arXiv subjects

Michiel van der Klis

Publications and source records attributed to Michiel van der Klis.

At least 73 records · Page 4Linked to original sources

arXiv2010

XTE J1701-462 and its Implications for the Nature of Subclasses in Low-Magnetic-Field Neutron Star Low-Mass X-Ray Binaries

We report on an analysis of RXTE data of the transient neutron star low-mass X-ray binary (NS-LMXB) XTE J1701-462, obtained during its 2006-2007 outburst. The X-ray properties of the source changed between those of various types of NS-LMXB subclasses. At high luminosities the source switched between two types of Z source behavior and at low luminosities we observed a transition from Z source to atoll source behavior. These transitions between subclasses primarily manifest themselves as changes in the shapes of the tracks in X-ray color-color and hardness-intensity diagrams, but they are accompanied by changes in the kHz quasi-periodic oscillations, broad-band variability, burst behavior, and/or X-ray spectra. We find that the low-energy X-ray flux is a good parameter to track the gradual evolution of the tracks in color-color and hardness-intensity diagrams, allowing us to resolve the evolution of the source in greater detail than before and relate the observed properties to other NS-LMXBs. We further find that during the transition from Z to atoll, characteristic behavior known as the atoll upper banana can equivalently be described as the final stage of a weakening Z source flaring branch, thereby blurring the line between the two subclasses. Our findings strongly suggest that the wide variety in behavior observed in NS-LXMBs with different luminosities can be linked through changes in a single variable parameter, namely the mass accretion rate, without the need for additional differences in the neutron star parameters or viewing angle. We briefly discuss the implications of our findings for the spectral changes observed in NS LMXBs and suggest that, contrary to what is often assumed, the position along the color-color tracks of Z sources is not determined by the instantaneous mass accretion rate.

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arXiv2009

Measuring the Accreting Stellar and Intermediate Mass Black Hole Populations in the Galaxy and Local Group

The population of stellar black holes (SBHs) in the Galaxy and galaxies generally is poorly known in both number and distribution. SBHs are the fossil record of the massive stars in galaxy evolution and may have produced some (if not all) of the intermediate mass (\gsim100\Msun) black holes (IMBHs) and, in turn, the central supermassive black holes (SMBHs) in galactic nuclei. For the first time, a Galaxy-wide census of accreting black holes, and their more readily recognizable tracer population, accreting neutron stars (NSs), could be measured with a wide-field hard X-ray imaging survey and soft X-ray and optical/IR prompt followup -- as proposed for the EXIST mission.

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arXiv2009

Linking jet emission and X-ray properties in the peculiar neutron star X-ray binary Circinus X-1

We present the results of simultaneous X-ray and radio observations of the peculiar Z-type neutron star X-ray binary Cir X-1, observed with the Rossi X-ray timing explorer satellite and the Australia Telescope Compact Array in 2000 October and 2002 December. We identify typical Z source behaviour in the power density spectra as well as characteristic Z patterns drawn in an X-ray hardness-intensity diagram. Power spectra typical of bright atoll sources have also been identified at orbital phases after the periastron passage, while orbital phases before the periastron passage are characterized by power spectra that are typical neither of Z nor of atoll sources. We investigate the coupling between the X-ray and the radio properties, focusing on three orbital phases when an enhancement of the radio flux density has been detected, to test the link between the inflow (X-ray) and the outflow (radio jet) to/from the compact object. In two out of three cases we associate the presence of the radio jet to a spectral transition in the X-rays, although the transition does not precede the radio flare, as detected in other Z sources. An analogous behaviour has recently been found in the black hole candidate GX 339-4. In the third case, the radio light curve shows a similar shape to the X-ray light curve. We discuss our results in the context of jet models, considering also black hole candidates.

