Low Mass Accretion Rate Research Articles

Analysing the atolls: X-ray spectral transitions of accreting neutron stars

Неге we investigate disc accreting neutron stars with low magnetic fields (atoll sources) by systematically analysing all the available X-ray spectra of these sources from the Rossi X- ray Timing Explorer database, provided by the High Energy Astrophysics Science Archive Research Center. From these we show that all atoll sources show broadly similar spectral evolution as a function of mass accretion rate with hard spectra at low luminosities switching to soft spectra at high luminosities. This is similar to the well known hard/soft spectral dichotomy in black holes. However, we also show that there are subtle but significant differences in the hard/soft transitions for individual atoll sources. There are two different types of hard/soft transition, those where the spectrum softens at all energies, leading to a diagonal track on a colour-colour diagram, and those where only the higher energy spectrum softens, giving a vertical track. The luminosity at which the transition occurs is correlated with this spectral behaviour, with the vertical transition at L/L(_Edd) ~ 0.02 while the diagonal one is at ~ 0.1. Superimposed on this is the well known hysteresis effect, but we show that classic, large scale hysteresis occurs only in the outbursting sources, indicating that its origin is in the dramatic rate of change of mass accretion rate during the disc instability. We show that the long term mass accretion rate correlates with the transition behaviour, and speculate that this is due to the magnetic field being able to emerge from the neutron star surface for low average mass accretion rates. While this is not strong enough to collimate the flow except in the millisecond pulsars, its presence may affect the inner accretion flow through changing the jet properties.

Discovery of X-ray burst triplets in EXO 0748-676

Type-I X-ray bursts are thermonuclear flashes that take place on the surface of accreting neutron stars. The wait time between consecutive bursts is set by the time required to accumulate the fuel needed to trigger a new burst; this is at least one hour. Sometimes secondary bursts are observed, approximately 10 min after the main burst. These short wait-time bursts are not yet understood. We observed the low-mass X-ray binary and X-ray burster EXO 0748-676 with XMM-Newton for 158 h, during 7 uninterrupted observations lasting up to 30 h each. We detect 76 X-ray bursts. Most remarkably, 15 of these bursts occur in burst triplets, with wait times of 12 min between the three components of the triplet, T1, T2, and T3. We also detect 14 doublets with similar wait times between the two components of the doublet, D1 and D2. We characterize this behavior to try and obtain a better understanding of bursts with short wait times. We measure the burst peak flux, fluence, wait time and time profile, and study correlations between these parameters and with the persistent flux representing the mass accretion rate. (i) For all bursts with a long wait time, the fluence is tightly correlated with the wait time, whereas burst with short wait times generally have higher fluences than expected from this relationship; (ii) wait times tend to be longer after doublets and triplets; (iii) the time profile of single bursts, S1, and of the first burst in a double or triple burst, D1 and T1, always contains a slow component which is generally absent in the D2, T2 and T3 bursts; (iv) the peak flux is highest for S1, D1 and T1 bursts, but this is still a factor of 7 lower than the highest peak flux ever seen for a burst in this system; (v) the persistent flux, representing the mass accretion rate onto the neutron star, is about 1% of Eddington, which is among the lowest value so far measured for this system. The amount of energy per gram of accreted mass liberated during bursts is consistent with a fuel mixture of hydrogen-rich material. The characteristics of the bursts indicate that possibly all bursts in this system are hydrogen-ignited, in contrast with most other frequent X-ray bursters in which bursts are helium-ignited, but consistent with the low mass accretion rate in EXO 0748-676. Possibly the hydrogen ignition is the determining factor for the occurrence of short wait-time bursts. For example the 12 min wait time may be associated with a nuclear beta decay timescale. Appendix is only available in electronic form at http://www.aanda.org

