Low-mass X-ray Binary XB Research Articles

A 2.15 hr ORBITAL PERIOD FOR THE LOW-MASS X-RAY BINARY XB 1832-330 IN THE GLOBULAR CLUSTER NGC 6652

We present a candidate orbital period for the low mass X-ray binary XB 1832-330 in the globular cluster NGC 6652 using a 6.5 hour Gemini South observation of the optical counterpart of the system. Light curves in g' and r' for two LMXBs in the cluster, sources A and B in previous literature, were extracted and analyzed for periodicity using the ISIS image subtraction package. A clear sinusoidal modulation is evident in both of A's curves, of amplitude ~0.11 magnitudes in g' and ~0.065 magnitudes in r', while B's curves exhibit rapid flickering, of amplitude ~1 magnitude in g' and ~0.5 magnitudes in r'. A Lomb-Scargle test revealed a 2.15 hour periodic variation in the magnitude of A with a false alarm probability less than 10^-11, and no significant periodicity in the light curve for B. Though it is possible saturated stars in the vicinity of our sources partially contaminated our signal, the identification of A's binary period is nonetheless robust.

Neutral absorber dips in the periodic burster LMXB XB 1323-619 from Suzaku

We present results of an observation with Suzaku of the dipping, periodic bursting low mass X-ray binary XB 1323-619. Using the energy band 0.8 - 70 keV, we show that the source spectrum is well-described as the emission of an extended accretion disk corona, plus a small contribution of blackbody emission from the neutron star. The dip spectrum is well-fitted by the progressive covering model in which the extended ADC is progressively overlapped by the absorbing bulge of low ionization state in the outer accretion disk and that dipping is basically due to photoelectric absorption in the bulge. An energy-independent decrease of flux at high energies (20 - 70 keV) is shown to be consistent with the level of Thomson scattering expected in the bulge. An absorption feature at 6.67 keV (Fe XXV) is detected in the non-dip spectrum and other possible weak features. In dipping, absorption lines of medium and highly ionized states are seen suggestive of absorption in the ADC but there is no evidence that the lines are stronger than in non-dip. We show that the luminosity of the source has changed substantially since the Exosat observation of 1985, increasing in luminosity between 1985 and 2003, then in 2003 - 2007 falling to the initial low value. X-ray bursting has again become periodic, which it ceased to do in its highest luminosity state, and we find that the X-ray bursts exhibit both the fast decay and later slow decay characteristic of the rp burning process. We present arguments against the recent proposal that the decrease of continuum flux in the dipping LMXB in general can be explained as absorption in an ionized absorber rather than in the bulge in the outer disk generally accepted to be the site of absorption.

Variations in the dip properties of the low-mass X-ray binary XB 1254-690 observed with XMM-Newton and INTEGRAL

We have analysed data from five XMM-Newton observations of XB 1254-690, one of them simultaneous with INTEGRAL, to investigate the mechanism responsible for the highly variable dip durations and depths seen from this low-mass X-ray binary. Deep dips were present during two observations, shallow dips during one and no dips were detected during the remaining two observations. At high (1–4 s) time resolution “shallow dips” are seen to include a few very rapid, deep dips whilst the “deep” dips consist of many similar very rapid, deep fluctuations. The folded V -band Optical Monitor light curves obtained when the source was undergoing deep, shallow and no detectable dipping exhibit sinusoid-like variations with different amplitudes and phases. We fit EPIC spectra obtained from “persistent” or dip-free intervals with a model consisting of disc-blackbody and thermal Comptonisation components together with Gaussian emission features at 1 and 6.6 keV modified by absorption due to cold and photo-ionised material. None of the spectral parameters appears to be strongly correlated with the dip depth except for the temperature of the disc blackbody which is coolest ( kT ~ 1.8 keV) when deep dips are present and warmest ( kT ~ 2.1 keV) when no dips are detectable. We propose that the changes in both disc temperature and optical modulation could be explained by the presence of a tilted accretion disc in the system. We provide a revised estimate of the orbital period of 0.16388875 ± 0.00000017 day.

XMM-Newton observations of the low-mass X-ray binary XB 1832–330 in the galactic globular cluster NGC 6652

Context. We report on two XMM-Newton observations performed in 2006 of the luminous low-mass X-ray binary XB 1832–330, which is located in the galactic globular cluster NGC 6652 and is probably an ultracompact binary (orbital period, Porb, of <1 h).

