arXiv subjects

Didier Barret

Publications and source records attributed to Didier Barret.

At least 37 records · Page 2Linked to original sources

arXiv2021

Follow-up Observations of the Prolonged, super-Eddington, Tidal Disruption Event Candidate 3XMM~J150052.0+015452: the Slow Decline Continues

The X-ray source 3XMM~J150052.0+015452 was discovered as a spectacular tidal disruption event candidate during a prolonged ($>11$ yrs) outburst (Lin et al. 2017). It exhibited unique quasi-soft X-ray spectra of characteristic temperature $kT\sim0.3$ keV for several years at the peak, but in a recent Chandra observation (10 yrs into the outburst) a super-soft X-ray spectrum of $kT\sim0.15$ keV was detected. Such dramatic spectral softening could signal the transition from the super-Eddington to thermal state or the temporary presence of a warm absorber. Here we report on our study of four new XMM-Newton follow-up observations of the source. We found that they all showed super-soft spectra, suggesting that the source had remained super-soft for $>5$ yrs. Then its spectral change is best explained as due to the super-Eddington to thermal spectral state transition. The fits to the thermal state spectra suggested a smaller absorption toward the source than that obtained in Lin et al. (2017). This led us to update the modeling of the event as due to the disruption of a 0.75 msun star by a massive black hole of a few$\times10^5$ msun. We also obtained two HST images in the F606W and F814W filters and found that the dwarf star-forming host galaxy can be resolved into a dominant disk and a smaller bulge. No central point source was clearly seen in either filter, ruling out strong optical emission associated with the X-ray activity.

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arXiv2021

Wind-luminosity evolution in NLS1 AGN 1H 0707-495

Ultra-fast outflows (UFOs) have been detected in the high-quality X-ray spectra of a number of active galactic nuclei (AGN) with fairly high accretion rates and are thought to significantly contribute to the AGN feedback. After a decade of dedicated study, their launching mechanisms and structure are still not well understood, but variability techniques may provide useful constraints. In this work, therefore, we perform a flux-resolved X-ray spectroscopy on a highly accreting and variable NLS1 AGN, 1H 0707-495, using all archival XMM-Newton observations to study the structure of the UFO. We find that the wind spectral lines weaken at higher luminosities, most likely due to an increasing ionization parameter as previously found in a few similar sources. Instead, the velocity is anticorrelated with the luminosity, which is opposite to the trend observed in the NLS1 IRAS 13224-3809. Furthermore, the detection of the emission lines, which are not observed in IRAS 13224-3809, indicates a wind with a larger opening angle in 1H 0707-495, presumably due to a higher accretion rate. The emitting gas is found to remain broadly constant with the luminosity. We describe the variability of the wind with a scenario where the strong radiation extends the launch radius outwards and shields the outer emitting gas, similarly to super-Eddington compact objects, although other possible explanations are discussed. Our work provides several hints for a multi-phase outflow in 1H 0707-495.

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arXiv2021

Timing Calibration of the NuSTAR X-ray Telescope

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission is the first focusing X-ray telescope in the hard X-ray (3-79 keV) band. Among the phenomena that can be studied in this energy band, some require high time resolution and stability: rotation-powered and accreting millisecond pulsars, fast variability from black holes and neutron stars, X-ray bursts, and more. Moreover, a good alignment of the timestamps of X-ray photons to UTC is key for multi-instrument studies of fast astrophysical processes. In this Paper, we describe the timing calibration of the NuSTAR mission. In particular, we present a method to correct the temperature-dependent frequency response of the on-board temperature-compensated crystal oscillator. Together with measurements of the spacecraft clock offsets obtained during downlinks passes, this allows a precise characterization of the behavior of the oscillator. The calibrated NuSTAR event timestamps for a typical observation are shown to be accurate to a precision of ~65 microsec.

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arXiv2021

Estimating, monitoring and minimizing the travel footprint associated with the development of the Athena X-ray Integral Field Unit -- An on-line travel footprint calculator released to the science community

