• 1
  • Jussi Ahoranta
  • Observations of off-centered turbulence distribution in velocity fields of NGC 4636 galaxy group IGM

    The cool core of galaxy group NGC 4636 has a complex morphology in X-ray region, exhibiting traces of AGN feedback with the surrounding hot Intra Group Medium IGM gas. Through detailed analysis of high resolution X-ray spectroscopy and imaging spectrometry, diverse information on the thermodynamic properties of the IGM gas can be obtained. We present our novel results of the IGM properties at NGC 4636 group core, based on the analysis of long exposure data of XMM-Newton Reflection Grating Spectrometer and Chadra ACIS instruments.

  • 2
  • Jessamyn Allen
  • Strongly Ionized and Variable Outflow in the Neutron Star Low-Mass X-Ray Binary GX 13+1

    We present the analysis of 7 Chandra HETG observations of GX 13+1, a persistent neutron star low-mass X-ray binary. The observations cover activity between 2002 and 2011, with 5 of the observations spanning a 2 week period in July-August 2010. The continuum was consistent with a two component model, either a blackbody plus powerlaw or a multicolor disk and blackbody across luminosities of 5-7x1037 erg/sec, modified by a neutral absorption column with a silicon overabundance. We have identified K-alpha Fe, Ca, S, and Si hydrogen-like lines in all observations, as well hydrogen-like Ar and Mg lines in the majority of observations. We did not find Mn or Cr lines in the 2010-2011 observations, which were previously reported in the 2002 Chandra observation. The absorption lines were found to be variable and strongly blue-shifted v_outflow 500 km/sec, indicating a significant ionized outflow.

  • 3
  • Laura Brenneman
  • The Athena/X-IFU View of the Energetic Universe

    I present a synopsis of the scientific objectives of Athenas Science Working Group on the Energetic Universe, of which I am a co-chair. Athena is ESAs next large X-ray observatory that has been selected for launch in 2028. One of the most important goals of the mission is to characterize the growth and evolution of supermassive black holes SMBHs over cosmic time through the measurement of their properties, how gas accretes and flows out of these systems in the form of winds and jets, and how the SMBH and its host galaxy influence each other. Two instruments will be utilized to address these questions: the Wide Field Imager WFI and the X-ray Integral Field Unit Calorimeter X-IFU. I focus herein on the investigations we expect to conduct with the X-IFU in our Working Group, and the observing strategies we will employ.

  • 4
  • Chia-Ying Chiang
  • A Test of the Nature of the Fe K Line in the Neutron Star Low-mass X-ray Binary Serpens X-1

    Broad Fe K emission lines have been widely discovered in the X-ray spectra of black hole systems, and in neutron star systems as well. The intrinsically narrow Fe K fluorescent line is generally believed to be part of the reflection spectrum originating in an illuminated accretion disk, and broadened by strong relativistic effects. However, the nature of the lines in neutron star LMXBs has been under debate. We therefore obtained the longest, high-resolution X-ray spectrum of a neutron star LMXB to date with a 300 ks Chandra HETGS observation of Serpens X-1. The observation was taken under the continuous clocking mode and thus free of photon pile-up effects. We carry out a systematic analyses and find that the blurred reflection model fits the Fe line of Serpens X-1 significantly better than a broad Gaussian component does, or a combination of broad and narrow Gaussians, implying that the relativistic reflection scenario is much preferred.

  • 5
  • Xiaohong Cui
  • X-ray spectra from kappa and Maxwellian distributions of electrons

    The electron velocity distribution in hot plasmas can significantly affect both the line and continuum X-ray spectrum emitted by the plasma. A relatively long-lasting stable but non-Maxwellian distribution, i.e., kappa distribution, was firstly identified in the Earth’s magnetosphere (Vasyliunas 1968) and subsequently can be rigorously derived via statistical mechanics (e.g. Livadiotis & McComas 2009). We propose to create a plasma model based on the κ distribution and the AtomDB atomic database including dielectronic satellite lines and inner-shell excitation arising from the high-energy electrons.