An in-depth study of the pre-polar candidate WX Leonis Minoris

Optical photometry, spectroscopy and XMM-Newton ultraviolet and X-ray observations with full phase coverage are used for an in-depth study of WXLMi, a system formerly termed as a low-accretion rate polar. We find a constant low mass accretion rate, M_dot ~ 1.5e-13 M_sun/yr, a peculiar accretion geometry with one spot not being accessible via Roche-lobe overflow, a low temperature of the white dwarf, Teff < 8000 K and the secondary likely being Roche-lobe underfilling. All this lends further support to the changed view on WXLMi and related systems as detached binaries, i.e. magnetic post-common envelope binaries without significant Roche-lobe overflow in the past. The transfer rate determined here is compatible with accretion from a stellar wind. We use cyclotron spectroscopy to determine the accretion geometry and to constrain the plasma temperatures. Both, cyclotron spectroscopy and X-ray plasma diagnostics reveal low plasma temperatures below 3 keV on both accretion spots. For the low m_dot, high B plasma at the accretion spots in WXLMi, cyclotron cooling is dominating thermal plasma radiation in the optical. Optical spectroscopy and X-ray timing reveal atmospheric, chromospheric and coronal activity at the saturation level on the dM4.5 secondary star.

The X-ray spectrum of the bursting atoll source 4U 1728-34 observed with INTEGRAL

We present for the first time a study of the 3–200 keV broad band spectra of the bursting atoll source 4U 1728-34 (GX 354-0) along its hardness intensity diagram. The analysis was done using the INTEGRAL public and Galactic Center deep exposure data ranging from February 2003 to October 2004. The spectra are well described by a thermal Comptonization model with an electron temperature from 35 keV to 3 keV and Thomson optical depth, τT, from 0.5 to 5 in a slab geometry. The source undergoes a transition from an intermediate/hard to a soft state where the source luminosity increases from 2 to 12% of Eddington. We have also detected 36 type I X-ray bursts two of which show photospheric radius expansion. The energetic bursts with photospheric radius expansion occurred at an inferred low mass accretion rate per unit area of u m ∼ 1.7 × 10 3 gc m −2 s −1 , while the others at a higher one between 2.4 × 10 3 −9.4 × 10 3 gm −2 s −1 . For 4U 1728-34 the bursts’ total fluence, and the bursts’ peak flux are anti-correlated with the mass accretion rate. The type I X-ray bursts involve pure helium burning either during the hard state, or during the soft state of the source.

The long period intermediate polar 1RXS J154814.5-452845

We present the first time resolved medium resolution optical spectroscopy of the recently identified peculiar Intermediate Polar (IP) 1RXS J154814.5-452845, which allows us to precisely determine the binary orbital period ( ± 0.03 h) and the white dwarf spin period ( ± 0.06 s). This system is then the third just outside the purported ~6–10 h IP orbital period gap and the fifth of the small group of long period IPs, which has a relatively high degree of asynchronism. From the presence of weak red absorption features, we identify the secondary star with a spectral type K2 ± 2 V, which appears to have evolved on the nuclear timescale. From the orbital radial velocities of emission and the red absorption lines a mass ratio ± 0.12 is found. The masses of the components are estimated to be and and the binary inclination . A distance between 540–840 pc is estimated. At this distance, the presence of peculiar absorption features surrounding Balmer emissions cannot be due to the contribution of the white dwarf photosphere and their spin modulation suggests an origin in the magnetically confined accretion flow. The white dwarf is also not accreting at a particularly high rate ( ), for its orbital period. The spin-to-orbit period ratio = 0.02 and the low mass accretion rate suggest that this system is far from spin equilibrium. The magnetic moment of the accreting white dwarf is found to be 1032 G cm3, indicating a low magnetic field system.

