Motoki Nakajima

The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images

The Monitor of All-sky X-ray Image (MAXI) mission is the first astronomical payload to be installed on the Japanese Experiment Module — Exposed Facility (JEM-EF or Kibo-EF) on the International Space Station. It has two types of X-ray slit cameras with wide FOVs and two kinds of X-ray detectors consisting of gas proportional counters covering the energy range of 2 to 30 keV and X-ray CCDs covering the energy range of 0.5 to 12 keV. MAXI will be more powerful than any previous X-ray All Sky Monitor payloads, being able to monitor hundreds of Active Galactic Nuclei. A realistic simulation under optimal observation conditions suggests that MAXI will provide all-sky images of X-ray sources of � 20 mCrab (� 7 � 10 � 10 erg cm � 2 s � 1 in the energy band of 2–30 keV) from observations during one ISS orbit (90 min), � 4.5 mCrab for one day, and � 2 mCrab for one week. The final detectability of MAXI could be � 0.2 mCrab for two years, which is comparable to the source confusion limit of the MAXI field of view (FOV). The MAXI objectives are: (1) to alert the community to X-ray novae and transient X-ray sources, (2) to monitor long-term variabilities of X-ray sources, (3) to stimulate multi-wavelength observations of variable objects, (4) to create unbiased X-ray source cataloges, and (5) to observe diffuse cosmic X-ray emissions, especially with better energy resolution for soft X-rays down to 0.5 keV.

Gas Slit Camera (GSC) onboard MAXI on ISS

The Gas Slit Camera (GSC) is an X-ray instrument on the MAXI (Monitor of All-sky X-ray Image) mission on the International Space Station. It is designed to scan the entire sky every 92-minute orbital period in the 2‐30 keV band and to achieve the highest sensitivity among the X-ray all-sky monitors ever flown so far. The GSC employs large-area position-sensitive proportional counters with the total detector area of 5350 cm 2 . The on-board data processor has functions to format telemetry data as well as to control the high voltage of the proportional counters to protect them from the particle irradiation. The paper describes the instruments, on-board data processing, telemetry data formats, and performance specifications expected from the ground calibration tests.

In-Orbit Performance of MAXI Gas Slit Camera (GSC) on ISS

We report the in-orbit performance of the Gas Slit Camera (GSC) on the MAXI (Monitor of All-sky X-ray Image) mission carried on the International Space Station (ISS). Its commissioning operation started on August 8, 2009, confirmed the basic performances of the effective area in the energy band of 2--30 keV, the spatial resolution of the slit-and-slat collimator and detector with 1.5 degree FWHM, the source visibility of 40-150 seconds for each scan cycle, and the sky coverage of 85% per 92-minute orbital period and 95% per day. The gas gains and read-out amplifier gains have been stable within 1%. The background rate is consistent with the past X-ray experiments operated at the similar low-earth orbit if its relation with the geomagnetic cutoff rigidity is extrapolated to the high latitude. We also present the status of the in-orbit operation and the calibration of the effective area and the energy response matrix using Crab-nebula data.

ENERGY-DEPENDENT HARMONIC RATIOS OF THE CYCLOTRON FEATURES OF X0331+53 IN THE 2004-2005 OUTBURST

We report on changes of the cyclotron resonance energies of the recurrent transient pulsar X0331+53 (V0332+53). All of the Rossi X-ray Timing Explorer data acquired in the 2004-2005 outburst were utilized. The 3-80 keV source luminosity varied between 1.7 × 1036 and 3.5 × 1038 erg s–1, assuming a distance of 7 kpc. We confirmed that the fundamental cyclotron resonance energy changed from ~22 to ~27 keV, in a clear anti-correlation with the source luminosity and without any hysteresis effects between the rising and declining phases of the outburst. In contrast, the second harmonic energy changed from ~49 to ~54 keV, implying a weaker fractional change as a function of the luminosity. As a result, the observed resonance energy ratio between the second harmonic and the fundamental was ~2.2 when the source was most luminous, whereas the ratio decreased to the nominal value of 2.0 at the least luminous state. Although the significance of this effect is model dependent, these results suggest that the fundamental and second harmonic resonances represent different heights in the accretion column, depending on the mass accretion rate.

THE 37 MONTH MAXI/GSC SOURCE CATALOG OF THE HIGH GALACTIC-LATITUDE SKY

We present a catalog of high Galactic-latitude (|b| > 10 Degree-Sign ) X-ray sources detected in the first 37 months of data of the Monitor of All-sky X-ray Image/Gas Slit Camera (MAXI/GSC). To achieve the best sensitivity, we develop a background model of the GSC that well reproduces the data based on the detailed on-board calibration. Source detection is performed through image fits with a Poisson likelihood algorithm. The catalog contains 500 objects detected with significances of s{sub D,4-10keV} {>=} 7 in the 4-10 keV band. The limiting sensitivity is Almost-Equal-To 7.5 Multiplication-Sign 10{sup -12} erg cm{sup -2} s{sup -1} ( Almost-Equal-To 0.6 mCrab) in the 4-10 keV band for 50% of the survey area, which is the highest ever achieved in an all-sky survey mission covering this energy band. We summarize the statistical properties of the catalog and results from cross matching with the Swift/BAT 70 month catalog, the meta-catalog of X-ray detected clusters of galaxies, and the MAXI/GSC 7 month catalog. Our catalog lists the source name (2MAXI), position and its error, detection significances and fluxes in the 4-10 keV and 3-4 keV bands, the hardness ratio, and the basic information of the likely counterpart available for 296morexa0» sources.«xa0less