Global warming imposes us to reflect on the way we carry research, embarking on the obligation to minimize the environmental impact of our research programs, with the reduction of our travel footprint being one of the easiest actions to implement, thanks to the advance of digital technology. The X-ray Integral Field Unit (X-IFU), the cryogenic spectrometer of the Athena space X-ray observatory of the European Space Agency will be developed by a large international consortium. The travel footprint associated with the development of the X-IFU is to be minimized. For that purpose, a travel footprint calculator has been developed and first released to the X-IFU consortium members. The calculator uses seven different emission factors and methods differing by up to a factor of ~5 for the same flying distance. The observed differences illustrate the lack of standards and regulations for computing the footprint of flight travels and are explained primarily, though partly, by different accountings of non-CO2 effects. The calculator enables us to compute the travel footprint of a large set of travels and can help identify a meeting place that minimizes the overall travel footprint for a large set of possible city hosts, e.g. cities with large airports. The calculator also includes the option for a minimum distance above which flying is considered the most suitable transport option ; below that chosen distance, the emission of train journeys are considered. To demonstrate its full capabilities, the calculator is first run on one of the largest scientific meetings; the fall meeting of the AGU and meetings of the IPCC for which it is used to compute the location that would minimize the travel footprint. Then the travel footprint of X-IFU is estimated to be ~500 tons of CO2-eq/yr. With this paper, the on-line travel footprint calculator is released to the science community (abridged).

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arXiv2020

The carbon footprint of large astronomy meetings

The annual meeting of the European Astronomical Society took place in Lyon, France, in 2019, but in 2020 it was held online only due the COVID-19 pandemic. The carbon footprint of the virtual meeting was roughly 3,000 times smaller than the face-to-face one, providing encouragement for more ecologically minded conferencing.

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arXiv2020

Studying the Reflection Spectra of the New Black Hole X-ray Binary Candidate MAXI J1631-479 Observed by NuSTAR: A Variable Broad Iron Line Profile

We present results from the Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the new black hole X-ray binary candidate MAXI J1631-479 at two epochs during its 2018-2019 outburst, which caught the source in a disk dominant state and a power-law dominant state. Strong relativistic disk reflection features are clearly detected, displaying significant variations in the shape and strength of the broad iron emission line between the two states. Spectral modeling of the reflection spectra reveals that the inner radius of the optically-thick accretion disk evolves from $<1.9$ $r_{\rm g}$ to $12\pm1$ $r_{\rm g}$ (statistical errors at 90% confidence level) from the disk dominant to the power-law dominant state. Assuming in the former case that the inner disk radius is consistent with being at the ISCO, we estimate a black hole spin of $a^*>0.94$. Given that the bolometric luminosity is similar in the two states, our results indicate that the disk truncation observed in MAXI J1631-479 in the power-law dominant state is unlikely to be driven by a global variation in the accretion rate. We propose that it may instead arise from local instabilities in the inner edge of the accretion disk at high accretion rates. In addition, we find an absorption feature in the spectra centered at $7.33\pm0.03$ keV during the disk dominant state, which is evidence for a rare case that an extremely fast disk wind ($v_{\rm out}=0.067^{+0.001}_{-0.004}~c$) is observed in a low-inclination black hole binary, with the viewing angle of $29\pm1^{\circ}$ as determined by the reflection modeling.

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arXiv2020

Multiwavelength Follow-up of the Hyperluminous Intermediate-mass Black Hole Candidate 3XMM J215022.4-055108

We recently discovered the X-ray/optical outbursting source 3XMM J215022.4-055108. It was best explained as the tidal disruption of a star by an intermediate-mass black hole of mass of a few tens of thousand solar masses in a massive star cluster at the outskirts of a large barred lenticular galaxy at D_L=247 Mpc. However, we could not completely rule out a Galactic cooling neutron star as an alternative explanation for the source. In order to further pin down the nature of the source, we have obtained new multiwavelength observations by XMM-Newton and Hubble Space Telescope (HST). The optical counterpart to the source in the new HST image is marginally resolved, which rules out the Galactic cooling neutron star explanation for the source and suggests a star cluster of half-light radius ~27 pc. The new XMM-Newton observation indicates that the luminosity was decaying as expected for a tidal disruption event and that the disk was still in the thermal state with a super-soft X-ray spectrum. Therefore, the new observations confirm the source as one of the best intermediate-mass black hole candidates.

