Dacheng Lin

Evaluating Spectral Models and the X-Ray States of Neutron Star X-Ray Transients

We analyze the X-ray spectra of the neutron star (NS) X-ray transients Aql X-1 and 4U 1608-52, obtained with RXTE during more than 20 outbursts. We test commonly used spectral models and evaluate their performance against desirability criteria, including LX ∝ T4 evolution for the multicolor disk (MCD) component and similarity to black holes (BHs) for correlated timing/spectral behavior. None of the classical models for thermal emission plus Comptonization perform well in the soft state. Instead, we devise a hybrid model: for the hard state, a single-temperature blackbody (BB) plus a broken power law (BPL), and for the soft state, two thermal components (MCD and BB) plus a constrained BPL. This model produces LX ∝ T4 tracks for both the MCD and BB, and it aligns the spectral/timing correlations of these NSs with the properties of accreting BHs. The visible BB emission area is very small (~1/16 of the NS surface), but it remains roughly constant over a wide range of LX that spans both the hard and soft states. We discuss implications of a small and constant boundary layer in terms of the presence of an innermost stable circular orbit that lies outside the NS. Finally, if the BB luminosity tracks the overall accretion rate, then we find that the Comptonization in the hard state has surprisingly high radiative efficiency compared to MCD emission in the soft state. Alternatively, if we assume that the radiative efficiency of a jet in the hard state must be less than the MCD efficiency in the soft state, while relaxing presumptions about the accretion rate, then our results may suggest substantial mass outflow in the jet.

The XMM-Newton serendipitous survey - VII. The third XMM-Newton serendipitous source catalogue

© ESO, 2016.Context. Thanks to the large collecting area (3 × ∼1500 cm2 at 1.5 keV) and wide field of view (30′ across in full field mode) of the X-ray cameras on board the European Space Agency X-ray observatory XMM-Newton, each individual pointing can result in the detection of up to several hundred X-ray sources, most of which are newly discovered objects. Since XMM-Newton has now been in orbit for more than 15 yr, hundreds of thousands of sources have been detected. Aims. Recently, many improvements in the XMM-Newton data reduction algorithms have been made. These include enhanced source characterisation and reduced spurious source detections, refined astrometric precision of sources, greater net sensitivity for source detection, and the extraction of spectra and time series for fainter sources, both with better signal-to-noise. Thanks to these enhancements, the quality of the catalogue products has been much improved over earlier catalogues. Furthermore, almost 50% more observations are in the public domain compared to 2XMMi-DR3, allowing the XMM-Newton Survey Science Centre to produce a much larger and better quality X-ray source catalogue. Methods. The XMM-Newton Survey Science Centre has developed a pipeline to reduce the XMM-Newton data automatically. Using the latest version of this pipeline, along with better calibration, a new version of the catalogue has been produced, using XMM-Newton X-ray observations made public on or before 2013 December 31. Manual screening of all of the X-ray detections ensures the highest data quality. This catalogue is known as 3XMM. Results. In the latest release of the 3XMM catalogue, 3XMM-DR5, there are 565 962 X-ray detections comprising 396 910 unique X-ray sources. Spectra and lightcurves are provided for the 133 000 brightest sources. For all detections, the positions on the sky, a measure of the quality of the detection, and an evaluation of the X-ray variability is provided, along with the fluxes and count rates in 7 X-ray energy bands, the total 0.2-12 keV band counts, and four hardness ratios. With the aim of identifying the detections, a cross correlation with 228 catalogues of sources detected in all wavebands is also provided for each X-ray detection. Conclusions. 3XMM-DR5 is the largest X-ray source catalogue ever produced. Thanks to the large array of data products associated with each detection and each source, it is an excellent resource for finding new and extreme objects.