  • 6
  • Renata Cumbee
  • Modeling X-ray Emission from C-Al ions due to Charge Exchange with H, He, and H2

    The fundamental collisional process of charge exchange CX has been been established as a primary source of X-ray emission from the heliosphere 1, planetary exospheres 2, and supernova remnants 3,4. In this process, X-ray emission results from the capture of an electron by a highly charged ion from a neutral atom or molecule, to form a highly-excited, high charge state ion. As the electron cascades down to the lowest energy level, photons are emitted, including X-rays. To accurately model the X-ray emission spectra and hence better understand the dynamics of these environments, it is essential to include the dominant collisional processes, including CX, for different ion and neutral interactions. To provide reliable CX-induced X-ray spectral models to realistically simulate these environments, line ratios and spectra are computed using theoretical cross-sections obtained with the multi-channel Landau-Zener, atomic-orbital close-coupling, and classical-trajectory Monte-Carlo methods. Using these calculations, X-ray line ratios, hardness ratios, and spectra will be presented for collisions of H and He-like C to Al with H, He, and H2 as their neutral target.This work was performed in collaboration with David Lyons, Patrick Mullen, David Schultz, Phillp Stancil, and Robin Shelton. Work at UGA was partially supported by NASA grants NNX09AC46G and NNX13AF31G.1 Henley, D. B. and Shelton, R. L. 2010, Astrophys. J. Suppl. 187, 388 2 Dennerl, k. et al. 2002, Astron. Astrophys. 386, 3193 Katsuda, S. et al. 2011, ApJ 730 244 Cumbee, R. S. et al. 2014, ApJ 787 L31

  • 7
  • Laura Delgado
  • X and gamma-ray Emission From Recurrent Novae

    Authors: L.Delgado, M. Hernanz ICE CSIC-IEECThe LAT instrument on board the Fermi satellite has detected seven nova explosions. The very high-energy gamma-ray emission is a consequence of pi0 decay and/or Inverse Compton, which are related to particle p and e- acceleration in the strong shock between the nova ejecta and the red giant wind. We will present the recurrent nova, V745 Sco, for which we have analysed Swift/XRT observations simultaneous to Fermi detection and a Chandra/LETG spectrum. This combined with IR and Radio emission allows us to get a global picture of the first days of nova outbursts and their relationship with particle acceleration. We will also present the analysis of the Chandra and XMM-Newton observations of the recurrent nova RS Oph, the first nova for which particle acceleration has been predicted.

  • 8
  • Adam Foster
  • High Resolution Spectroscopic Diagnostics for Future X-ray Missions

    New mission concepts involve new mission technologies or larger instruments which enable the exploration of new phenomena. There are significant trade offs between increasing the effective area, the sensitivity of the instruments, and the energy resolution, with the exact combination determining the range of physics which is available. In this work, we present a summary of astrophysical diagnostics available, and the energy resolution required to observe them. This will provide a summary of possible physics avenues for exploration, independent of any currently proposed mission concept. This work is based on data in AtomDB and the XSTAR database, and is therefore principally, though not exclusively aimed at the X-ray regime.

  • 9
  • Christina Graefe
  • X-raying Centaurus A

    Centaurus A is the closest radio-loud active galactic nucleus AGN and a perfect laboratory to study the role of AGN feedback into interstellar gas. With the high angular resolution of Chandra/ACIS we disentangle the X-ray emission from different components of the core, as well as a population of point sources in the host galaxy of Centaurus A. We study these sources, which are likely X-ray binaries, in observations spanning 16 years. Accounting for all the point sources then allows us to isolate the extended diffuse emission of Cen A and thus gain insight into the influence the active nucleus has on the galaxy. We analyze Chandra/LETGS and NuSTAR data to model the continuum components and to constrain temperature and density properties of the diffuse plasma.

  • 10
  • Victoria Grinberg
  • The clumpy focussed wind in the Cyg X-1 / HDE 226868 system

    Binary systems with a compact object are a unique chance to investigate the strong, clumpy, line-driven winds of early type supergiants by using the compact object's X-rays as a probe of the wind structure. Here, we analyze the two-component wind of HDE 226868, the O9.7Iab giant companion of the black hole Cyg X-1.
    Using Chandra-HETG we separate signatures of the hot gas phase from those of the cold, dense clumps. Observations at different orbital phases reveal orbital variability in clump occurrence. We can further probe this variability and thus the large-scale spatial distribution of the wind components using ~4.8 Msec of RXTE data. In particular, we traces changes in absorption on timescales of 2 ksec over 16 years of observations throughout different accretion regimes of the black hole. The exceptional time coverage of RXTE allows us, for the first time, to directly compare our observations to simulations of clumpy winds in O/B type stars and we find a good agreement for a porosity length on the order of the stellar radius.