Truncated disc versus extremely broad iron line in XTE J1650--500

There is growing evidence from both spectral and timing properties that there is a truncated inner accretion disc in low mass accretion rate Galactic black hole systems. The detection of extremely smeared relativistic iron lines in some of these systems is the only current piece of evidence which conflicts with this geometrical interpretation of the low/hard state. Here we show that the line width in the BeppoSAX data of a bright low/hard state of the transient black hole XTE J1650-500 is indeed consistent with extreme relativistic effects. However, the relativistic smearing can be significantly reduced if there is also resonance iron K line absorption from an outflowing disc wind. The iron line smearing is then completely compatible with a truncated disc, so gives no information on the black hole spin.

The circumstellar disc around the Herbig AeBe star HD169142

We present 7 mm and 3.5 cm wavelength continuum observations toward the Herbig AeBe star HD169142 performed with the Very Large Array (VLA) with an angular resolution of ~1". We find that this object exhibits strong (~4.4 mJy), unresolved (~1") 7 mm continuum emission, being one of the brightest isolated Herbig AeBe stars ever detected with the VLA at this wavelength. No emission is detected at 3.5 cm continuum, with a 3 sigma upper limit of ~0.08 mJy. From these values, we obtain a spectral index of ~2.5 in the 3.5 cm to 7 mm wavelength range, indicating that the observed flux density at 7mm is most likely dominated by thermal dust emission coming from a circumstellar disc. We use available photometric data from the literature to model the spectral energy distribution (SED) of this object from radio to near-ultraviolet frequencies. The observed SED can be understood in terms of an irradiated accretion disc with low mass accretion rate, 10^{-8} solar masses per year, surrounding a star with an age of ~10 Myr. We infer that the mass of the disc is ~0.04 solar masses, and is populated by dust grains that have grown to a maximum size of 1 mm everywhere, consistent with the lack of silicate emission at 10 microns. These features, as well as indications of settling in the wall at the dust destruction radius, led us to speculate the disc of HD169142 is in an advanced stage of dust evolution, particularly in its inner regions.

Detection of Hard X‐Ray Pulsations and a Strong Iron Kβ Emission Line during an Extended Low State of GX 1+4

We present here results obtained from a detailed timing and spectral analysis of three BeppoSAX observations of the binary X-ray pulsar GX 1+4 carried out in 1996 August, 1997 March, and 2000 August. In the middle of the 2000 August observation, the source was in a rare low-intensity state that lasted for about 30 hr. Although the source does not show pulsations in the soft X-ray band (1.0-5.5 keV) during the extended low state, pulsations are detected in the 5.5-10.0 keV energy band of the MECS detector and in the hard X-ray energy bands (15-150 keV) of the PDS instrument. Comparing the 2-10 keV flux during this low state with the previously reported low states in GX 1+4, we suggest that the propeller regime in GX 1+4 occurs at a lower mass accretion rate than reported earlier. Broadband (1.0-150 keV) pulse-averaged spectroscopy reveals that the best-fit model comprises a Comptonized continuum along with an iron Kα emission line. A strong iron Kβ emission line is detected for the first time in GX 1+4 during the extended low state of the 2000 observation with an equivalent width of ~550 eV. The optical depth and temperature of the Comptonizing plasma are found to be identical during the high- and low-intensity states, whereas the hydrogen column density and the temperature of the seed photons are higher during the low state. We also present results from pulse-phase-resolved spectroscopy during the high- and low-flux episodes.

Jet dominated states in X-ray binaries

Abstract We demonstrate that at relatively low mass accretion rates, black hole candidate (BHC) X-ray binaries (XRBs) should enter ‘jet-dominated’ states, in which the majority of the liberated accretion power is in the form of a (radiatively inefficient) jet and not dissipated as X-rays in the accretion flow. This result follows from the empirically established non-linear relation between radio and X-ray power from low/hard state BHC XRBs, which we assume also to hold for neutron star (NS) XRBs. Conservative estimates of the jet power indicate that all BHC XRBs in ‘quiescence’ should be in this jet-dominated regime. In combination with an additional empirical result, namely that BHC XRBs are more ‘radio loud’ than NS XRBs, we find that in quiescence NS XRBs should be up to two orders of magnitude more luminous in X-rays than BHC XRBs, without requiring any significant advection of energy into a black hole. This ratio is as observed, and such observations should therefore no longer be considered as direct evidence for the existence of black hole event horizons. Furthermore, even if BHCs do contain black holes with event horizons, this work demonstrates that there is no requirement for the advection of significant amounts of accretion energy across the horizon.