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arXiv2009

The first outburst of the new magnetar candidate SGR 0501+4516

We report here on the outburst onset and evolution of the new Soft Gamma Repeater SGR 0501+4516. We monitored the new SGR with XMM-Newton starting on 2008 August 23, one day after the source became burst-active, and continuing with 4 more observations, with the last one on 2008 September 30. Combining the data with the Swift-XRT and Suzaku data, we modelled the outburst decay over 160 days, and we found that the source flux decreased exponentially with a timescale of t_c=23.8 days. In the first XMM-Newton observation a large number of short X-ray bursts were observed, the rate of which decayed drastically in the following observations. We found large changes in the spectral and timing behavior of the source during the outburst, with softening emission as the flux decayed, and the non-thermal soft X-ray spectral component fading faster than the thermal one. Almost simultaneously to our XMM-Newton observations (on 2008 August 29 and September 2), we observed the source in the hard X-ray range with INTEGRAL, which clearly detected the source up to ~100keV in the first pointing, while giving only upper limits during the second pointing, discovering a variable hard X-ray component fading in less than 10 days after the bursting activation. We performed a phase-coherent X-ray timing analysis over about 160 days starting with the burst activation and found evidence of a strong second derivative period component (\ddot{P} = -1.6(4)x10^{-19} s/s^{-2}). Thanks to the phase-connection, we were able to study the the phase-resolved spectral evolution of SGR 0501+4516 in great detail. We also report on the ROSAT quiescent source data, taken back in 1992 when the source exhibits a flux ~80 times lower than that measured during the outburst, and a rather soft, thermal spectrum.

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arXiv2009

Discovery of burst oscillations in the intermittent accretion-powered millisecond pulsar HETE J1900.1-2455

We report the discovery of burst oscillations from the intermittent accretion-powered millisecond pulsar (AMP) HETE J1900.1-2455, with a frequency approximately 1 Hz below the known spin frequency. The burst oscillation properties are far more similar to those of the non-AMPs and Aql X-1 (an intermittent AMP with a far lower duty cycle), than those of the AMPs SAX J1808.4-3658 and XTE J1814-338. We discuss the implications for models of the burst oscillation and intermittency mechanisms.

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arXiv2009

Accretion torques and motion of the hot spot on the accreting millisecond pulsar XTE J1807-294

We present a coherent timing analysis of the 2003 outburst of the accreting millisecond pulsar XTE J1807-294. We find an upper limit for the spin frequency derivative of 5E-14 Hz/s. The sinusoidal fractional amplitudes of the pulsations are the highest observed among the accreting millisecond pulsars and can reach values of up to 27% (2.5-30 keV). The pulse arrival time residuals of the fundamental follow a linear anti-correlation with the fractional amplitudes that suggests hot spot motion over the surface of the neutron star both in longitude and latitude. An anti-correlation between residuals and X-ray flux suggests an influence of accretion rate on pulse phase, and casts doubts on the use of standard timing techniques to measure spin frequencies and torques on the neutron star.

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arXiv2009

An alternative interpretation of the timing noise in accreting millisecond pulsars

The measurement of the spin frequency in accreting millisecond X-ray pulsars (AMXPs) is strongly affected by the presence of an unmodeled component in the pulse arrival times called 'timing noise'. We show that it is possible to attribute much of this timing noise to a pulse phase offset that varies in correlation with X-ray flux, such that noise in flux translates into timing noise. This could explain many of the pulse frequency variations previously interpreted in terms of true spin up or spin down, and would bias measured spin frequencies. Spin frequencies improved under this hypothesis are reported for six AMXPs. The effect would most easily be accounted for by an accretion rate dependent hot spot location.

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arXiv2009

The Dynamic X-ray Sky of the Local Universe

Over the next decade, we can expect time domain astronomy to flourish at optical and radio wavelengths. In parallel with these efforts, a dedicated transient "machine" operating at higher energies (X-ray band through soft gamma-rays) is required to reveal the unique subset of events with variable emission predominantly visible above 100 eV. Here we focus on the transient phase space never yet sampled due to the lack of a sensitive, wide-field and triggering facility dedicated exclusively to catching high energy transients and enabling rapid coordinated multi-wavelength follow-up. We first describe the advancements in our understanding of known X-ray transients that can only be enabled through such a facility and then focus on the classes of transients theoretically predicted to be out of reach of current detection capabilities. Finally there is the exciting opportunity of revealing new classes of X-ray transients and unveiling their nature through coordinated follow-up observations at longer wavelengths.

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arXiv2008

The Swift capture of a long X-ray burst from XTE J1701-407

XTE J1701-407 is a new transient X-ray source discovered on June 8th, 2008. More than one month later it showed a rare type of thermonuclear explosion: a long type I X-ray burst. We report herein the results of our study of the spectral and flux evolution during this burst, as well as the analysis of the outburst in which it took place. We find an upper limit on the distance to the source of 6.1 kpc by considering the maximum luminosity reached by the burst. We measure a total fluence of 3.5*10^{-6} erg/cm^2 throughout the ~20 minutes burst duration and a fluence of 2.6*10^{-3} erg/cm^2 during the first two months of the outburst. We show that the flux decay is best fitted by a power law (index ~1.6) along the tail of the burst. Finally, we discuss the implications of the long burst properties, and the presence of a second and shorter burst detected by Swift ten days later, for the composition of the accreted material and the heating of the burning layer.