Discovery of absorption features of the accretion disc corona and systematic acceleration of the X-ray burst rate in XB 1323-619

We present results from analysis of the observation of the dipping, quasi-periodic bursting low mass X-ray binary XB 1323-619 made with XMM-Newton in 2003 January. In spectral analysis of the non-dip, non-burst EPIC PN spectrum, a number of absorption lines were discovered, notably at 6.70 and 6.98 keV, which we identify with scattering by high ionization state ions Fe xxv and Fe xxvi. Such features have been seen in other dipping sources, but their origin was not understood. Curve of growth analysis provided a consistent solution in which the line ratio was reproduced assuming collisional ionization with kT = 31 keV, close to the electron temperature we previously determined for the accretion disc corona (ADC) in this source. We thus propose that the absorption lines in the dipping low mass X-ray binaries (LMXB) are produced in the ADC. Spectral evolution in dipping was well described by the progressive covering model, which we have previously shown to give very good explanations of many dipping sources. We discuss the proposal of Boirin et al. (2004b), based on analysis of the present observation, that spectral evolution in all of the dipping LMXB may be explained by subjecting the continuum to a highly ionized absorber. This would require a decrease in X-ray intensity by a factor of ∼3 in dipping at energies where photoelectric absorption is not effective (40-100 keV), which previous BeppoSAX analysis of several sources firmly rules out. In XB 1323-619, any decrease was less than 10 ± 10 per cent and so the ionized absorber proposal can be ruled out. We find a remarkable linear increase in the rate of X-ray bursts with time over the 14-yr period since 1989 and a systematic non-linear increase in source luminosity (L). The linear variation of burst rate with L shows that the burst rate is proportional to mass accretion rate and if continued implies that the gap between bursts will become zero on 2008 January 11. In reality, we expect the source to undergo a transition from X-ray bursting to X-ray flaring, which would confirm the suggestion we previously made that flaring is unstable nuclear burning effectively consisting of a superposition of X-ray bursts.

Discovery of X-ray absorption features from the dipping low-mass X-ray binary XB 1916-053 with XMM-Newton

We report the discovery of narrow Fe XXV and Fe XXVI K alpha X-ray absorption lines at 6.65 and 6.95 keV in the persistent emission of the dipping low-mass X-ray binary (LMXB) XB 1916-053 during an XMM-Newton observation performed in September 2002. In addition, there is marginal evidence for absorption features at 1.48 keV, 2.67 kev, 7.82 keV and 8.29 keV consistent with Mg XII, S XVI, Ni XXVII K alpha and Fe XXVI K beta transitions, respectively. Such absorption lines from highly ionized ions are now observed in a number of high inclination (ie. close to edge-on) LMXBs, such as XB 1916-053, where the inclination is estimated to be between 60-80 degrees. This, together with the lack of any orbital phase dependence of the features (except during dips), suggests that the highly ionized plasma responsible for the absorption lines is located in a cylindrical geometry around the compact object. Using the ratio of Fe XXV and Fe XXVI column densities, we estimate the photo-ionization parameter of the absorbing material to be 10^{3.92} erg cm s^{-1}. Only the Fe XXV line is observed during dipping intervals and the upper-limits to the Fe XXVI column density are consistent with a decrease in the amount of ionization during dipping intervals. This implies the presence of cooler material in the line of sight during dipping. We also report the discovery of a 0.98 keV absorption edge in the persistent emission spectrum. The edge energy decreases to 0.87 keV during deep dipping intervals. The detected feature may result from edges of moderately ionized Ne and/or Fe with the average ionization level decreasing from persistent emission to deep dipping. This is again consistent with the presence of cooler material in the line of sight during dipping.

Dip Spectroscopy of the Low‐Mass X‐Ray Binary XB 1254−690

We observed the low-mass X-ray binary XB 1254-690 with the Rossi X-Ray Timing Explorer in 2001 May and December. During the first observation, strong dipping on the 3.9 hr orbital period and a high degree of variability were observed, along with "shoulders" ~15% deep during extended intervals on each side of the main dips. The first observation also included pronounced flaring activity. The nondip spectrum obtained using the PCA instrument was well described by a two-component model consisting of a blackbody with kT = 1.30 ± 0.10 keV plus a cutoff power-law representation of Comptonized emission with power-law photon index 1.10 ± 0.46 and a cutoff energy of 5.9 keV. The intensity decrease in the shoulders of dipping is energy-independent, consistent with electron scattering in the outer ionized regions of the absorber. In deep dipping, the depth of dipping reached 100% in the energy band below 5 keV, indicating that all emitting regions were covered by absorber. Intensity-selected dip spectra were well fitted by a model in which the pointlike blackbody is rapidly covered, while the extended Comptonized emission is progressively overlapped by the absorber, with the covering fraction rising to 95% in the deepest portion of the dip. The intensity of this component in the dip spectra could be modeled by a combination of electron scattering and photoelectric absorption. Dipping did not occur during the 2001 December observation, but remarkably, both bursting and flaring were observed contemporaneously.