  • 11
  • Ralf Heilmann
  • Next-Generation Grating Spectrometer with Critical-Angle Transmission CAT Gratings

    The high energy astrophysics community continues to rely on the High Energy Transmission Grating Spectrometer on board Chandra and the Reflection Grating Spectrometers on board XMM-Newton both launched in 1999 and built with technology from the 80s and early 90s for soft x-ray spectroscopy of celestial sources. Looking ahead toward the 2020s almost a generation after those launches - it is time to prepare for the next decadal review and for the implementation of the latest technologies that will enable orders of magnitude improved performance metrics for soft E 2 keV x ray absorption and emission spectroscopy in order to address high priority science questions. Examples are the growth of the large scale structure of the universe and its interaction with active galactic nuclei, the kinematics of galactic outflows, and coronal emission from stars. We have developed high-efficiency blazed transmission gratings that combine the advantages of traditional transmission gratings low mass, relaxed alignment tolerances, high transparency for hard x rays and blazed reflection gratings high diffraction efficiency, high resolving power due to blazing into higher diffraction orders. Blazing is achieved through reflection off of the smooth sidewalls of freestanding, ultra-high aspect-ratio silicon grating bars. A hierarchy of integrated low-blockage supports allows for the fabrication of lightweight, large-area gratings that can be used to tile a large telescope aperture with minimal mass. The dispersed photons are detected by order-sorting linear readout cameras in the focal plane, offset from an imaging detector at the telescope focus. Grating effective area on the order of 4,000 cm2 and spectral resolving power R= 61548614876150861548 of up to 5,000 are conceivable for an X-ray Surveyor mission, while a probe-class mission could easily exceed 1,000 cm2 with R 3,000. Using a microcalorimeter as the imaging detector in tandem with the high transparency of CAT gratings to hard x rays allows for effective simultaneous observation and high-resolution spectroscopy across the soft and hard x-ray bands.We report on the latest fabrication and x-ray test results of our prototype CAT gratings.

  • 12
  • Young-Dae Jung
  • Turbulent and plasma screening effects on the bremsstrahlung spectrum

    The influence of turbulence and plasma screening on the electron-ion bremsstrahlung spectrum is investigated in astrophysical turbulent plasmas. It is found that the bremsstrahlung cross section including the influence of turbulence and plasma screening decreases with an increase of the thermal energy for small impact parameters and, however, increases with an increase of the diffusion coefficient. It is also found that the plasma screening effect enhances the bremsstrahlung radiation cross section for small thermal energies. In addition, the plasma screening effect on the bremsstrahlung cross section increases with an increase of the diffusion coefficient and, however, decreases with increasing thermal energy in astrophysical turbulent plasmas.

  • 13
  • Vinay Kashyap
  • TANAGRA: Timing Analysis of Grating Data of Stellar Coronal Sources

    We have developed a comprehensive database of spectro-temporal properties of stellar coronal sources that have been observed with gratings with Chandra. A number of useful variables are computed, including light curves, flux curves, 2D change points and flare distributions. Analysis is carried out for both broadband as well as for diagnostically useful spectral lines.

  • 14
  • Walter Maksym
  • Shredding Stars at High Resolution

    X-rays are a critical regime in which to study the tidal disruption of stars by massive black holes. But thus far most observations of these events have been sufficiently photon-starved that high-resolution X-ray spectroscopy has typically been impractical. With sufficient photon statistics, it should be possible to investigate the role of winds and the evolving geometry of stellar debris streams in these encounters. Enabled by timely follow-up of a low-redshift tidal disruption flare from the new generation of all-sky time domain surveys, we discuss the first example of high-resolution X-ray spectroscopy of a tidal disruption event which shows numerous discrete spectral features. We also discuss the implications for future large-area missions such as Athena+, which will transform X-ray spectroscopy into a versatile tool for understanding the interactions between black holes and stellar debris.

  • 15
  • Junjie Mao
  • Parameterisation for the Level Resolved Radiative Recombination Data

    The level resolved radiative recombination (RR) rate coefficients for H-like to Na-like from H (Z=1) up to and including Zn (Z=30) are studied here. For H-like sequence, the quantum-mechanic exact photoionisation cross sections, provided by Strorey&Hummer 1986, for nonrelativistic hydrogenic systems are used for calculating the RR rate coefficients under the principle of detailed balance. While for He-like to Na-like, the archival data on ADAS are adopted. Parameterisations are made for the direct capture rates in the 10-10^8 K temperature range. The fitting accuracies are better than 5% for the majority of the levels, while several levels from the singly ionised ions and/or high shell (n=4 or 5) levels yield accuracies up to ~10%. The RR data will be incorporated to the high-resolution spectral analysis package SPEX (Kaastra et al. 1996).

