Didier Barret

The Nuclear Spectroscopic Telescope Array (NuSTAR) High-Energy X-Ray Mission

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a National Aeronautics and Space Administration (NASA) Small Explorer mission that carried the first focusing hard X-ray (6-79 keV) telescope into orbit. It was launched on a Pegasus rocket into a low-inclination Earth orbit on June 13, 2012, from Reagan Test Site, Kwajalein Atoll. NuSTAR will carry out a two-year primary science mission. The NuSTAR observatory is composed of the X-ray instrument and the spacecraft. The NuSTAR spacecraft is three-axis stabilized with a single articulating solar array based on Orbital Sciences Corporations LEOStar-2 design. The NuSTAR science instrument consists of two co-aligned grazing incidence optics focusing on to two shielded solid state CdZnTe pixel detectors. The instrument was launched in a compact, stowed configuration, and after launch, a 10-meter mast was deployed to achieve a focal length of 10.15 m. The NuSTAR instrument provides sub-arcminute imaging with excellent spectral resolution over a 12-arcminute field of view. The NuSTAR observatory will be operated out of the Mission Operations Center (MOC) at UC Berkeley. Most science targets will be viewed for a week or more. The science data will be transferred from the UC Berkeley MOC to a Science Operations Center (SOC) located at the California Institute of Technology (Caltech). In this paper, we will describe the mission architecture, the technical challenges during the development phase, and the post-launch activities.

An intermediate-mass black hole of over 500 solar masses in the galaxy ESO 243-49

Ultraluminous X-ray sources are extragalactic objects located outside the nucleus of the host galaxy with bolometric luminosities exceeding 1039 erg s-1. These extreme luminosities—if the emission is isotropic and below the theoretical (Eddington) limit, where the radiation pressure is balanced by the gravitational pressure—imply the presence of an accreting black hole with a mass of ∼102–105 solar masses (). The existence of such intermediate-mass black holes is in dispute, and though many candidates have been proposed, none are widely accepted as definitive. Here we report the detection of a variable X-ray source with a maximum 0.2–10 keV luminosity of up to 1.1 × 1042 erg s-1 in the edge-on spiral galaxy ESO 243-49, with an implied conservative lower limit for the mass of the black hole of ∼500.

HARD X-RAY EMISSION FROM LOW-MASS X-RAY BINARIES

We report on Rossi X-Ray Timing Explorer observations of four type I X-ray bursters, namely, 1E 1724-3045, GS 1826-238, SLX 1735-269, and KS 1731-260. The first three were in a low state, with 1-200 keV X-ray luminosities in the range ~0.05-0.1LEdd (LEdd: Eddington luminosity for a neutron star, LEdd = 2.5 × 1038 ergs s-1), whereas KS 1731-260 was in a high state, with luminosity ~0.35LEdd. The low-state sources have very similar power spectra, displaying high-frequency noise up to ~200 Hz. For KS 1731-260, its power spectrum is dominated by noise at frequencies 20 Hz; in addition a quasi-periodic oscillation at 1200 Hz is detected in a segment of the observation. The 1-200 keV spectra of the low-state sources are all consistent with resulting from thermal Comptonization with an electron temperature (kTe) around 25-30 keV. For KS 1731-260, the spectrum is also dominated by thermal Comptonization, but with a much lower kTe ~ 3 keV and no significant hard X-ray emission. With the exception of GS 1826-238, they each have an underlying soft component, carrying at most ~25% of the total 1-200 keV luminosity. For all sources, we have detected an iron Kα line at 6.4 keV (although it is weak and marginal in 1E 1724-3045). A reflection component is present in the spectra of GS 1826-238 and SLX 1735-269, and for both we find that the reflecting medium subtends only a small solid angle (Ω/2π ~ 0.15, 0.28). The origin of the line and the reflection component is most likely to be irradiation of the accretion disk by the X-ray source. We suggest a model in which the region of main energy release, where hard X-rays are produced, would be an optically thin boundary layer merged with an advection-dominated accretion flow (ADAF) and would be responsible for the rapid variability observed. The soft component observed probably represents the unscattered emission from an optically thick accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks and the strength of the reflected component and associated iron line increase. At the same time, the Comptonization region cools off in response to an increased cooling flux from the accretion disk and from the reprocessed/reflected component, thus leading progressively to a quenching of the hard X-ray emission. If low-state neutron stars (NSs) accrete via ADAFs, the observation of X-ray bursts, indicating that all the accreting matter actually accumulates onto the NS surface, argues against the existence of strong winds from such accretion flows. Finally, we discuss two criteria recently proposed to distinguish between nonquiescent black holes (BHs) and NSs that are not contradicted by existing observations. The first one states that, when thermal Comptonization is responsible for the hard X-ray emission, only BHs have kTe larger than ~50 keV. However, this criterion is weakened by the fact that there are NSs displaying nonattenuated power laws extending up to at least 200 keV, possibly implying nonthermal Comptonization or thermal Comptonization with kTe larger than 50 keV. The second criterion stipulates that only BHs are capable of emitting hard X-ray tails with 20-200 keV luminosities 1.5 × 1037 ergs s-1.