X-RAY VARIABILITY AND HARDNESS OF ESO 243-49 HLX-1: CLEAR EVIDENCE FOR SPECTRAL STATE TRANSITIONS

The ultraluminous X-ray (ULX) source ESO 243-49 HLX-1, which reaches a maximum luminosity of 1042 erg s–1 (0.2-10 keV), currently provides the strongest evidence for the existence of intermediate-mass black holes (IMBHs). To study the spectral variability of the source, we conduct an ongoing monitoring campaign with the Swift X-ray Telescope (XRT), which now spans more than two years. We found that HLX-1 showed two fast rise and exponential decay type outbursts in the Swift XRT light curve with increases in the count rate of a factor ~40 separated by 375 ± 13 days. We obtained new XMM-Newton and Chandra dedicated pointings that were triggered at the lowest and highest luminosities, respectively. From spectral fitting, the unabsorbed luminosities ranged from 1.9 × 1040 to 1.25 × 1042 erg s–1. We confirm here the detection of spectral state transitions from HLX-1 reminiscent of Galactic black hole binaries (GBHBs): at high luminosities, the X-ray spectrum showed a thermal state dominated by a disk component with temperatures of 0.26 keV at most, and at low luminosities the spectrum is dominated by a hard power law with a photon index in the range 1.4-2.1, consistent with a hard state. The source was also observed in a state consistent with the steep power-law state, with a photon index of ~3.5. In the thermal state, the luminosity of the disk component appears to scale with the fourth power of the inner disk temperature, which supports the presence of an optically thick, geometrically thin accretion disk. The low fractional variability (rms of 9% ± 9%) in this state also suggests the presence of a dominant disk. The spectral changes and long-term variability of the source cannot be explained by variations of the beaming angle and are not consistent with the source being in a super-Eddington accretion state as is proposed for most ULX sources with lower luminosities. All this indicates that HLX-1 is an unusual ULX as it is similar to GBHBs, which have non-beamed and sub-Eddington emission, but with luminosities three orders of magnitude higher. In this picture, a lower limit on the mass of the black hole of >9000 M ☉ can be derived, and the relatively low disk temperature in the thermal state also suggests the presence of an IMBH of a few 103 M ☉.

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 Rossi X-Ray Timing Explorer 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 (CD) and hardness-intensity diagrams (HID), but they are accompanied by changes in the kHz quasi-periodic oscillations, broadband variability, burst behavior, and/or X-ray spectra. We find that for most of the outburst the low-energy X-ray flux is a good parameter to track the gradual evolution of the tracks in CD and HID, 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.

Classification of X-Ray Sources in the XMM-Newton Serendipitous Source Catalog

We carry out classification of 4330 X-ray sources in the 2XMMi-DR3 catalog. They are selected under the requirement of being a point source with multiple XMM-Newton observations and at least one detection with the signal-to-noise ratio larger than 20. For about one-third of them we are able to obtain reliable source types from the literature. They mostly correspond to various types of stars (611), active galactic nuclei (AGNs, 753), and compact object systems (138) containing white dwarfs, neutron stars, and stellar-mass black holes. We find that about 99% of stars can be separated from other source types based on their low X-ray-to-IR flux ratios and frequent X-ray flares. AGNs have remarkably similar X-ray spectra, with the power-law photon index centered around 1.91 ± 0.31, and their 0.2-4.5 keV flux long-term variation factors have a median of 1.48, with 98.5% being less than 10. In contrast, 70% of compact object systems can be very soft or hard, highly variable in X-rays, and/or have very large X-ray-to-IR flux ratios, separating them from AGNs. Using these results, we derive a source type classification scheme to classify the other sources and find 644 candidate stars, 1376 candidate AGNs, and 202 candidate compact object systems, whose false identification probabilities are estimated to be about 1%, 3%, and 18%, respectively. There are still 320 sources associated with nearby galaxies and 151 in the Galactic plane, which we expect to be mostly compact object systems or background AGNs. We also have 100 candidate ultraluminous X-ray sources. They are found to be much less variable than other accreting compact objects.

