes of extended emission regions to be determined, notably for the Accretion Disk Corona which is responsible for Comptonization in X-ray binaries. In LMXB, the radius of the ADC is shown to be between ~ 10^9 and ~ 5x10^10 cm, an appreciable fraction of the accretion disk radius. This is inconsistent with Comptonization models requiring a localized Comptonizing region, for example, in the immediate neighbourhood of the neutron star. Results from a survey of LMXB using ASCA and BeppoSAX reveal an approximate equality between the height of the blackbody emission region on the neutron star and the height of the inner radiatively-supported disk, suggesting either that there is a direct causal link, such as a radial accretion flow between the inner disk edge and the star, or an indirect link, as in the case of accretion flow creep on the surface of the neutron star as suggested by Inogamov & Sunyaev. Finally, the survey shows that the blackbody cannot originate on the accretion disk as the required inner radii in many sources are substantially less than the neutron star radius.

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arXiv2000

The Ephemeris and Dipping Spectral Behavior of X1624-490

We present striking results from Rossi X-ray Timing Explorer (RXTE) observations of the 21-hr low mass X-ray binary X1624-490, showing five complex dips in unprecedented detail. For the first time, dipping is detected up to 15 keV. Prominent flares are also observed in the light curves, limited to energies above 8 keV. Spectra selected by intensity during dip episodes can be well fit with a two-component model consisting of a point-like blackbody from the neutron star and progressive covering of an extended Comptonized region, presumably an accretion disk corona (ADC), corrected for photons scattered into and out of the X-ray beam by a interstellar dust halo. We find that the outer regions of the absorber are highly ionized and that electron scattering is totally responsible for the X-ray attenuation during shallow dipping. The timescales of dip ingress and egress indicate that the envelope of material absorbing the ADC has smaller angular size than the ADC itself, and that the ADC is likely limited to a height-to-radius ratio of 10%, rather than being spherical in extent. In addition, we have analyzed 4.5 yrs of RXTE All Sky Monitor (ASM) coverage to derive the first accurate orbital ephemeris for X1624-490, with phase zero (the time of dip centers) well-described by the relation 2450088.63918(69) + N*0.869907(12) (JD).

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arXiv2000

A study of the dipping low mass X-ray binary Xþ1624-490 from the broadband BeppoSAX observation

A study of the luminous dipping LMXB X1624-490 was made using BeppoSAX. From modelling the MECS radial profile in several energy bands, the optical depth to dust scattering at 1 keV was found to be 2.4+/-0.4. The non-dip and dip spectra of X1624-490 in the energy band 1-100 keV are well-described by emission consisting of a point-like blackbody from the neutron star plus extended Comptonized emission from an ADC. The blackbody temperature was 1.31+/-0.07 keV and the Comptonized emission had photon power law index 2.0+0.5/-0.7 and cut-off energy ~12 keV. Good fits to the dip spectra were obtained by allowing the Comptonized emission to be progressively covered by an extended absorber while the blackbody was rapidly absorbed and a constant halo component accounted for dust scattering into the line-of-sight. From the dip ingress time, a diameter of the extended Comptonized emission region of 5.3+/-0.8x10^10 cm was derived, consistent with a hot, X-ray emitting corona extending to ~50% of the accretion disk radius. The source luminosity for a distance of 15 kpc is 7.3x10^37 erg/s, an appreciable fraction of the Eddington limit making X1624-490 the most luminous dipping LMXB. The half-height of the blackbody emitting region on the neutron star of 6.8+/-1.8 km agrees well with the half-height of the radiatively supported inner accretion disk of 6.3+/-2.9 km. Together with similar agreement recently obtained for 13 other LMXB in an ASCA survey (Church & Balucinska-Church 2000), this strongly supports the identification of the neutron star as the origin of the blackbody emission in LMXB and suggests a mechanism in which the disk height is the main factor determining the level of blackbody emission. Finally, from RXTE ASM data, we derive an improved orbital period of 20.87+/-0.01 hr.