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arXiv2020

Spectral evolution of the ultraluminous X-ray sources M82 X-1 and X-2

M82 hosts two well-known ultraluminous X-ray sources (ULXs). X-1, an intermediate-mass black hole (IMBH) candidate, and X-2, an ultraluminous X-ray pulsar (ULXP). Here we present a broadband X-ray spectral analysis of both sources based on ten observations made simultaneously with Chandra and NuSTAR. Chandra provides the high spatial resolution to resolve the crowded field in the 0.5--8 keV band, and NuSTAR provides the sensitive hard X-ray spectral data, extending the bandpass of our study above 10 keV. The observations, taken in the period 2015--2016, cover a period of flaring from X-1, allowing us to study the spectral evolution of this source with luminosity. During four of these observations, X-2 was found to be at a low flux level, allowing an unambiguous view of the emission from X-1. We find that the broadband X-ray emission from X-1 is consistent with that seen in other ULXs observed in detail with NuSTAR, with a spectrum that includes a broadened disk-like component and a high-energy tail. We find that the luminosity of the disk scales with inner disk temperature as L~T^-3/2 contrary to expectations of a standard accretion disk and previous results. These findings rule out a thermal state for sub-Eddington accretion and therefore do not support M82 X-1 as an IMBH candidate. We also find evidence that the neutral column density of the material in the line of sight increases with L$_X$, perhaps due to an increased mass outflow with accretion rate. For X-2, we do not find any significant spectral evolution, but we find the spectral parameters of the phase-averaged broadband emission are consistent with the pulsed emission at the highest X-ray luminosities.

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arXiv2019

The Athena space X-ray Observatory and the astrophysics of hot plasma

The properties (temperature, density, chemical composition, velocity) of hot astrophysical plasma and the physical processes affecting them (heating/cooling, turbulence, shocks, acceleration) can be probed by high resolution X-ray spectroscopy, to be complemented by high spatial resolution imaging. The paper presents a status of the ESA's Advanced Telescope for High Energy Astrophysics (Athena) mission, particularly focusing on the science performance of its two focal plane instruments for the studies of extended X-ray sources: the Wide Field Imager (WFI) and the X-ray Integral Field Unit (X-IFU). This paper then provides a brief summary of the breakthroughs expected with Athena on the astrophysics of hot plasma, building on the vast heritage of the discoveries and revolutionary results obtained by Chandra and XMM-Newton in this field. As of November 12th, 2019, Athena successfully concluded its feasibility study, and has since then moved into the definition phase, with a launch date scheduled in the early 2030s.

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arXiv2019

The high energy universe at ultra-high resolution: the power and promise of X-ray interferometry

We propose the development of X-ray interferometry (XRI), to reveal the universe at high energies with ultra-high spatial resolution. With baselines which can be accommodated on a single spacecraft, XRI can reach 100 $μ$as resolution at 10 Å(1.2 keV) and 20 $μ$as at 2 Å(6 keV), enabling imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby accreting supermassive black holes (SMBH) and the SMBH `shadow'; SMBH accretion flows and outflows; X-ray binary winds and orbits; stellar coronae within ~100 pc and many exoplanets which transit across them. For sufficiently luminous sources XRI will resolve sub-pc scales across the entire observable universe, revealing accreting binary SMBHs and enabling trigonometric measurements of the Hubble constant with X-ray light echoes from quasars or explosive transients. A multi-spacecraft `constellation' interferometer would resolve well below 1 $μ$as, enabling SMBH event horizons to be resolved in many active galaxies and the detailed study of the effects of strong field gravity on the dynamics and emission from accreting gas close to the black hole.

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arXiv2019

Inferring black hole spins and probing accretion/ejection flows in AGNs with the Athena X-ray Integral Field Unit

Active Galactic Nuclei (AGN) display complex X-ray spectra which exhibit a variety of emission and absorption features, that are commonly interpreted as a combination of i) a relativistically smeared reflection component, resulting from the irradiation of an accretion disk by a compact hard X-ray source, ii) one or several warm/ionized absorption components produced by AGN-driven outflows crossing our line of sight, and iii) a non relativistic reflection component produced by more distant material. Disentangling these components via detailed model fitting can thus be used to constrain the black hole spin, the geometry and characteristics of the accretion flow, as well as of the outflows and surroundings of the black hole. We investigate how a high throughput high resolution X-ray spectrometer, such as the Athena X-ray Integral Field Unit (X-IFU) can be used to this aim, using the state of the art reflection model Relxill in a lamp post geometrical configuration. We simulate a representative sample of AGN spectra, including all necessary model complexities, as well as a range of model parameters going from standard to more extreme values, and considered X-ray fluxes that are representative of known AGN and Quasars (QSOs) populations. We also present a method to estimate the systematic errors related to the uncertainties in the calibration of the X-IFU. In a conservative setting, in which the reflection component is computed self consistently by the Relxill model from the pre-set geometry and no iron over abundance, the mean errors on the spin and height of the irradiating source are <0.05 and ~0.2 Rg. Similarly the absorber parameters are measured to an accuracy typically less than ~5%. Extending the simulations to include blue shifted ultra fast outflows, we show that X-IFU could measure their velocity with statistical errors <1% even for high redshift objects (abridged).

