JaeSub Hong

NuSTAR DISCOVERY OF A 3.76 s TRANSIENT MAGNETAR NEAR SAGITTARIUS A

We report the discovery of 3.76 s pulsations from a new burst source near Sgr A^* observed by the NuSTAR observatory. The strong signal from SGR J1745–29 presents a complex pulse profile modulated with pulsed fraction 27% ± 3% in the 3-10 keV band. Two observations spaced nine days apart yield a spin-down rate of Ṗ =(6.5 ± 1.4) × 10^(–12). This implies a magnetic field B = 1.6 × 10^(14) G, spin-down power Ė =5 × 10^(33) erg s^(–1), and characteristic age P/2Ṗ =9 × 10^3 yr for the rotating dipole model. However, the current Ṗ may be erratic, especially during outburst. The flux and modulation remained steady during the observations and the 3-79 keV spectrum is well fitted by a combined blackbody plus power-law model with temperature kT_(BB) = 0.96 ± 0.02 keV and photon index Γ = 1.5 ± 0.4. The neutral hydrogen column density (N_H ~ 1.4 × 10^(23) cm^(–2)) measured by NuSTAR and Swift suggests that SGR J1745–29 is located at or near the Galactic center. The lack of an X-ray counterpart in the published Chandra survey catalog sets a quiescent 2-8 keV luminosity limit of L_x ≾ 10^(32) erg s^(–1). The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3.5 × 10^(35) erg s^(–1) for a distance of 8 kpc. SGR J1745–29 joins a growing subclass of transient magnetars, indicating that many magnetars in quiescence remain undetected in the X-ray band or have been detected as high-B radio pulsars. The peculiar location of SGR J1745–29 has important implications for the formation and dynamics of neutron stars in the Galactic center region.

NEW SPECTRAL CLASSIFICATION TECHNIQUE FOR X-RAY SOURCES: QUANTILE ANALYSIS

We present a new technique, quantile analysis, to classify the spectral properties of X-ray sources with limited statistics. Quantile analysis is superior to conventional approaches, such as X-ray hardness ratio or X-ray color analysis, for studying relatively faint sources or investigating a certain phase or state of a source in detail, where poor statistics do not allow spectral fitting using a model. Instead of working with predetermined energy bands, we determine the energy values that divide the detected photons into predetermined fractions of the total counts, such as the median (50%), terciles (33% and 67%), and quartiles (25% and 75%). We use these quantiles as an indicator of the X-ray hardness or color of the source. We show that the median is an improved substitute for the conventional X-ray hardness ratio. The median and other quantiles form a phase space, similar to the conventional X-ray color-color diagrams. The quantile-based phase space is more evenly sensitive over various spectral shapes than the conventional color-color diagrams, and it is naturally arranged to properly represent the statistical similarity of various spectral shapes. We demonstrate the new technique in the 0.3-8 keV energy range using the Chandra ACIS-S detector response function and a typical aperture photometry involving background subtraction. The technique can be applied in any energy band, provided the energy distribution of photons can be obtained.

X-Ray Processing of ChaMPlane Fields: Methods and Initial Results for Selected Anti-Galactic Center Fields

We describe the X-ray analysis procedure of the ongoing Chandra Multiwavelength Plane (ChaMPlane) Survey and report the initial results from the analysis of 15 selected anti-Galactic center observations (90° < l < 270°). We describe the X-ray analysis procedures for ChaMPlane using custom-developed analysis tools appropriate for Galactic sources but also of general use: optimum photometry in crowded fields using advanced techniques for overlapping sources, rigorous astrometry and 95% error circles for combining X-ray images or matching to optical/IR images, and application of quantile analysis for spectral analysis of faint sources. We apply these techniques to 15 anti-Galactic center observations (of 14 distinct fields), in which we have detected 921 X-ray point sources. We present log N-log S distributions and quantile analysis to show that in the hard band (2-8 keV) active galactic nuclei dominate the sources. Complete analysis of all ChaMPlane anti-Galactic center fields will be given in a subsequent paper, followed by papers on sources in the Galactic center and bulge regions.