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arXiv1999

Discovery of a red and blue shifted iron disk line in the galactic jet source GRO J1655-40

We report the discovery of emission features in the X-ray spectrum of GRO J1655-40 obtained with RXTE during the observation of 1997, Feb 26. We have fitted the features firstly by two Gaussian lines which in four spectra analysed have average energies of 5.85 +/- 0.08 keV and 7.32 +/- 0.13 keV, strongly suggestive that these are the red and blue shifted wings of an iron disk line. These energies imply a velocity of ~0.33 c. The blue wing is less bright than in the calculated profiles of disk lines near a black hole subject to Doppler boosting, however known Fe absorption lines in GRO J1655-40 at energies between ~7 and 8 keV can reduce the apparent brightness of the blue wing. Secondly, we have fitted the spectra using the disk line model of Laor based on a full relativistic treatment plus an absorption line, and show that good fits are obtained. This gives a restframe energy of the disk line between 6.4 and 6.8 keV indicating that the line is iron K_alpha emission probably of significantly ionized material. The Laor model shows that the line originates in a region of the accretion disk extending from ~10 Schwarzschild radii from the black hole outwards. The line is direct evidence for the black hole nature of the compact object and is the first discovery of a highly red and blue shifted iron disk line in a Galactic source.

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arXiv1999

The Distribution of X-ray Dips with Orbital Phase in Cygnus X-1

We present results of a comprehensive study of the distribution of absorption dips with orbital phase in Cygnus X-1. Firstly, the distribution was obtained using archival data from all major X-ray observatories and corrected for the selection effect that phase zero (superior conjunction of the black hole) has been preferentially observed. Dip occurrence was seen to vary strongly with orbital phase ϕ, with a peak at ϕ~ 0.95, i.e. was not symmetric about phase zero. Secondly, the RXTE ASM has provided continuous coverage of the Low State of Cygnus X-1 since Sept. 1996, and we have selected dip data based on increases in hardness ratio. The distribution, with much increased numbers of dip events, confirms that the peak is at ϕ~ 0.95, and we report the discovery of a second peak at ϕ~ 0.6. We attribute this peak to absorption in an accretion stream from the companion star HDE 226868. We have estimated the ionization parameter at different positions showing that radiative acceleration of the wind is suppressed by photoionization in particular regions in the binary system. To obtain the variation of column density with phase, we make estimates of neutral wind density for the extreme cases that acceleration of the wind is totally suppressed, or not suppressed at all. An accurate description will lie between these extremes. In each case, a strong variation of column density with orbital phase resulted, similar to the variation of dip occurrence. This provides evidence that formation of the blobs in the wind which lead to absorption dips depends on the density of the neutral component in the wind, suggesting possible mechanisms for blob growth.

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arXiv1999

An X-ray study of the dipping low mass X-ray binary XB 1323-619

During a BeppoSAX observation of the low-mass X-ray binary dip source XB 1323-619 a total of 10 type I X-ray bursts and parts of 12 intensity dips were observed. During non-bursting, non-dipping intervals, the 1-150 keV BeppoSAX spectrum can be modelled by a cutoff power-law with a photon index of 1.48 +/- 0.01, a cutoff energy of 44.1 +5.1/-4.4 keV together with a blackbody with kT of 1.77 +/- 0.25 keV contributing ~15% of the 2-10 keV flux. Absorption equivalent to 3.88 +/- 0.16x10^22 H atom cm^(-2) is required. The dips repeat with a period of 2.938 +/- 0.020 hr and span 40% of the orbital cycle. During dips the maximum reduction in 2-10 keV intensity is ~65%. The spectral changes during dips are complex and cannot be modelled by a simple absorber because of the clear presence of part of the non-dip spectrum which is not absorbed. Spectral evolution in dipping can be well modelled by progressive covering of the cutoff power-law component which must be extended, plus rapid absorption of the point-source blackbody. One of the bursts is double and 4 of the bursts occurred during dipping intervals. These bursts have 2-10 keV peak count rates reduced by only 22% on average from those occurring outside the dips, and are not heavily absorbed. One explanation for this lack of absorption is that the bursts temporarily ionize the absorbing material responsible for the dips.