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 signicantly in this source, and the observation made with BeppoSAX on 1997 August was used to provide MECS radial intensity proles at several energies. From these, the halo fractions were obtained and thus an optical depth to dust scattering of 1:8 0: 4d e- rived. 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 emis- sion consists of point-like blackbody emission identied with the neutron star, plus Comptonized emission from an extended ADC. The blackbody temperature is 1:79 0:21 keV and the cut-o 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 eects of dust scattering and of the X-ray pulsar 1SAX J1324.4{6200 also in the eld of view are included in the tting. We detect an iron line at6.4 keV and its probable origin in the ADC is discussed.

Progressive covering of the accretion disc 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 (LMXB) XB 1916--053. The dipping is 100 per cent 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 kTBB = 1.95+0.740.34 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 disc 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 0 to ∼1.0. Progressive covering shows that the Comptonized emission region is extended, consistent with it originating in the accretion disc 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 ROSAT Position Sensitive Proportional Counter (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.

The optical light curve of the low-mass X-ray binary XB 1254-690

view Abstract Citations (55) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Optical Light Curve of the Low-Mass X-Ray Binary XB 1254-690 Motch, Christian ; Pedersen, Holger ; Beuermann, Klaus ; Pakull, Manfred W. ; Courvoisier, Thierry J. -L. Abstract The discovery of periodic optical variations from the counterpart of the low-mass X-ray binary XB 1254-690 is reported. The period is 0.163890 + or - 0.000009 days, identical to the recurrence period of X-ray dips. The full amplitude of the light curve is 0.4 mag and the mean V-magnitude is 19.1. From spectrophotometric observations the authors derive a B-V color index of 0.31 + or - 0.05 and an interstellar reddening of E(B-V) = 0.4 + or - 0.1. The optical minimum occurs 0.15 in phase after X-ray dips. Most of the optical light curve is probably due to varying aspect of the X-ray-heated secondary atmosphere. The contribution of the X-ray-heated bulge does not dominate the optical variability. The X-ray-heated secondary explanation supports the interpretation that the X-ray dips are due to eclipses by a bulge located on the edge of the accretion disk and indicates that at least in the case of XB 1254-690 the accretion disk does not shadow completely the companion star. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/165017 Bibcode: 1987ApJ...313..792M Keywords: Light Curve; Optical Measurement; Periodic Variations; Visual Photometry; X Ray Binaries; Accretion Disks; Interstellar Extinction; Stellar Color; Stellar Mass; Stellar Models; Stellar Spectrophotometry; Astrophysics; STARS: INDIVIDUAL ALPHANUMERIC: XB 1254-690; X-RAYS: BINARIES full text sources ADS | data products SIMBAD (1)

The discovery of 39 hour periodic dips in the X-ray intensity of XB 1254 - 690

view Abstract Citations (68) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Discovery of 3.9 Hour Periodic Dips in the X-Ray Intensity of XB 1254-690 Courvoisier, T. J. -L. ; Parmar, A. N. ; Peacock, A. ; Pakull, M. Abstract The low-mass X-ray binary XB 1254 - 690 has been discovered to show irregular dips in X-ray intensity that recur with a period of 3.9 hr consistent with the optical period derived independently. The spectral changes observed during dipping are complex and can be successfully described using a two-component model. One of these components may be X-rays scattered into the line of sight by an accretion disk corona. In contrast to other X-ray dipping sources for which the metallicity has been measured, the abundance of the material responsible for the dips is close to that of cosmic material. Publication: The Astrophysical Journal Pub Date: October 1986 DOI: 10.1086/164598 Bibcode: 1986ApJ...309..265C Keywords: Variable Stars; X Ray Binaries; X Ray Spectra; Accretion Disks; Exosat Satellite; Light Curve; Metallicity; Neutron Stars; Astrophysics; STARS: INDIVIDUAL ALPHANUMERIC: XB 1254-690; X-RAYS: BINARIES full text sources ADS | data products SIMBAD (3)