  • 16
  • Michael McCollough
  • Cygnus X-3 Chandra Spectra of The Fe K-Alpha Region

    Cygnus X-3 Cyg X-3 is a unique X-ray binary XRB. Its X-ray emission shows a strong 4.8 hour orbital modulation, typical for a low-mass XRB, but its mass-donating companion is a Wolf-Rayet WR star which makes it a high-mass XRB. During the Chandra mission there has been several HETG Chandra observations of Cygnus X-3 taken in a variety of spectral states. The Chandra spectra of Cygnus X-3 show a rich photoionized spectrum with a large number of H-like and He-like lines. In this presentation we will examine the Fe K-alpha region of the spectrum. We will look at how the observed H-like, He-like, and neutral Fe lines vary both as a function of time and spectral state. In particular, we will examine both the 1st order and 3rd order HETG spectra with specific attention paid to the He-like Fe triplet.

  • 17
  • Brendan McLaughlin
  • Benchmarking X-ray Cross Sections for Astrophysical Applications

    Photoionization of atomic and molecular elements is an important process in determining the ionization balance and hence the abundances of elements in photoionized astrophysical nebulae. It has recently become possible to detect neutron n-capture elements (atomic number Z>30) in a large number of ionized nebulae [1,2]. Measuring the abundances of these elements helps to reveal their dominant production sites in the Universe, as well as details of stellar structure, mixing and nucleosynthesis. Astrophysical observations provide an impetus to determine the photoionization and recombination properties of n-capture elements. Accurate assessment of elemental abundances in astrophysical nebulae can be made from the direct comparison of the observed spectra with synthetic non-local thermodynamic equilibrium (NLTE) spectra, if the atomic data for electron and photon interaction processes are known with sufficient accuracy. Experiments on light systems and on trans-Fe atomic ions and molecular species at third generation synchrotron radiation source, such the Advanced Light Source (ALS) in Berkeley, California, USA, SOLEIL in Saint-Aubin, France, ASTRID II in Aarhus, Denmark and PETRA III, in Hamburg, Germany, highlighted the need for high quality theoretical work to fully interpret experimental results. Recently developed methods for atomic systems using parallel computing architectures [3-5,9,10] (incorporating the necessary relativistic effects within either a Breit-Pauli or a Dirac formulation) has been used to perform detailed photoionization cross section calculations on a variety of atomic species, e.g.; Fe [3], Se [4], Kr [5,6], Ar [7], Xe [5,13], W [8-12], Si[14], S[15], Carbon[16], Nitrogen[17] and Oxygen[18], in neutral or low stages of ionization. Similar ab initio R-matrix methods for molecular species have also been implemented, e.g. N2, NO, and CO2 [19-25]. Comparison of theoretical results with experiments performed at third generation synchrotron light sources serve as the ultimate benchmark in order to have confidence in the atomic and molecular data to be incorporated into standard astrophysical modelling codes such as CLOUDY, XSTAR, CHIANTI and ATOMDB.

  • 18
  • Matthew Miller
  • Revealing Milky Way Hot Gas Kinematics with High Resolution X-ray Spectroscopy

    The Milky Way hosts a hot 106 K, low-density plasma extending at least 50 kpc from the Galactic disk based on detections of z0 OVII and OVIII absorption and emission lines. This radial gas distribution is determined from absorption and emission line strengths that are unresolved with current X-ray spectrographs. The Doppler b parameter is expected to be 100 km/s, but we show that if the halo gas is rotating, which is predicted in models, the line widths can be wider and they will have structure. The line profiles, which have analogs to HI profiles, encode the hot gas rotation curve, the net inflow or outflow accretion or wind of hot gas, and the hot gas angular momentum as a function of radius. Modest optical depth effects also are important in determining the line shape, especially for lines of sight across the Galaxy. Determining these properties will offer broad insights into galaxy formation and the current state of activity in the halo today. These effects will be observable with an instrument that has a spectral resolution of about 3000, a goal that is technically possible today.