Modeling the Heating and Cooling of WZ Sagittae Following the 2001 July Outburst

Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope Space Telescope Imaging Spectograph spectra of the dwarf nova WZ Sge, obtained during and following the early superoutburst of 2001 July over a time span of 20 months, monitor changes in the components of the system during its different phases. The synthetic spectral fits to the data indicate a cooling in response to the outburst of about 12,000 K, from ≈28,000 down to ≈16,000 K. The cooling timescale τ (of the white dwarf temperature excess) is on the order of ≈100 days in the early phase of the cooling period, and increases to τ ≈ 850 days toward the end of the second year following the outburst. In the present work, we numerically model the accretional heating and subsequent cooling of the accreting white dwarf in WZ Sge. The best compressional heating model fit is obtained for a 1.2 M☉ white dwarf accreting at a rate of 9 × 10-9 M☉ yr-1 for 52 days. However, if one assumes a lower mass accretion rate or a lower white dwarf mass, then compressional heating alone cannot account for the observed temperature decline, and other sources of heating have to be included to increase the temperature of the model to the observed value. We quantitatively check the effect of boundary layer irradiation as such an additional source.

Jet-dominated states: an alternative to advection across black hole event horizons in 'quiescent' X-ray binaries

We demonstrate that at relatively low mass accretion rates, black hole X-ray binaries should enter ‘jet-dominated’ states, in which the majority of the liberated accretion power is in the form of a (radiatively inefcient) jet and not dissipated as X-rays in the accretion o w. This result follows from the empirically established non-linear relation between radio and X-ray power from hard state black holes, which we assume also to hold for neutron stars. Conservative estimates of the jet power indicate that all ‘quiescent’ black holes should be in this jetdominated regime. In combination with an additional empirical result, namely that black hole X-ray binaries are more ‘radio loud’ than neutron stars, we nd that quiescent neutron stars should be up to two orders of magnitude more luminous in X-rays than the black hole systems, without requiring any signicant advection of energy into a black hole. This ratio is as observed, and such observations should therefore no longer be considered as direct evidence for the existence of black hole event horizons. Furthermore, even if black hole candidates do contain black holes with event horizons, this work demonstrates that there is no requirement for the advection of signicant amounts of accretion energy across the horizon.

Accretion in Very Low Mass Young Objects

We have investigated evidence for active accretion in a sample of ~ 30 young, very low mass objects, including at least 10 brown dwarfs. About 30% of the sample exhibits broad, asymmetric Hα emission line profiles, indicative of gas accretion via magnetospheric infall. There is a distinct lack of associated optical continuum veiling in these accretors, suggesting very low mass accretion rates. Our models yield an upper limit to the accretion rates that is several orders of magnitude smaller than typical of higher-mass T Tauri stars, suggesting a dependence of accretion rates with stellar mass.

A Comprehensive Study of the L1551 IRS 5 Binary System

We model the Class I source L1551 IRS 5, adopting a flattened infalling envelope surrounding a binary disk system and a circumbinary disk. With our composite model, we calculate self-consistently the spectral energy distribution of each component of the L1551 IRS 5 system, using additional constraints from recent observations by ISO, the water ice feature from observations with SpeX, the SCUBA extended spatial brightness distribution at submillimeter wavelengths, and the VLA spatial intensity distributions at 7 mm of the binary disks. We analyze the sensitivity of our results to the various parameters involved. Our results show that a flattened-envelope collapse model is required to explain simultaneously the large-scale fluxes and the water ice and silicate features. On the other hand, we find that the circumstellar disks are optically thick in the millimeter range and are inclined so that their outer parts hide the emission along the line of sight from their inner parts. We also find that these disks have lower mass accretion rates than the infall rate of the envelope.