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arXiv2008

Coherence of burst oscillations and accretion-powered pulsations in the accreting millisecond pulsar XTE J1814-338

X-ray timing of the accretion-powered pulsations during the 2003 outburst of the accreting millisecond pulsar XTE J1814-338 has revealed variation in the pulse time of arrival residuals. These can be interpreted in several ways, including spin-down and wandering of the fuel impact point around the magnetic pole. In this Letter we show that the burst oscillations of this source are coherent with the persistent pulsations, to the level where they track all of the observed fluctuations. Only one burst, which occurs at the lowest accretion rates, shows a significant phase offset. We discuss what might lead to such rigid phase-locking between the modulations in the accretion and thermonuclear burst emission, and consider the implications for spin variation and the burst oscillation mechanism. Wandering of the fuel impact hot spot around a fixed magnetic pole seems the most likely cause for the accretion-powered pulse phase variations. This means that the burst asymmetry is coupled to the hot spot, not the magnetic pole. If premature ignition at this point (due to higher local temperatures) triggers a burning front that stalls before spreading over the entire surface, the resulting localized nuclear hot spot may explain the unusual burst and burst oscillation properties of this source.

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arXiv2008

Phase-coherent timing of the accreting millisecond pulsar SAX J1748.9-2021

We present a phase-coherent timing analysis of the intermittent accreting millisecond pulsar SAX J1748.9-2021. A new timing solution for the pulsar spin period and the Keplerian binary orbital parameters was achieved by phase connecting all episodes of intermittent pulsations visible during the 2001 outburst. We investigate the pulse profile shapes, their energy dependence and the possible influence of Type I X-ray bursts on the time of arrival and fractional amplitude of the pulsations. We find that the timing solution of SAX J1748.9-2021 shows an erratic behavior when selecting different subsets of data, that is related to substantial timing noise in the timing post-fit residuals. The pulse profiles are very sinusoidal and their fractional amplitude increases linearly with energy and no second harmonic is detected. The reason why this pulsar is intermittent is still unknown but we can rule out a one-to-one correspondence between Type I X-ray bursts and the appearance of the pulsations.

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arXiv2008

Simultaneous X-ray/radio observations of Cir X-1

We analysed simultaneous X-ray/radio observations of Circinus X-1 collected respectively with RXTE and ATCA in 2000 October and 2002 December and identified radio flares close to phase 0.0 and 0.5 of the orbital period. To date, there is only circumstantial evidence for radio flares near phase 0.5. Moreover, in our data set, we clearly associated both a radio flare and X-ray spectral timing changes with phase 0.0. While for black hole X-ray binaries the picture of the association between the X-ray and the radio bands is quite well understood, for neutron star X-ray binaries a clear and complete picture is still missing.

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arXiv2008

Multiwavelength observations of the black hole candidate Swift J1753.5-0127

We present preliminary results from the analysis of simultaneous multiwavelength observations of the black hole candidate Swift J1753.5-0127. The source is still continuing its outburst started in May 2005, never leaving the Low/Hard State. In the X-ray energy spectra we confirm evidence for a thermal component at a very low luminosity possibly extending close to but not at the innermost stable orbit. This is unusual for black hole candidates in the Low/Hard State. Furthermore, we confirm that its radio emission is significantly fainter than expected from the relation observed in other black hole candidates between the observed radio/X-ray fluxes.

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arXiv2008

Timing and spectral properties of the accreting millisecond pulsar SWIFT J1756.9-2508

SWIFT J1756.9-2508 is one of the few accreting millisecond pulsars (AMPs) discovered to date. We report here the results of our analysis of its aperiodic X-ray variability, as measured with the Rossi X-ray Timing Explorer during the 2007 outburst of the source. We detect strong (~35%) flat-topped broadband noise throughout the outburst with low characteristic frequencies (~0.1 Hz). This makes SWIFT J1756.9-2508 similar to the rest of AMPs and to other low luminosity accreting neutron stars when they are in their hard states, and enables us to classify this AMP as an atoll source in the extreme island state. We also find a hard tail in its energy spectrum extending up to 100 keV, fully consistent with such source and state classification.