SUZAKU AND BeppoSAX X-RAY SPECTRA OF THE PERSISTENTLY ACCRETING NEUTRON-STAR BINARY 4U 1705-44

United States. National Aeronautics and Space Administration (NASA Grant NNX08AC02G, Suzaku guest observer program)

The kilohertz quasi-periodic oscillations during the Z and atoll phases of the unique transient XTE J1701-462

We analysed 866 observations of the neutron star low-mass X-ray binary XTE J1701-462 during its 2006-2007 outburst. XTE J1701-462 is the only example so far of a source that during an outburst showed, beyond any doubt, spectral and timing characteristics both of the Z and atoll type. There are 707 RXTE observations (similar to 2.5 Ms) of the source in the Z phase and 159 in the atoll phase (similar to 0.5 Ms). We found, respectively, pairs of kilohertz quasi-periodic oscillations (kHz QPOs) in eight observations during the Z phase and single kHz QPO in six observations during the atoll phase. Using the shift-and-add technique, we identified the QPO in the atoll phase as the lower kHz QPO. We found that the lower kHz QPO in the atoll phase has a significantly higher coherence and fractional rms amplitude than any of the kHz QPOs seen during the Z phase, and that in the same frequency range, atoll lower kHz QPOs show coherence and fractional rms amplitude, respectively, two and three times larger than the Z kHz QPOs. Out of the 707 observations in the Z phase, there is no single observation in which the kHz QPOs have a coherence or rms amplitude similar to those seen when XTE J1701-462 was in the atoll phase, even though the total exposure time was about five times longer in the Z than in the atoll phase. Since it is observed in the same source, the difference in QPO coherence and rms amplitude between the Z and atoll phase cannot be due to neutron star mass, magnetic field, spin, inclination of the accretion disc, etc. If the QPO frequency is a function of the radius in the accretion disc in which it is produced, our results suggest that in XTE J1701-462 the coherence and rms amplitude are not uniquely related to this radius. Here we argue that this difference is instead due to a change in the properties of the accretion flow around the neutron star. Regardless of the precise mechanism, our result shows that effects other than the geometry of space-time around the neutron star have a strong influence on the coherence and rms amplitude of the kHz QPOs, and therefore the coherence and rms amplitude of the kHz QPOs cannot be simply used to deduce the existence of the innermost stable circular orbit around a neutron star.

TYPE I X-RAY BURSTS FROM THE NEUTRON-STAR TRANSIENT XTE J1701-462

The neutron-star X-ray transient XTE J1701-462 was observed for ~3 Ms with RXTE during its 2006-2007 outburst. Here we report on the discovery of three type-I X-ray bursts from XTE J1701-462. They occurred as the source was in transition from the typical Z-source behavior to the typical atoll-source behavior, at ~10% of the Eddington luminosity. The first burst was detected in the Z-source flaring branch (FB); the second in the vertex between the FB and normal branches; and the third in the atoll-source soft state. The detection of the burst in the FB cast doubts on earlier speculations that the FB is due to unstable nuclear burning of accreted matter. The last two of the three bursts show photospheric radius expansion, from which we estimate the distance to the source to be 8.8 kpc with a 15% uncertainty. No significant burst oscillations in the range 30-4000 Hz were found during these three bursts.

The Rates of Type I X-Ray Bursts from Transients Observed with RXTE: Evidence for Black Hole Event Horizons

We measure the rates of type I X-ray bursts from a likely flux-limited sample of 37 nonpulsing Galactic transients observed with RXTE during 1996-2004. These sources are well categorized in the literature as either neutron star systems or black hole candidates. Our goals are to test the burst model for neutron stars and to investigate whether black holes have event horizons. Target selection is one of several differences between the present study and the investigation of the event horizon question by Tournear and coworkers. Burst rates are measured as a function of bolometric luminosity, and the results are compared with augmented versions of the burst model developed by Narayan & Heyl. For a given mass, we consider a range in both the radius and the temperature at the boundary below the accretion layer. We find 135 spectrally confirmed type I bursts in 3.7 Ms of PCA exposures for the neutron star group (13 sources), and the burst rate function is generally consistent with the model predictions. However, for the black hole groups (18 sources), there are no confirmed type I bursts in 6.5 Ms of exposure, and the upper limits in the burst function are inconsistent with the model predictions for heavy compact objects with a solid surface. The consistency probability is ~2 × 10-7 for dynamical black hole binaries, falling to 3 × 10-13 with the added exposures of black hole candidates. Furthermore, there are systematic spectral differences between the neutron star and black hole groups, supporting the presumption that physical differences underly the classifications in our sample. These results provide indirect evidence that black holes do have event horizons.

RBS 1032: A TIDAL DISRUPTION EVENT IN ANOTHER DWARF GALAXY?

RBS 1032 is a supersoft (