  • 19
  • Patrick Mullen
  • Streamlined Multi-Channel Landau-Zener CX Calculations for Cometary X-ray Emission

    Charge exchange CX has emerged in X-ray emission modeling as a dominant process that must be considered in many astrophysical environments -- such as comets, supernova remnants, the heliosphere, astrospheres of stars, and generally, highly ionized regions of the interstellar medium. With a motivation to bring resolution to the current lack of atomic and molecular data for such a vital process, we have applied quantum defect theory and the multi-channel Landau-Zener MCLZ approach to quickly provide charge exchange data for any single electron capture SEC system. By implementing the resulting nlS-resolved cross sections into a cascade model for X-ray emission, we are able to generate theoretical emission lines and spectra for charge exchange. By comparing MCLZ cross sections and theoretical spectra for Fe XXV and Fe XXVI SEC collisions to an electron beam ion trap EBIT study, as well as other theoretical methods such as CTMC -- classical trajectory Monte Carlo, we demonstrate very good agreement among theory and experiment, thus giving merit to our streamlined program set and its use in astrophysical modeling. Thus, we pursue the case study of calculating charge exchange nlS-resolved cross sections and theoretical line ratios for a variety of bare and non-bare ion SEC collisions between C, N, O, Ne, Mg, Al, and Si with H2O, CO, CO2, O, and OH in efforts to model cometary X-ray emission. This work was performed in collaboration with R. S. Cumbee, D. Lyons, P. C. Stancil, and B. J. Wargelin. Work at UGA was partially funded by NASA grant NNX13AF31G.

  • 20
  • Joey Neilsen
  • Infrared to X-ray Emission from a Compton-Thick Wind in GRO J1655-40

    During its 2005 outburst, GRO J1655-40 was observed at high spectral resolution with the Chandra HETGS, revealing a spectrum rich with blueshifted absorption lines of elements ranging from oxygen to nickel, including exotic metals like titanium and scandium. It has been argued that magnetic fields must be responsible for the dense accretion disk wind responsible for these deep absorption lines. But questions about this outburst remain, because the presence of this exotic wind coincides with extremely curved X-ray spectra, remarkable X-ray variability, and bright, unexpected optical/infrared emission that varies on the orbital period. We argue that the unusual features of this hypersoft state see Uttley et al. 2015 are natural consequences of a Compton-thick wind from the disk.

  • 21
  • Joy Nichols
  • Time-Resolved Variability Studies with ACIS HETG Spectra

    Custom time slices of ACIS HETG stellar spectra are used to study variability of emission lines and/or continuum. We present studies of several types of stars, demonstrating the method of creating the time slices and analysis methods used for determining variability. The techniques described search for emission line shape, width, and flux variability.

  • 22
  • Michael Nowak
  • Leveraging High Resolution Spectra to Understand the Disk and Relativistic Iron Line of Cygnus X-1

    In April 2008 we conducted an observation of the black hole candidate Cygnus X-1 that was performed simultaneously with every X-ray and gamma-ray satellite flying at that time, including Chandra-HETG. The HETG spectra are crucial for modeling the ionized absorbtion from the focused-wind of the secondary, which is present and must be accounted for in all of our spectra. These features, however, are unresolved in the non-gratings instruments e.g., RXTE, Suzaku, Swift, XMM-EPIC, INTEGRAL. Similarly, we must account for differences in spatial resolution. The X-ray scattering dust halo, which is usually ignored in most analyses, is spatially resolved in the Chandra and XMM-Newton spectra, but is unresolved in the other instruments. Thus one must account for dust scattering loss in the high spatial resolution spectra, and the scattering back into our line of site for the low resolution spectra. In this poster, we attempt to arrive at a joint model for these spectra, and further comment on the cross calibration of each of the X-ray instruments participating in this campaign.

  • 23
  • Marina Orio
  • Chandra Grating Observation of Nova Mon 2012

    Nova Mon 2012 was observed with the Chandra LETG grating in December of 2012, after it had entered the supersoft X-ray phase. This spectrum can be compared with that obtained with the HETG grating by another group two months earlier, when only the ejecta, and not the white dwarf, seemed to be emitting X-rays. This nova has been well observed in several wavelength range, including gamma rays with Fermi shortly after the outburst, and recent radio imaging has clearly shown the geometry of the remnant. We find interesting, quite precise constraints on the abundances with Chandra. Among other results, we confirm that the nova should have occurred on a NeO white dwarf.

  • 24
  • Francesca Senatore
  • The z=0 warm-hot absorber in-and-around the Milky Way

    At z=0, almost half of the baryons are missing. Most likely, they are concentrated in a warm-hot phase, possibly surrounding galaxies, as also predicted by simulations. Our own Galaxy represents an ideal laboratory to search for these baryons.Here we present a systematic search of z=0 Li- to H-like metal absorption in the high-resolution X-ray spectra of two S/N-complete samples of Galactic and extragalactic targets. We show that, while highly ionized absorption is ubiquitously present against both Galactic and extragalactic targets, the detection of more than one transition from the same ion in several spectra of our sample allows us to establish that these two absorbers are different and therefore likely located at different distances from the Galactic center.