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arXiv2019

Broadband X-ray Spectral and Timing Analyses of the Black Hole Binary Candidate Swift J1658.2-4242: Rapid Flux Variation and the Turn-on of a Transient QPO

We report results from joint NuSTAR, Swift and XMM-Newton observations of the newly discovered black hole X-ray binary candidate Swift J1658.2-4242 in the intermediate state. We observe a peculiar event in this source, with its X-ray flux rapidly decreasing by $\sim$45\% in $\sim$40~s, accompanied by only subtle changes in the shape of the broadband X-ray spectrum. In addition, we find a sudden turn-on of a transient QPO with a frequency of $6-7$~Hz around the time of the flux change, and the total fractional rms amplitude of the power spectrum increases from $\sim$2\% to $\sim$10\%. X-ray spectral and timing analyses indicate that the flux decrease is driven by intrinsic changes in the accretion flow around the black hole, rather than intervening material along the line of sight. In addition, we do not significantly detect any relativistic disk reflection component, indicating it is much weaker than previously observed while the source was in the bright hard state. We propose accretion disk instabilities triggered at a large disk radius as the origin of the fast transition in spectral and timing properties, and discuss possible causes of the unusual properties observed in Swift J1658.2-4242. The prompt flux variation detected along with the emergence of a QPO makes the event an interesting case for investigating QPO mechanisms in black hole X-ray binaries.

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arXiv2019

A broadband look at the old and new ULXs of NGC 6946

Two recent observations of the nearby galaxy NGC 6946 with NuSTAR, one simultaneous with an XMM-Newton observation, provide an opportunity to examine its population of bright accreting sources from a broadband perspective. We study the three known ultraluminous X-ray sources (ULXs) in the galaxy, and find that ULX-1 and ULX-2 have very steep power-law spectra with $Γ=3.6^{+0.4}_{-0.3}$ in both cases. Their properties are consistent with being super-Eddington accreting sources with the majority of their hard emission obscured and down-scattered. ULX-3 (NGC 6946 X-1) is significantly detected by both XMM-Newton and NuSTAR at $L_{\rm X}=(6.5\pm0.1)\times10^{39}$ erg s$^{-1}$, and has a power-law spectrum with $Γ=2.51\pm0.05$. We are unable to identify a high-energy break in its spectrum like that found in other ULXs, but the soft spectrum likely hinders our ability to detect one. We also characterise the new source, ULX-4, which is only detected in the joint XMM-Newton and NuSTAR observation, at $L_{\rm X}=(2.27\pm0.07)\times10^{39}$ erg s$^{-1}$, and is absent in a Chandra observation ten days later. It has a very hard cut-off power-law spectrum with $Γ=0.7\pm0.1$ and $E_{\rm cut}=11^{+9}_{-4}$ keV. We do not detect pulsations from ULX-4, but its transient nature can be explained either as a neutron star ULX briefly leaving the propeller regime or as a micro-tidal disruption event induced by a stellar-mass compact object.

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arXiv2019

Unlocking the Capabilities of Future High-Resolution X-ray Spectroscopy Missions Through Laboratory Astrophysics

Thanks to high-resolution and non-dispersive spectrometers onboard future X-ray missions such as XRISM and Athena, we are finally poised to answer important questions about the formation and evolution of galaxies and large-scale structure. However, we currently lack an adequate understanding of many atomic processes behind the spectral features we will soon observe. Large error bars on parameters as critical as transition energies and atomic cross sections can lead to unacceptable uncertainties in the calculations of e.g., elemental abundance, velocity, and temperature. Unless we address these issues, we risk limiting the full scientific potential of these missions. Laboratory astrophysics, which comprises theoretical and experimental studies of the underlying physics behind observable astrophysical processes, is therefore central to the success of these missions.

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arXiv2018

Accretion in Strong Field Gravity with eXTP

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process.