New Evidence for Black Hole Event Horizons from Chandra

Previously we claimed that black hole X-ray novae (BHXNs) in quiescence are much less luminous than equivalent neutron star X-ray novae (NSXNs). This claim was based on the quiescent detection of a single short-period BHXN (A0620-00, Porb = 7.8 hr) and two longer period BHXNs (GRO J1655-40, Porb = 62.9 hr; V404 Cygni, Porb = 155.3 hr), along with sensitive upper limits. Here we announce the detection of two more short-period BHXNs (GRO J0422+32, Porb = 5.1 hr; GS 2000+25, Porb = 8.3 hr), an upper limit for a third that is improved by 2 orders of magnitude (4U 1543-47, Porb = 27.0 hr), and a new, much lower quiescent measurement of GRO J1655-40. Taken together, these new Chandra Advanced CCD Imaging Spectrometer measurements confirm that the quiescent X-ray luminosities of BHXNs are significantly lower than those of NSXNs. We argue that this provides strong evidence for the existence of event horizons in BHXNs.

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.

The X-Ray Spectra of Black Hole X-Ray Novae in Quiescence as Measured by Chandra

We present Chandra observations of black hole X-ray novae V404 Cyg, A0620-00, GRO J1655-40, and XTE J1550-564 in quiescence. Their quiescent spectra can be well fitted by a power-law model with number slope α ~ 2. While a coronal (Raymond-Smith) model is also a statistically acceptable representation of the spectra, the best-fit temperatures of these models is ~5 times higher than that seen in active stellar coronae. These four spectra of quiescent X-ray novae are all consistent with that expected for accretion via an advection-dominated accretion flow and inconsistent with that expected from a stellar corona. This evidence for continued accretion in quiescence further strengthens the case for the existence of event horizons in black holes. Both A0620-00 and GRO J1655-40 were fainter than in previous observations, while V404 Cyg was more luminous and varied by a factor of 2 in a few kiloseconds. A reanalysis of the X-ray data for XTE J1550-564 shows that (like V404 Cyg and A0620-00) its luminosity exceeds the maximum prediction of the coronal model by a large factor. The 0.3-7 keV luminosities of the four sources studied are in the range from ~1030 to 1033 ergs s-1.

Asymmetries in core-collapse supernovae from maps of radioactive 44 Ti in Cassiopeia A

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive 44Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium, directly probes the explosion asymmetries. Cassiopeia A is a young, nearby, core-collapse remnant from which 44Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed 44Ti emission to estimated 56Ni emission, from optical light echoes, and from jet-like features seen in the X-ray and optical ejecta. Here we report spatial maps and spectral properties of the 44Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the 44Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.

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 ☉.

The Ultraluminous X-Ray Sources NGC 1313 X-1 and X-2: A Broadband Study with NuSTAR and XMM-Newton

We present the results of NuSTAR and XMM-Newton observations of the two ultraluminous X-ray sources: NGC 1313 X-1 and X-2. The combined spectral bandpass of the two satellites enables us to produce the first spectrum of X-1 between 0.3 and 30 keV, while X-2 is not significantly detected by NuSTAR above 10 keV. The NuSTAR data demonstrate that X-1 has a clear cutoff above 10 keV, whose presence was only marginally detectable with previous X-ray observations. This cutoff rules out the interpretation of X-1 as a black hole in a standard low/hard state, and it is deeper than predicted for the downturn of a broadened iron line in a reflection-dominated regime. The cutoff differs from the prediction of a single-temperature Comptonization model. Further, a cold disk-like blackbody component at ~0.3 keV is required by the data, confirming previous measurements by XMM-Newton only. We observe a spectral transition in X-2, from a state with high luminosity and strong variability to a lower-luminosity state with no detectable variability, and we link this behavior to a transition from a super-Eddington to a sub-Eddington regime.

An abrupt drop in the coherence of the lower kHz quasi-periodic oscillations in 4U 1636-536

Using archival data from the Rossi X-ray Timing Explorer (RXTE), we study in a systematic way the variation of the quality factor (Q = ν/�ν , �ν , full width at half maximum) and amplitude of the lower and upper kHz quasi-periodic oscillations (QPOs) in the low-mass X-ray binary 4U 1636−536, over a frequency range from ∼550 to ∼1200 Hz. When represented in a quality factor versus frequency diagram, the upper and lower QPOs follow two different tracks, suggesting that they are distinct phenomena, although not completely independent because the frequency difference of the two QPOs, when detected simultaneously, remains within ∼60 Hz of half the neutron star spin frequency (at ν spin = 581 Hz). The quality factor of the lower kHz QPO increases with frequency up to a maximum of Q ≈ 200 at ν lower ≈ 850 Hz, then drops precipitously to Q ≈ 50 at the highest detected frequencies ν lower ≈ 920 Hz. A ceiling of the lower QPO frequencies at 920 Hz is also clearly seen in a frequency versus count rate diagram. At the same time, the quality factor of the upper kHz QPO increases steadily from Q ∼ 5t o∼15 all the way to ν upper ≈ 1150 Hz, which is the highest detectable QPO frequency. The rms amplitudes of both the upper and lower kHz QPOs decrease steadily towards higher frequencies. The quality factor provides a measure of the coherence of the underlying oscillator. For exponentially damped sinusoidal shots, the highest Q observed corresponds to an oscillator coherence time of 1/π�ν ∼ 0.1 s. All existing QPO models face challenges in explaining such a long coherence time and the significantly different behaviours of the quality factors of the upper and lower QPOs reported here. It is therefore difficult to be certain of the implications of the abrupt change in the lower QPO at ∼850 Hz. We discuss various possible causes, including that the drop in coherence is ultimately caused by effects related to the innermost stable circular orbit.