POSSIBLE DETECTION OF AN EMISSION CYCLOTRON RESONANCE SCATTERING FEATURE FROM THE ACCRETION-POWERED PULSAR 4U 1626−67

We present analysis of 4U 1626-67, a 7.7 s pulsar in a low-mass X-ray binary system, observed with the hard X-ray detector of the Japanese X-ray satellite Suzaku in 2006 March for a net exposure of {approx}88 ks. The source was detected at an average 10-60 keV flux of {approx}4 Multiplication-Sign 10{sup -10} erg cm{sup -2} s{sup -1}. The phase-averaged spectrum is reproduced well by combining a negative and positive power-law times exponential cutoff (NPEX) model modified at {approx}37 keV by a cyclotron resonance scattering feature (CRSF). The phase-resolved analysis shows that the spectra at the bright phases are well fit by the NPEX with CRSF model. On the other hand, the spectrum in the dim phase lacks the NPEX high-energy cutoff component, and the CRSF can be reproduced by either an emission or an absorption profile. When fitting the dim phase spectrum with the NPEX plus Gaussian model, we find that the feature is better described in terms of an emission rather than an absorption profile. The statistical significance of this result, evaluated by means of an F test, is between 2.91 Multiplication-Sign 10{sup -3} and 1.53 Multiplication-Sign 10{sup -5}, taking into account the systematic errors in the backgroundmorexa0» evaluation of HXD-PIN. We find that the emission profile is more feasible than the absorption one for comparing the physical parameters in other phases. Therefore, we have possibly detected an emission line at the cyclotron resonance energy in the dim phase.«xa0less

Discovery of a Cyclotron Resonance Feature in the X-Ray Spectrum of GX 304−1 with RXTE and Suzaku during Outbursts Detected by MAXI in 2010

We report the discovery of a cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of GX 304-1, obtained by RXTE and Suzaku during major outbursts detected by MAXI in 2010. The peak intensity in August reached 600 mCrab in the 2-20 keV band, which is the highest ever observed from this source. The RXTE observations on more than twenty occasions and one Suzaku observation revealed a spectral absorption feature at around 54 keV, which is the first CRSF detection from this source. The estimated strength of surface magnetic field,

The First MAXI/GSC Catalog in the High Galactic-Latitude Sky

4.7 times 10^{12}

Revisit of Local X-Ray Luminosity Function of Active Galactic Nuclei with the MAXI Extragalactic Survey

G, is one of the highest among binary X-ray pulsars from which CRSFs have ever been detected. The RXTE spectra taken during the August outburst also suggest that the CRSF energy changed over 50-54 keV, possibly in a positive correlation with the X-ray flux. The behavior is qualitatively similar to that observed from Her X-1 on long time scales, or from A 0535+26, but different from the negative correlation observed from 4U 0115+63 and X 0331+53.

Spectral Evolution of a New X-Ray Transient MAXI J0556−332 Observed by MAXI, Swift, and RXTE

AbstractWe present the first unbiased source catalog of the Monitor of All-sky X-ray Image (MAXI) mission athigh Galactic latitudes (|b|>10 ◦ ), produced from the first 7-month data (2009 September 1 to 2010 March31) of the Gas Slit Camera in the 4–10 keV band. We develop an analysis procedure to detect faint sourcesfrom the MAXI data, utilizing a maximum likelihood image fitting method, where the image response,background, and detailed observational conditions are taken into account. The catalog consists of 143X-ray sources above 7 sigma significance level with a limiting sensitivity of ∼ 1.5×10 −11 ergs cm −2 s −1 (1.2 mCrab) in the 4–10 keV band. Among them, we identify 38 Galactic/LMC/SMC objects, 48 galaxyclusters, 39 Seyfert galaxies, 12 blazars, and 1 galaxy. Other 4 sources are confused with multiple objects,and one remains unidentified. The log N - log S relation of extragalactic objects is in a good agreementwith the HEAO-1 A-2 result, although the list of the brightest AGNs in the entire sky has significantlychanged since that in 30 years ago.Key words:catalogs — surveys — galaxies: active — X-rays: galaxies1. INTRODUCTIONAll-sky X-ray surveys are powerful tools to investigatethe whole populations of active and hot phenomena inthe universe at the brightest flux end. The strong X-ray emitters include Galactic objects such as active stars,SNRs, pulsars, CVs, low mass and high mass X-ray bi-naries (with a neutron star or a black hole as the pri-mary), and extragalactic objects, mainly active galacticnuclei (AGNs; Seyfert galaxies and blazars) and clustersof galaxies. The source catalog consisting of a statisticallywell-defined sample detected from an unbiased survey isa primary product on which many subsequent studies arebased. For extragalactic populations, in particular, theseresults define the “local” sample in the present universe,the end point of their cosmological evolution. Thus, toestablish the statistical properties of bright X-ray sourcesusing the best quality data over the entire sky has alwaysbeen a key issue in high energy astrophysics.Past all-sky X-ray surveys indeed brought valuable in-formation on the X-ray source populations. In the softX-ray band, the ROSAT mission conducted an all-sky sur-vey in the 0.1–2.4 keV band, producing the ROSAT All-Sky Survey (RASS) Bright Source Catalog (BSC; Voges