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arXiv1998

The Orbital Period of HDE226868/Cyg X-1

We present epoch 1996, high-quality radial velocity data for HDE 226868, the optical counterpart of Cygnus X-1. Combining our results with all published historical data, we have derived a new ephemeris for the system of HJD2450235.29 + n x 5.5998, which allows accurate orbital phase calculations to be made for any X-ray observations over the last 30 years. We find no evidence for any period change as has been suggested by Ninkov, Walker & Yang (1987). We discuss the shortcomings of previous work in establishing the period and orbital elements.

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arXiv1998

Progressive Covering of the Accretion Disk Corona during Dipping in the LMXB XB 1916-053

Results are reported for analysis of the extensive Rosat observation of the dipping low mass X-ray binary XB 1916-053. Dipping is 100% deep showing that the emission regions are completely covered by the absorber. A good fit to the non-dip spectrum is obtained using a model consisting of a blackbody with kT_BB = 1.95 +0.74 -0.34 keV and a power law with photon index 2.32 +/- 0.04. These components are identified with emission from the neutron star, and Comptonized emission from an extended accretion disk corona (ADC). Dip spectra are well-fitted by rapid absorption of the blackbody, and progressive covering of the extended component, as the absorber moves across the source, with a covering fraction that increases smoothly from zero to ~1.0. Progressive covering shows that the Comptonized emission region is extended, consistent with it originating in the accretion disk corona. The strong unabsorbed component in the dip spectra is well-modelled as the uncovered part of the Comptonized emission at all stages of dipping. There is no detectable change in the low energy cut-off of the spectrum in dipping which supports the identification of the unabsorbed part of the spectrum with the uncovered part of the ADC emission. The absorbed part of the ADC emission is rapidly removed from the 0.1 - 2.0 keV band of the PSPC, which therefore selects only the uncovered part of the emission, and so the spectral evolution in dipping as viewed by the PSPC depends only on the covering fraction, determined by the geometric overlap between the source and absorber.

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arXiv1998

Discovery of a new 170 s X-ray pulsar 1SAX J1324.4-6200

We report the discovery with BeppoSAX of a new 170.84 +/- 0.04 s X-ray pulsar, 1SAX J1324.4-6200, found serendipitously in the field of the X-ray binary X1323-619 in 1997 August. The source and periodicity are also detected in archival ASCA data taken in 1994. The X-ray spectrum is modeled by a power-law with a photon index of 1.0 +/- 0.4 and absorption of (7.8 +2.7 -1.1) 10^22 atom cm-2. The source is located close to the galactic plane and within 3 arcmin of the direction of the dark cloud DC 306.8+0.6. The measured interstellar absorption and cloud size imply a distance >3.4 kpc. This implies a 1-10 keV source luminosity of >1.1 10^34 erg/sec during the BeppoSAX observation. The source is not detected in earlier Einstein IPC and EXOSAT CMA observations, most probably due to reduced detector efficency and lower sensitivity to highly absorbed sources. The X-ray properties suggest that 1SAX J1324.4-6200 is an accreting neutron star with a Be star companion.

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arXiv1998

Progressive covering in dipping and Comptonization in the spectrum of XB1916-053 from the BeppoSAX observation

We report results of a BeppoSAX observation of the low-mass X-ray binary (LMXB) dipping source XB 1916-053. The source joins the small group of LMXB detected at energies ~100 keV. The non-dip spectrum is well fitted by an absorbed blackbody with a temperature of 1.62+/-0.05 keV and an absorbed cut-off power law with a photon index of 1.61+/- 0.01 and a cut-off energy of 80+/-10 keV. Below 10 keV, where photoelectric absorption is dominant, the dramatic spectral changes observed during dips can be simply modelled by progressive covering of the blackbody and cut-off power law components. The blackbody component is very rapidly absorbed during dips, consistent with it being point-like, while the cutoff power law is more gradually absorbed, consistent with it being extended. The most likely locations for the blackbody component are the surface of the neutron star or the boundary layer between the neutron star and the accretion disk. The extended emission most probably originates in an accretion disk corona. Above 10 keV, dipping is detected up to \~40 keV, and there is some evidence for an energy-independent reduction in intensity of up to 15%. This reduction could be caused by electron scattering or obscuration. In the first case, the change is consistent with an electron column density of ~2.9e23cm^-2, several times smaller than the average hydrogen column measured simultaneously.

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