Chandra Multiwavelength Plane (ChaMPlane) Survey: An Introduction

We introduce the Chandra Multiwavelength Plane (ChaMPlane) survey, designed to measure or constrain the populations of low-luminosity (LX 1031 ergs s-1) accreting white dwarfs, neutron stars, and stellar mass black holes in the Galactic plane and bulge. ChaMPlane incorporates two surveys, X-ray (Chandra) and optical (NOAO 4 m Mosaic imaging), and a follow-up spectroscopy and IR identification program. The survey has now extended through the first 6 yr of Chandra data using serendipitous sources detected in 105 distinct ACIS-I and ACIS-S fields observed in 154 pointings and covered by 65 deep Mosaic images in V, R, I, and Hα. ChaMPlane incorporates fields with Galactic latitude 12° and selected to be devoid of bright point or diffuse sources, with exposure time 20 ks and (where possible) minimum NH. We describe the scientific goals and introduce the X-ray and optical/IR processing and databases. We derive preliminary constraints on the space density or luminosity function of cataclysmic variables (CVs) from the X-ray/optical data for 14 fields in the Galactic anticenter. The lack of ChaMPlane CVs in these anticenter fields suggests that their space density is ~3 times below the value (3 × 10-5 pc-3) found for the solar neighborhood by previous X-ray surveys. Companion papers describe the X-ray and optical processing in detail, optical spectroscopy of ChaMPlane sources in selected anticenter fields, and IR imaging results for the Galactic center field. An appendix introduces the ChaMPlane Virtual Observatory (VO) for online access to the X-ray and optical images and source catalogs for ready display and further analysis.

Extended Hard-X-Ray Emission in the Inner Few Parsecs of the Galaxy

The Galactic Centre hosts a puzzling stellar population in its inner few parsecs, with a high abundance of surprisingly young, relatively massive stars bound within the deep potential well of the central supermassive black hole, Sagittarius A* (ref. 1). Previous studies suggest that the population of objects emitting soft X-rays (less than 10 kiloelectronvolts) within the surrounding hundreds of parsecs, as well as the population responsible for unresolved X-ray emission extending along the Galactic plane, is dominated by accreting white dwarf systems. Observations of diffuse hard-X-ray (more than 10 kiloelectronvolts) emission in the inner 10 parsecs, however, have been hampered by the limited spatial resolution of previous instruments. Here we report the presence of a distinct hard-X-ray component within the central 4 × 8 parsecs, as revealed by subarcminute-resolution images in the 20–40 kiloelectronvolt range. This emission is more sharply peaked towards the Galactic Centre than is the surface brightness of the soft-X-ray population. This could indicate a significantly more massive population of accreting white dwarfs, large populations of low-mass X-ray binaries or millisecond pulsars, or particle outflows interacting with the surrounding radiation field, dense molecular material or magnetic fields. However, all these interpretations pose significant challenges to our understanding of stellar evolution, binary formation, and cosmic-ray production in the Galactic Centre.

RADIAL DISTRIBUTION OF X-RAY POINT SOURCES NEAR THE GALACTIC CENTER

We present the log N-log S and spatial distributions of X-ray point sources in seven Galactic bulge (GB) fields within 4? from the Galactic center (GC). We compare the properties of 1159 X-ray point sources discovered in our deep (100 ks) Chandra observations of three low extinction Window fields near the GC with the X-ray sources in the other GB fields centered around Sgr B2, Sgr C, the Arches Cluster, and Sgr A* using Chandra archival data. To reduce the systematic errors induced by the uncertain X-ray spectra of the sources coupled with field-and-distance-dependent extinction, we classify the X-ray sources using quantile analysis and estimate their fluxes accordingly. The result indicates that the GB X-ray population is highly concentrated at the center, more heavily than the stellar distribution models. It extends out to more than 14 from the GC, and the projected density follows an empirical radial relation inversely proportional to the offset from the GC. We also compare the total X-ray and infrared surface brightness using the Chandra and Spitzer observations of the regions. The radial distribution of the total infrared surface brightness from the 3.6 band ?m images appears to resemble the radial distribution of the X-ray point sources better than that predicted by the stellar distribution models. Assuming a simple power-law model for the X-ray spectra, the closer to the GC the intrinsically harder the X-ray spectra appear, but adding an iron emission line at 6.7 keV in the model allows the spectra of the GB X-ray sources to be largely consistent across the region. This implies that the majority of these GB X-ray sources can be of the same or similar type. Their X-ray luminosity and spectral properties support the idea that the most likely candidate is magnetic cataclysmic variables (CVs), primarily intermediate polars (IPs). Their observed number density is also consistent with the majority being IPs, provided the relative CV to star density in the GB is not smaller than the value in the local solar neighborhood.

Spectral Response of THM Grown CdZnTe Crystals

The spectral response of several crystals grown by the Traveling Heater Method (THM) were investigated. An energy resolution of 0.98% for a Pseudo Frisch-Grid of 4 times 4 times 9 mm3 and 2.1% FWHM for a coplanar-grid of size 11 times 11 times 5 mm3 were measured using 137Cs-662 keV. In addition a 4% FWHM at 122 keV has also been measured on 20 times 20 X 5 mm3 monolithic pixellated devices. The material shows great potential toward producing large-volume detectors with spectral performance that meets the requirement for high-resolution gamma-ray spectroscopy.