  • 25
  • Randall Smith
  • Arcus: The next generation of high-resolution X-ray grating spectra

    Arcus is an X-ray grating spectrometer mission to be deployed on the International Space Station, proposed in response to NASAs Astrophysics Division 2014 SMEX call. The baseline design uses sub-apertured X-ray silicon pore optics feeding into off-plane gratings to achieve both high spectral resolution with a large effective area. The detector focal plane uses Suzaku-type CCDs. The mission would be ready to be launched and mounted on the ISS in 2020. The mission parameters are R2000 and effective area 400 sq. cm at the critical O VII wavelength near 21 0.5 keV, with an overall bandpass from 8-52 0.25-1.5 keV, enabling a wide range of science objectives. These values are similar to those of the grating spectrometers considered as part of the proposed Constellation-X and IXO missions, which were highly ranked by two Decadal surveys.

  • 26
  • Igone Urdampilleta
  • X-ray emission from thin plasma. Collisional ionization for atoms and ions of H to Zn.

    Every observation of astrophysical objects involving a spectra requires some aspects of atomic data for the interpretation of line fluxes, ratios and ionization state of the emitting plasma. One of the thermal radiative process which determines it, partially, is the collisional ionization. In this work, the following processes has been taken into account: direct ionization DI, excitation-autoionization EA, resonance excitation double autoionization REDA and multi ionisation. The most recent assessments have been performed by Dere 2007, AA 466, 771 for H to Zn sequences, Arnaud. Raymond 1992 ,ApJ. 398, 394 for iron and Arnaud Rothenflug 1985, AAS, 60, 425. However, in the last years new laboratory measurements and theoretical calculations of ionization cross sections have become accessible. Our main goal is to provide a review, extension and update of these previous works and be able of fitting the cross sections of all inner shells of all ions from H to Zn. Once all dataset available are identified, they have been fitted using an extension of Youngers Younger 1981 formula for DI and Mewe formula Mewe 1972 for EA, suitable for integration over a Maxwellian velocity distribution to derive the ionization rate coefficients. For the elements with non available data, the cross section has been interpolated or extrapolated. The results of the present work will be included in SPEX SW, utilized for X-ray spectra modeling, fitting and analysis. SPEX will be extensively used for the analysis of the measurements obtained with the Astro-H satellite.

  • 27
  • Sascha Zeegers
  • Investigating dense interstellar environments in the X-rays

    We present the modelling of the silicon absorption edge due to dust absorption along the line of sight of bright X-ray sources. The shape and observed energy of these edges can reveal the composition and abundance of the intervening dust grains in different environments of our Galaxy. Here we present, as a test case, the interpretation of a high-quality spectrum of a bright X-ray binary located in the vicinity of the Galactic Center Zeegers et al. 2015, in prep. This spectroscopic study, performed using data from the Chandra-HETG, benefits from new laboratory data in the X-ray band of several silicate compounds Costantini et al. 2013.

  • 28
  • Shuo Zhang
  • NuSTAR Observations of Galactic Center Molecular Clouds: Reconstructing Sgr A X-ray Outburst History

    The Galactic Center GC supermassive black hole Sgr A radiates at a level about 9 orders of magnitude lower than its Eddington luminosity. But indication of its glorious past has come from X-ray observations of surrounding molecular clouds. Their varying neutral iron lines and X-ray continuum emission could be due to reflection of Sgr A X-ray outbursts in the past few centuries. However, such observational facts can also be explained by low energy cosmic ray electron LECRe bombardment. In this talk, I present the NuSTAR observations of the GC molecular clouds. Their substructures at energies greater than 10 keV are revealed at sub-arcminute scale for the first time. With the broadband 3-79 keV spectrum, we are able to test physical models of X-ray reflection and LECRe. Results show that the X-ray reflection model is preferred over the LECRe model for all the clouds. The X-ray emission from cloud Sgr B2 points to a faraway illuminating source with a photon index of 2.2 and a luminosity of 6x1038 erg/s, most likely due to a past giant X-ray outburst from Sgr A. The Sgr A clouds requires a fainter and more recent X-ray outburst, revealing different stages of the Sgr A X-ray outbursts.

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