X-ray Dips Research Articles

A Hard Tail in the Broadband Spectrum of the Dipper XB 1254−690

We report on the results of spectral analysis of the dipping source XB 1254-690 observed by the BeppoSAX satellite. We find that the X-ray dips are not present during the BeppoSAX observation, in line with recent RXTE results. The broadband (0.1-100 keV) energy spectrum is well fitted by a three-component model consisting of a multicolor disk blackbody with an inner disk temperature of ~0.85 keV, a Comptonized spectrum with an electron temperature of ~2 keV, and bremsstrahlung at a temperature of ~20 keV. Adopting a distance of 10 kpc and taking into account a spectral hardening factor of ~1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is Rin 1/2 ~ 11 km, where i is the inclination angle of the system with respect to the line of sight. The Comptonized component could originate in a spherical corona or boundary layer, surrounding the neutron star, with optical depth ~19. The bremsstrahlung emission, contributing ~4% of the total luminosity, probably originates in an extended accretion disk corona with radius ~1010 cm. In this scenario, we calculated that the optical depth of this region is ~0.71 and its mean electron density is Ne ~ 2.7 × 1014 cm-3. This last component might also be present in other low-mass X-ray binaries, but, because of its low intrinsic luminosity, it is not easily observable. We also find an absorption edge at ~1.27 keV with an optical depth of ~0.15. Its energy could correspond to the L edge of Fe XVII or K edge of Ne X or neutral Mg.

Detection of a Series of X‐Ray Dips Associated with a Radio Flare in GRS 1915+105

We report the detection of a series of X-ray dips in the Galactic black hole candidate GRS 1915+105 during 1999 June 6-17 from observations carried out with the pointed proportional counters of the Indian X-ray Astronomy Experiment on board the Indian Remote Sensing Satellite (IRS-P3). The observations were made after the source made a transition from a steady low-hard state to a chaotic state, which occurred within a few hours. Dips of about 20-160 s in duration were observed on most of the days. The X-ray emission outside the dips shows a quasi-periodic oscillation (QPO) at ~4 Hz that has characteristics similar to the ubiquitous 0.5-10 Hz QPO seen during the low-hard state of the source. During the onset of dips this QPO is absent, and, also, the energy spectrum is soft and the variability is low compared to the nondip periods. These features gradually reappear as the dip recovers. The onset of the occurrence of a large number of such dips followed the start of a huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as the cause for mass ejection due to the evacuation of matter from an accretion disk around the black hole. We propose that a superposition of a large number of such dip events produces a huge radio jet in GRS 1915+105.

Peculiar x-ray dips in the superluminal source GRS 1915+105 observed in the soft state

This paper reports the detection of a series of X-ray dips in the superluminal black hole source GRS 1915+105 during 1999 June 6–17 from observations carried out with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment on board the Indian satellite IRS—P3. The observations made in the soft state after the source had made a transition from a low-hard state to a high-soft state, reveal large number of x-ray dips each lasting for about 100– 150 seconds on most of the days. Quasi periodic oscillation (QPOs) with characteristics similar to those of the 0.5–10 Hz QPOs, seen during the low-hard state of the source, were detected in the non-dip portions of the data. The QPOs are, however, not present during the onset or in the dip-period data. During the dips the energy spectrum is soft and the variability is low compared to the non-dip periods. These features re-appear as the dip gradually recovers. Coincident with the occurrence of a large number of X-ray dips, a huge radio flare of strength 0.48 Jy (at 2.25 GHz) was found from the NRAO archival data. It is suggested that the X-ray dips represent mass ejection due to the evacuation of matter from an accretion disk around the black hole and that a super-position of a large number of such dip events leads to production of huge radio jet in GRS 1915+105.

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 ∼ 1 × 10 9 and ∼ 5 × 10 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.

On the multi-periodicities in the X-ray dipper XB 1916-053

Using the Rossi X-ray Timing Explorer and the Nordic Optical Telescope we have obtained the highest ever quality X-ray/white-light high-speed photometry of XB 1916-053. We refine the X-ray period (P_X) to 3000.6+/-0.2s via a restricted cycle counting approach. Using our complete optical lightcurve, we have extended the optical period (P_opt) ephemeris by another 4 years, providing further evidence for its stability, although a slightly longer period of 3027.555+/-0.002s now provides a marginally better fit. Moreover, modulations at both P_X and P_opt are present in the optical data, with the former dominating the nightly lightcurves (i.e. a few cycles of data). We have also attempted to determine the ``beat'' period, as seen in the repeating evolution of the X-ray dip structure, and the variation in primary dip phase. We find that a quasi-period of 4.74+/-0.05d provides the best fit to the data, even then requiring phase shifts between cycles, with the expected 3.90d ``beat'' of P_X and P_opt appearing to be less likely. Finally, considering the nature of each of these temporal phenomena, we outline possible models, which could explain all of the observed behaviour of this enigmatic source, focusing on which of P_X or P_opt is the binary period.