Dominance of magnetic cataclysmic variables in the resolved Galactic ridge X‐ray emission of the limiting window

The diffuse appearance of the Galactic ridge X-ray emission has been puzzling since its discovery due to the lack of compelling theories for sustainable hot diffuse X-ray emission in the Galactic plane. Recently, Revnivtsev et al. claimed that ∼90 per cent of the 6.5–7.1 keV X-ray flux from a small section of a low-extinction region at 1°.4 south of the Galactic Centre has been resolved to discrete sources with erg s−1 cm−2, using ultradeep (1 Ms) observations made by the Chandra X-ray Observatory. They also concluded that coronally active stars such as active binaries (ABs) contribute ∼60 per cent of the resolved flux. However, our recent discovery of a large population of magnetic cataclysmic variables (MCVs) in the same region suggests their significant role in the resolved hard X-ray flux. In addition, deep X-ray surveys of other several Galactic bulge fields over the past decade have indicated that MCVs are likely the major contributor in the hard X-ray emission above 2–3 keV. To solve this mystery, we have conducted an independent in-depth analysis of discrete X-ray sources in the low-extinction region. The total fraction of the 6.5–7.1 keV flux we can confidently claim as resolved is ∼70–80 per cent, which largely agrees with Revnivtsev et al., but leaves some room for diffuse components. However, despite the various attempts, we consistently find that the resolved hard X-ray flux above 3 keV is dominated by relatively bright, hard X-ray sources such as MCVs, whereas the contribution from relatively faint, soft sources such as ABs is below 20 per cent. We describe in detail our analysis procedure in order to elucidate possible origins of the discrepancy.

NuSTAR HARD X-RAY SURVEY OF THE GALACTIC CENTER REGION. I. HARD X-RAY MORPHOLOGY AND SPECTROSCOPY OF THE DIFFUSE EMISSION

We present the first sub-arcminute images of the Galactic Center above 10 keV, obtained with NuSTAR. NuSTAR resolves the hard X-ray source IGR J17456–2901 into non-thermal X-ray filaments, molecular clouds, point sources, and a previously unknown central component of hard X-ray emission (CHXE). NuSTAR detects four non-thermal X-ray filaments, extending the detection of their power-law spectra with Γ ~ 1.3–2.3 up to ~50 keV. A morphological and spectral study of the filaments suggests that their origin may be heterogeneous, where previous studies suggested a common origin in young pulsar wind nebulae (PWNe). NuSTAR detects non-thermal X-ray continuum emission spatially correlated with the 6.4 keV Fe Kα fluorescence line emission associated with two Sgr A molecular clouds: MC1 and the Bridge. Broadband X-ray spectral analysis with a Monte-Carlo based X-ray reflection model self-consistently determined their intrinsic column density (~10^(23) cm^(−2)), primary X-ray spectra (power-laws with Γ ~ 2) and set a lower limit of the X-ray luminosity of Sgr A* flare illuminating the Sgr A clouds to L_X ≳ 10^(38) erg s^(−1). Above ~20 keV, hard X-ray emission in the central 10 pc region around Sgr A* consists of the candidate PWN G359.95–0.04 and the CHXE, possibly resulting from an unresolved population of massive CVs with white dwarf masses M_(WD) ~ 0.9 M_⊙. Spectral energy distribution analysis suggests that G359.95–0.04 is likely the hard X-ray counterpart of the ultra-high gamma-ray source HESS J1745–290, strongly favoring a leptonic origin of the GC TeV emission.

Optimizing wide-field coded aperture imaging: radial mask holes and scanning

Imaging at hard X-ray energies (~10-600 keV) over very large fields of view (~60° per telescope) is required to conduct a high sensitivity all-sky and all-time survey for black holes. The proposed Energetic X-ray Imaging Survey Telescope (EXIST) could achieve the high sensitivity required for the mission science objectives by scanning an array of wide-field coded aperture telescopes with aperture mask holes radially aligned to minimize auto-collimation by the thick (~7mm) masks required for high energy imaging. Simulation results from a preliminary design study are reported which quantify the improvement in off-axis imaging sensitivity vs. the conventional case with mask holes all perpendicular to the mask. Such masks can be readily constructed from a stacked laminate of thin (1mm) laser-etched W sheets. An even more dramatic increase in coded aperture imaging sensitivity, and dynamic range, for a realistic telescope and imaging detector with typical systematic errors can be achieved by continuously scanning the field of view of the telescope over the source region to be imaged. Simulation results are reported for detectors with systematic errors 1-10%, randomly distributed but unknown in each detector pixel. For the simplified case of a 1-D coded aperture telescope scanning along its pattern, the systematics are removed identically. Results are also presented for the 2-D case with both 1-D and partial 2-D scanning which demonstrate the feasibility of a coded aperture scanning telescope with systematic errors achieving nearly Poisson-limited sensitivity for signal/background ratios S/B ~ 10-4, in constrast to limits typically ~10-100X worse that have been actually achieved by pointed or dithered coded aperture telescopes flown (or proposed) previously.