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arXiv2018

The Hard State of the Highly Absorbed High Inclination Black Hole Binary Candidate Swift J1658.2-4242 Observed by NuSTAR and Swift

We present a spectral and timing analysis of the newly reported Galactic X-ray transient Swift J1658.2-4242 observed by NuSTAR and Swift. The broad-band X-ray continuum is typical of a black hole binary in the bright hard state, with a photon index of $Γ=1.63\pm0.02$ and a low coronal temperature of $kT_{\rm e}=22\pm1$ keV, corresponding to a low spectral cutoff well constrained by NuSTAR. Spectral modeling of the relativistic disk reflection features, consisting of a broad Fe K$α$ line and the Compton reflection hump, reveals that the black hole is rapidly spinning with the spin parameter of $a^{*}>0.96$, and the inner accretion disk is viewed at a high inclination angle of $i=64^{+2}_{-3}{^\circ}$ (statistical errors, 90% confidence). The high inclination is independently confirmed by dips in the light curves, which can be explained by absorbing material located near the disk plane temporarily obscuring the central region. In addition, we detect an absorption line in the NuSTAR spectra centered at $7.03^{+0.04}_{-0.03}$ keV. If associated with ionized Fe K absorption lines, this provides evidence for the presence of outflowing material in the low/hard state of a black hole binary candidate. A timing analysis shows the presence of a type-C QPO in the power spectrum, with the frequency increasing from $\sim0.14$ Hz to $\sim0.21$ Hz during the single NuSTAR exposure. Our analysis reveals that Swift J1658.2-4242 displays characteristics typical for a black hole binary that is viewed at a high inclination angle, making it a good system for studying the accretion geometry in black hole binaries.

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arXiv2018

The Athena X-ray Integral Field Unit

The X-ray Integral Field Unit (X-IFU) is the high resolution X-ray spectrometer of the ESA Athena X-ray observatory. Over a field of view of 5' equivalent diameter, it will deliver X-ray spectra from 0.2 to 12 keV with a spectral resolution of 2.5 eV up to 7 keV on ~5 arcsecond pixels. The X-IFU is based on a large format array of super-conducting molybdenum-gold Transition Edge Sensors cooled at about 90 mK, each coupled with an absorber made of gold and bismuth with a pitch of 249 microns. A cryogenic anti-coincidence detector located underneath the prime TES array enables the non X-ray background to be reduced. A bath temperature of about 50 mK is obtained by a series of mechanical coolers combining 15K Pulse Tubes, 4K and 2K Joule-Thomson coolers which pre-cool a sub Kelvin cooler made of a 3He sorption cooler coupled with an Adiabatic Demagnetization Refrigerator. Frequency domain multiplexing enables to read out 40 pixels in one single channel. A photon interacting with an absorber leads to a current pulse, amplified by the readout electronics and whose shape is reconstructed on board to recover its energy with high accuracy. The defocusing capability offered by the Athena movable mirror assembly enables the X-IFU to observe the brightest X-ray sources of the sky (up to Crab-like intensities) by spreading the telescope point spread function over hundreds of pixels. Thus the X-IFU delivers low pile-up, high throughput (>50%), and typically 10 eV spectral resolution at 1 Crab intensities, i.e. a factor of 10 or more better than Silicon based X-ray detectors. In this paper, the current X-IFU baseline is presented, together with an assessment of its anticipated performance in terms of spectral resolution, background, and count rate capability. The X-IFU baseline configuration will be subject to a preliminary requirement review that is scheduled at the end of 2018.

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arXiv2018

Reproducibility and monitoring of the instrumental particle background for the X-Ray Integral Field Unit

The X-ray Integral Field Unit (X-IFU) is the cryogenic imaging spectrometer on board the future X-ray observatory \textsl{Athena}. With a hexagonal array of 3840 AC-biased Transition Edge Sensors (TES), it will provide narrow-field observations (5$^{\prime}$ equivalent diameter) with unprecedented high spectral resolution (2.5 eV up to 7 keV) over the 0.2 - 12 keV bandpass. Throughout its observations, the X-IFU will face various sources of X-ray background. Specifically, the so-called Non-X-ray Background (NXB) caused by the interaction of high-energy cosmic rays with the instrument, may lead to a degradation of its sensitivity in the observation of faint extended sources (e.g. galaxy clusters outskirts). To limit this effect, a cryogenic anti-coincidence detector (CryoAC) will be placed below the detector plane to lower the NXB level down to the required level of $5 \times 10^{-3}$ cts/s/cm$^{2}$/keV over 2 - 10 keV. In this contribution, we investigate ways to accurately monitor the NXB and ensure the highest reproducibility in-flight. Using the limiting science case of the background-dominated observation of galaxy clusters outskirts, we demonstrate that a reproducibility of 2\% on the absolute knowledge of the background is required to perform driving science objectives, such as measuring abundances and turbulence in the outskirts. Monitoring of the NXB in-flight through closed observations, the detector's CryoAC or the companion instrument (Wide Field Imager) will be used to meet this requirement.

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