A Cessation of X‐Ray Dipping Activity in X1254−690

We present results from a campaign of simultaneous X-ray and optical observations of X1254-690 conducted using the Rossi X-ray Timing Explorer and the CTIO 1.5m telescope. We find that the usually-observed deep X-ray dipping is not seen during the times of our observations, with an upper limit of ~2% on any X-ray orbital variation, and that the mean optical variability has declined in amplitude from 0.40+-0.02 mag (in V) to 0.28+-0.01 mag. These findings indicate that the vertical structure on the disk edge associated with the impact point of the accretion stream has decreased in angular size from 17-25 degrees to <10 degrees, and support the suggestions of previous modeling work that the bulge provides 35-40% of the contribution to the overall optical modulation. The average optical and X-ray brightnesses are comparable to their values during dipping episodes, indicating that the mean accretion rate and disk radius remain unchanged.

ASCAObservation of the Dipping X‐Ray Source XB 1916−053

We present the results of timing and spectral studies of the dipping X-ray source XB 1916-053, observed by ASCA during its performance verification phase. The detected dipping activity is consistent with previous observations, with a period of 3008 s and an intermittent secondary dip observed roughly 0.4 out of phase with the primary dip. The energy spectra of different intensity states are fitted with a power law with partial covering fraction absorption and interstellar absorption. The increase in the hardness ratio during the primary and secondary dips, and the increase in the covering fraction and column density with decreasing X-ray intensity, all imply that the dipping is caused by the photoabsorbing materials that have been suggested to be where the accreted flow hits the outer edge of the disk materials. The spectra at all intensity levels show no apparent evidence for Fe or Ne emission lines. This may be due to the low metal abundance in the accretion flow. Alternatively, the X-ray luminosity of the central source may be too weak to excite emission lines, which are assumed to be produced by X-ray photoionization of the disk materials.

Simultaneous optical polarimetry and X-ray observations of the magnetic CV CP Tuc (AX J2315-592)

CP Tuc (AX J2315–592) shows a dip in X-rays which lasts for approximately half the binary orbit and is deeper in soft X-rays compared with hard X-rays. It has been proposed that this dip is due to the accretion stream obscuring the accretion region from view. If CP Tuc were a polar, as has been suggested, then the length of such a dip would make it unique amongst polars since in those polars in which a dip is seen in hard X-rays the dip lasts for only 0.1 of the orbit. We present optical polarimetry and RXTE observations of CP Tuc which show circular polarization levels of ∼10 per cent and find evidence for only one photometric period. These data confirm CP Tuc as a polar. Our modelling of the polarization data implies that the X-ray dip is due to the bulk of the primary accretion region being self-eclipsed by the white dwarf. The energy dependence of the dip is due to a combination of this self-eclipse and also the presence of an X-ray temperature gradient over the primary accretion region.

An X‐Ray Dip in the X‐Ray Transient 4U 1630−47

An x-ray dip was observed during a 1996 Rossi X-Ray Timing Explorer observation of the recurrent x-ray transient 4U 1630-47. During the dip, the 2-60 keV x-ray flux drops by a factor of about three, and, at the lowest point of the dip, the x-ray spectrum is considerably softer than at non-dip times. We find that the 4U 1630-47 dip is best explained by absorption of the inner part of an accretion disk, while the outer part of the disk is unaffected. The spectral evolution during the dip is adequately described by the variation of a single parameter, the column density obscuring the inner disk.