The X-ray spectrum of the atoll source 4U 1608--52

The transient atoll source 4U 1608−52 was extensively observed by the Rossi X-ray Timing Explorer (RXTE) during its 1998 outburst. We analyse its X-ray spectra as a function of inferred accretion rate from both the 1998 outburst and the 1996 and 1998 non-outburst data. We can fit all the spectra by a model in which seed photons from the neutron star surface are Comptonized in a boundary layer. The Comptonized emission illuminates the accretion disc surface, producing an ionized, relativistically broadened reflection signature, while the direct emission from the accretion disc can also be seen. The evolution of the source can be explained if the main parameter driving the spectral evolution is the average mass accretion rate, which determines the truncation radius of the inner accretion disc. At low mass accretion rates, in the island state, the disc truncates before reaching the neutron star surface and the inner accretion flow/boundary layer is mostly optically thin. The disc emission is at too low a temperature to be observed in the RXTE spectra, but some of the seed photons from the neutron star can be seen directly through the mostly optically thin boundary layer. At higher mass accretion rates, in the banana state, the disc moves in and the boundary layer becomes much more optically thick so its temperature drops. The disc can then be seen directly, but the seed photons from the neutron star surface cannot, as they are buried beneath the increasingly optically thick boundary layer.

Where have all the black holes gone?

We have calculated stationary models for accretion disks around super-massive black holes in galactic nuclei. Our models show that below a critical mass flow rate of310 3 _ MEdd advection will dominate the energy budget while above that rate all the viscously liberated energy is radiated. The radiation eciency declines steeply below that critical rate. This leads to a clear dichotomy between AGN and normal galaxies which is not so much given by dierences in the mass flow rate as by the radiation eciency. At very low mass accretion rates below 510 5 _ MEdd synchrotron emission and Bremsstrahlung dominate the SED, while above 210 4 _ MEdd the inverse Compton radiation from synchrotron seed photons produce flat to inverted SEDs from the radio to X-rays. Finally we discuss the implications of these ndings for AGN duty cycles and the long-term AGN evolution.

Phase-resolved X-ray spectroscopy of the millisecond pulsar SAX J1808.4−3658

We present new results based on RXTE observations of the millisecond pulsar SAX J1808.4−3658, carried out during the decay of the 1998 April outburst. The X-ray spectrum can be fitted by a two-component model. We interpret the soft component as blackbody emission from a heated spot on the neutron star, and the hard component as coming from Comptonization in plasma heated by the accretion shock as the material collimated by the magnetic field impacts on to the neutron star surface. The hotspot is probably the source of seed photons for Comptonization. The hard component illuminates the disc, giving rise to a reflected spectrum. The amount of reflection indicates that the disc is truncated at fairly large radii (20–40Rg), consistent with the lack of relativistic smearing of the spectral features. The inferred evolution of the inner radius is not consistent with the magnetic field truncating the disc. Instead, it seems more likely that the inner disc radius is set by some much longer time-scale process, most probably connected to the overall evolution of the accretion disc. This disc truncation mechanism would then have to be generic in all low-mass accretion rate flows both in disc accreting neutron stars and black hole systems. The phase-resolved spectra show clearly that the blackbody and hard Comptonized spectra pulse independently. This obviously gives an energy-dependent phase-lag. Full general relativistic effects are not required to explain this. The soft blackbody component is optically thick, so its variability is dominated by its changing projected area, while the Doppler shifts (which are maximized 90° before the maximum in projected area) are somewhat stronger for the translucent column. We do not detect Compton reflection from the neutron star surface, though we predict that it should be present in the X-ray spectrum. This would give an unambiguous observational measure of M/R if there is any iron on the neutron star surface which is not completely ionized.