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arXiv2007

The long-term evolution of the spin, pulse shape, and orbit of the accretion-powered millisecond pulsar SAX J1808.4-3658

We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658, an X-ray transient with a recurrence time of ~2 yr, using data from the Rossi X-ray Timing Explorer covering 4 transient outbursts (1998-2005). We verify that the 401 Hz pulsation traces the spin frequency fundamental and not a harmonic. Substantial pulse shape variability, both stochastic and systematic, was observed during each outburst. Analysis of the systematic pulse shape changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar surface drifts longitudinally and a second hot spot may appear. The overall pulse shape variability limits the ability to measure spin frequency evolution within a given X-ray outburst (and calls previous nudot measurements of this source into question), with typical upper limits of |nudot| < 2.5x10^{-14} Hz/s (2 sigma). However, combining data from all the outbursts shows with high (6 sigma) significance that the pulsar is undergoing long-term spin down at a rate nudot = (-5.6+/-2.0)x10^{-16} Hz/s, with most of the spin evolution occurring during X-ray quiescence. We discuss the possible contributions of magnetic propeller torques, magnetic dipole radiation, and gravitational radiation to the measured spin down, setting an upper limit of B < 1.5x10^8 G for the pulsar's surface dipole magnetic field and and Q/I < 5x10^{-9} for the fractional mass quadrupole moment. We also measured an orbital period derivative of Pdot = (3.5+/-0.2)x10^{-12} s/s. This surprising large Pdot is reminiscent of the large and quasi-cyclic orbital period variation observed in the so-called "black widow" millisecond radio pulsars, supporting speculation that SAX J1808.4-3658 may turn on as a radio pulsar during quiescence. In an appendix we derive an improved (0.15 arcsec) source position from optical data.

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arXiv2007

Relativistic iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii

Using Suzaku observations of three neutron star low-mass X-ray binaries (Ser X-1, 4U 1820-30 and GX 349+2) we have found broad, asymmetric, relativistic Fe K emission lines in all three objects. These Fe K lines can be well fit by a model for lines from a relativistic accretion disk ('diskline'), allowing a measurement of the inner radius of the accretion disk, and hence an upper limit on the neutron star radius. These upper limits correspond to 14.5 - 16.5 km for a 1.4 M(solar) neutron star. The inner disk radii we measure with Fe K lines are in good agreement with the inner disk radii implied by kHz QPOs observed in both 4U 1820-30 and GX 349+2, supporting the inner disk nature of kHz QPOs. Additionally, the Fe K lines observed in these neutron stars are narrower than those in the black holes that are thought to be close to maximally spinning, as one would expect if inferences for spin are robust.

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arXiv2007

Non-Detection of Gravitationally Redshifted Absorption Lines in the X-ray Burst Spectra of GS 1826-24

During a 200 ks observation with the XMM-Newton Reflection Grating Spectrometer, we detected 16 type-I X-ray bursts from GS 1826-24. We combined the burst spectra in an attempt to measure the gravitational redshifts from the surface of the neutron star. We divided the composite GS 1826-24 burst spectrum into three groups based on the blackbody temperature during the bursts. The spectra do not show any obvious discrete absorption lines. We compare our observations with those of EXO 0748-676.

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arXiv2007

Peak Luminosities of the Hard States of GX 339-4: Implications for the Accretion Geometry, Disk Mass, and Black Hole Mass

We have analyzed observations of the black hole transient {GX 339$-$4} made with the {\it Rossi} X-ray Timing Explorer (RXTE) and the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma-ray Observatory (CGRO). We have found a nearly linear relation between the peak flux during the low/hard (LH) state that occurs at the beginning of an outburst and the time since the flux peak of the latest LH state identified in the previous outburst. Assuming that the rate at which mass accumulates in the accretion disk between these peaks is constant and that any mass that remains in the disk after an outburst has a negligible effect on the next outburst, this nearly linear relation suggests that the peak flux during the LH state that occurs at the beginning of an outburst is related to the mass in the disk, and thus that the entire disk is probably involved in powering these LH states. We have also found a positive correlation between the peak luminosities of the LH state in the three recent outbursts of {GX 339$-$4} and the peak luminosities of the following HS state. This correlation is similar to the correlations reported previously for {Aql X$-$1}, {4U 1705$-$44}, and {XTE J1550$-$564}, providing further support that the accretion flow that powers the LH state is related to the accretion flow that powers the following HS state. Although the luminosity of the LH-to-HS transition varies by up to an order of magnitude, the neutron stars and the black holes are distinguishable in the state transition luminosity. We discuss the implications for the mass determination of the compact stars in the ultraluminous X-ray sources (ULXs).

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