The Dipping Source X1254-690 Observed with GINGA

The results of Ginga observations of the dipping X-ray source X1254-690 are presented. The persistent emission spectrum cannot be represented with conventional two-component models; an additional spectral structure, which can be represented either by a broad emission line or by reflection by warm matter, is required. The energy spectra during dips can be represented by the sum of a non-absorbed component, which has the same spectral shape as the persistent emissions just before or after the dip, and an absorbed component, which is described by the persistent spectrum with additional absorption. The intensity of the non-absorbed component first decreases with decreasing intensity; then, the decrease saturates at about 8% of the persistent intensity. This strongly supports the existence of an accretion-disk corona of an electron-scattering optical depth of about 0.1. The column density of the absorbed component obtained from spectral fit stays at about 1 × 1023 cm−2 when the intensity of the non-absorbed component is above about 20% of the persistent intensity, but increases to 1 × 1024 cm−2 at the bottom of the dip. The electron-scattering optical depth of the accretion-disk corona and the column density of the absorber are roughly consistent with the double-phase model of X-ray irradiated accretion matter.

Pulsations and Line Profile Changes in the Ultraviolet Spectrum of Hercules X‐1: Results from a Multiwavelength Campaign

We report simultaneous X-ray and ultraviolet observations of the X-ray binary pulsar Hercules X-1, obtained over nearly half of a 1.7 day binary orbit using the ASCA X-ray telescope and the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope. The GHRS resolved, for the first time, two moving emission components. One feature in the N V λλ1238.8, 1242.8 doublet is broad (FWHM ≈ 1000 km s–1) and the other is narrow (FWHM ≈ 150 km s–1). We attribute the broad emission line to the X-ray—illuminated surface of a Keplerian accretion disk and the narrow line to the X-ray-heated atmosphere of the companion star, HZ Her. Near orbital phase orb = 0.80 (simultaneous with a pre-eclipse X-ray dip), the N V λ1242.8 component became stronger than the N V λ1238.8 component. Later in the orbit, the blue edge of the broad lines diminished as the approaching side of the accretion disk was occulted by HZ Her. UV continuum pulsations were detected for the first time, with an amplitude ≈ 0.5% of the steady flux. The pulse shapes of the UV continuum and hard X-rays were similar at orb = 0.83 but not at orb = 0.56. We found evidence for a stronger pulsation (amplitude ≈ 15%) within a narrow (0.25 A) segment of the broad 1242.8 A N v component, only at W erb = 0.80. We briefly discuss some implications of these observations for the gas flows in the Her X-1 system.

New Features of the X-Ray Dip Source 1755-338

New Features of the X-Ray Dip Source 1755-338

Optical properties of the new polar RXJ1940.2–1025

We present detailed optical spectroscopic and photometric observations of the recently discovered X-ray source, RXJ1940.2–1025. This object, whose periodic X-ray emission had previously been confused with the X-ray signal from the nearby active galaxy, NGC 6814, is almost certainly an AM Her binary. Its 3.4-h orbital light curve is strongly modulated, and possesses a prominent dip feature lasting about 850s. The dip ingress and egress are well resolved (lasting about 200s each) at optical wavelengths. Blueward of 5500 Å the profile is flat-bottomed and near total. In contrast, we find that a reduced but significant residual flux is present in the Balmer and HeII emission lines throughout the dip. The line fluxes, however, recover much more slowly than the continuum at the end of the dip. At wavelengths beyond 5500 Å, the clear signature of an M4 dwarf secondary star becomes visible during dip minimum, yielding an estimate of 230 pc for the distance to RXJ1940.2–1025. Using our broad-band photometry, we demonstrate that the dip feature recurs with a period of 12116.2±0.4s. The accuracy with which we can determine this period is sufficient to extrapolate back to the epoch of the ROSAT PSPC observations. We can thus demonstrate that the optical dip occurs in the middle of the X-ray ‘low state’, and is not associated with the prominent broader minimum evident in the soft X-ray light curves. We find, however, that there is evidence for an X-ray dip feature corresponding to the optical dip both in the ROSAT data, and in earlier Ginga hard X-ray data. Under the assumption that the X-ray and optical dips are the same phenomenon, we derive a refined ephemeris for the dips, yielding a dip period of 12116.30±0.01s. Whilst, on balance, the X-ray and optical properties of the dip suggest absorption in the accretion flow rather than an eclipse by the companion star as the cause, the inferred wavelength dependence of the optical opacity of the absorber does not follow that expected for free-free absorption in a homogeneous absorber. This may be circumvented by invoking an inhomogeneous accretion flow.