The Burst Behavior of the Eclipsing Low‐Mass X‐Ray Binary MXB 1659−298

We present a detailed study of the correlations between the burst properties and the inferred mass accretion rate for the X-ray transient MXB 1659-298. The bursts which exhibited oscillations were observed when the source was at relatively high mass accretion rate, similar to what has been seen for other sources. However, due to the limited number of observations at lower mass accretion rates, no bursts were observed at such accretion rates and it is still possible that when MXB 1659-298 accretes at such low mass accretion rates, bursts can occur which might still exhibit burst oscillations. No clear correlations were found between the different burst properties and accretion rate, in contrast to what has been found for KS 1731-260 and 4U 1728-34, but similar to what has been reported for Aql X-1. However, this lack of correlation for MXB 1659-298 and Aql X-1 might be due to the limited range of mass accretion rate observed for those sources compared to KS 1731-260 and 4U 1728-34.

Discovery of a Cyclotron Resonant Scattering Feature in theRossi X‐Ray Timing ExplorerSpectrum of 4U 0352+309 (X Persei)

We have discovered a ~29 keV cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of 4U 0352+309 (X Per) using observations taken with the Rossi X-Ray Timing Explorer. The source 4U 0352+309 is a persistent low-luminosity (LX = 4.2 × 1034 ergs s-1) X-ray pulsar with a 837 s period, which accretes material from the Be star X Per. The X-ray spectrum, unusual when compared to brighter accreting pulsars, may be due to the low mass accretion rate and could be typical of the new class of persistent low-luminosity Be/X-ray binary pulsars. We attempted spectral fits with continuum models used historically for 4U 0352+309 and found that all were improved by the addition of a CRSF at ~29 keV. The model that best fitted the observations is a combination of a 1.45 ± 0.02 keV blackbody with a 5.4 × 108 cm2 area and a power law with a 1.83 ± 0.03 photon index modified by the CRSF. In these fits the CRSF energy is 28.6 keV, implying a magnetic field strength of 2.5(1 + z) × 1012 G in the scattering region (where z is the gravitational redshift). Phase-resolved analysis shows that the blackbody and cyclotron line energies are consistent with being constant through the pulse.

Nuclear reaction rates and the nova outburst

In this paper we examined the consequences of improving the nuclear reaction library on our simulations of TNRs on 1.25M, WD and 1.35M, WDS. We have found that the changes in the rates have affected the nucleosynthesis predictions of our calculations but not, to any great extent, the gross features. In addition, we have used a lower mass accretion rate than in our previous studies in order to accrete (and eject) more material. This has, as expected, caused the peak values of some important parameters to increase over our previous studies at the same WD mass. However, because some important reaction rates have declined in the new compilation this has not increased the abundances for nuclei above aluminum and, in fact, they have declined while the abundances of both {sup 26}Al and {sup 27}Al have increased at both WD masses. In contrast, the abundance of {sup 22}Na has declined at both WD masses over the values predicted in our earlier work. This has important implications with respect to predictions of the observability of novae with INTEGRAL.

Quasi-periodic oscillations and noise in neutron star and black-hole X-ray binaries

Before the launch of the Rossi X-ray Timing Explorer (RXTE) satellite, the differences in the rapid X-ray variability between the two main types of neutron star binaries (i.e., the Z and atoll sources) could be explained by invoking different mass accretion rates and magnetic field strengths. However, the results obtained with RXTE now show that these systems are more similar than previously thought and although differences in mass accretion rate are still likely, the differences in the magnetic field strength have become questionable. The great similarities between the neutron star systems and the black-hole candidates at low mass accretion rates also point towards a similar origin of their timing phenomena indicating that the presence or absence of a solid surface, a magnetic field, or an event horizon do not play a significant role in the production mechanisms for the rapid X-ray variability.