The four periodicities of the cataclysmic variable TV Columbae

I present six nights of simultaneous optical photometry and spectroscopy aimed at observing the four periodicities of TV Col. The 32-min white dwarf spin period (known from the X-ray band) is not convincingly detected in the optical. The 5.5-h orbital period is manifest both photometrically and in the emission-line radial velocities. The emission lines show disc emission with a superimposed S-wave. Near orbital phase 0.8, the S-wave is at inferior conjunction, the photometric variation is at minimum, there is absorption in the line centres and we see X-ray dips. All these findings suggest that the rim of the disc is thickened at this location. The 4-d and 5.2-h cycles dominate the continuum variations but do not change the emission-line profiles. These modulations are not absolutely coherent

X-ray binaries

The various types and classes of X-ray binary are reviewed high-lighting recent results. The high mass X-ray binaries (HMXRBs) can be used to probe the nature of the mass loss from the OB star in these systems. Absorption measurements through one orbital cycle of the supergiant system X1700-37 are well modelled by a radiation driven wind and also require a gas stream trailing behind the X-ray source. In Cen X-3 the gas stream is accreted by the X-ray source via an accretion disk. Changes in the gas stream can cause the disk to thicken and the disk to obscure the X-ray source. How close the supergiant is to corotation seems to be as much a critical factor in these systems as how close it is to filling its Roche lobe. In the Be star X-ray binaries a strong correlation between the neutron stars rotation period and its orbital period has been explained as due to the neutron star being immersed in a dense, slow moving equatorial wind from the Be star. For the X-ray pulsars in the transient Be X-ray binaries a centrifugal barrier to accretion is important in determining the X-ray lightcurve and the spin evolution. The X-ray orbital modulations from the low mass X-ray binaries, LMXRBs, include eclipses by the companion and/or periodic dipping behaviour from structure at the edge of the disk. The corresponding optical modulations show a smooth sinusoidal like component and in some cases a sharp eclipse by the companion. The orbital period of the LMXRB XB1916-05 is 1% longer in the optical compared to that given by the X-ray dip period. The optical period has been interpreted as the orbital period, but this seems inconsistent with the well established view of the origin of the X-ray modulations in LMXRB. A new model is presented that assumes the X-ray dip period is the true orbital period. The 5.2 h eclipsing LMXRB XB2129+47 recently entered a low state and optical observations unexpectedly reveal an F star which is too big to fit into the binary. This is probably the first direct evidence that an X-ray binary is part of a hierarchical triple. Finally the class of X-ray binaries containing black hole candidates is reviewed focusing on the value of using X-ray signatures to identify new candidates.

Discovery of a possible X-ray triple - 4U 1915-05

The optical counterpart of the 50 minute binary and X-ray burst source 4U 1915-05 has been identified with a 21st mag blue object. Extensive CCD photometric observations show that the optical modulation of the system defines a period of 50.4567 minutes with high precision and is thus probably the true orbital period, whereas the range of X-ray ray dip period values (49.7-50.1 minutes) reported previously are excluded. A model is described whereby the optical modulation is due to partial eclipse of the rim of the accretion disk by a low-mass companion star and the X-ray dips are due to absorption and scattering by blobs of material above and below the disk plane and with density proportional to the current mass transfer rate from the companion star. The 1 percent difference in periods may suggest that the system is a hierarchical triple with a third companion star of low-mass orbiting the binary ion in an about 2.5 day retrograde orbit. The possible origin of such a system by tidal capture in a globular cluster, since disrupted, is briefly discussed.

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)

EXOSAT observations of V471 Tauri - A 9.25 minute white dwarf pulsation and orbital phase dependent X-ray dips

New results obtained from a 28 hr continuous observation of V471 Tauri with the EXOSAT satellite are reported. The detection of soft X-ray fluxes from both the white dwarf and the K dwarf, the discovery of a 9.25 minute pulsation from the white dwarf, and the discovery of orbital phase-related soft X-ray dips are discussed. The dips may be correlated with the triangular Lagrangian points of the binary orbit. The X-ray flux from the white dwarf is consistent with thermal models for a white dwarf photosphere with T(eff) of about 35,000 K, log g = 8.0-8.5, and log N(H) = 18.65 + or - 0.2.

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)

Simultaneous X-ray and optical observations of the X-ray dip source X1755 – 338

Les donnees RX confirment que les creux de X 1755-338 sont peu profonds et ne sont pas associes a l'absorption photoelectrique. L'explication la plus plausible en est que les elements responsables de l'absorption des rayons X 0,5-10,0 keV ont une tres faible abondance dans la matiere qui obscurcit la source RX