X-ray timing beyond the Rossi X-ray Timing Explorer

With its ability to look at bright galactic X-ray sources with sub-millisecond time resolution, the Rossi X-ray Timing Explorer (RXTE) discovered that the X-ray emission from accreting compact stars shows quasi-periodic oscillations on the dynamical timescales of the strong field region. RXTE showed also that waveform fitting of the oscillations resulting from hot spots at the surface of rapidly rotating neutron stars constrain their masses and radii. These two breakthroughs suddenly opened up a new window on fundamental physics, by providing new insights on strong gravity and dense matter. Building upon the RXTE legacy, in the Cosmic Vision exercise, testing General Relativity in the strong field limit and constraining the equation of state of dense matter were recognized recently as key goals to be pursued in the ESA science program for the years 2015–2025. This in turn identified the need for a large (10 m² class) aperture X-ray observatory. In recognition of this need, the XEUS mission concept which has evolved into a single launch L2 formation flying mission will have a fast timing instrument in the focal plane. In this paper, I will outline the unique science that will be addressed with fast X-ray timing on XEUS.     

Challenges in X-ray binary timing: current and future

Millisecond X-ray time variability studies of accreting low-magnetic-field neutron stars and stellar-mass black holes in X-ray binaries probe the motion of matter in regions of strong gravity. In these regions, general relativity (GR) is no longer a small correction to the classical laws of motion, but instead dominates the dynamics: we are studying motion in strongly curved spacetime. Such millisecond X-ray variability studies can therefore provide unique tests of GR in the strong-field regime. The same studies also constrain neutron-star parameters such as stellar mass and radius, and thereby the equation of state (EOS) of ultradense matter. I briefly review the status, and discuss the prospects for mapping out space-time near accreting stellar-mass compact objects, and measuring the EOS of dense matter, through millisecond timing, particularly with an eye towards future missions. The overwhelming consideration for timing sensitivity is collecting area: contrary to most applications, the signal-to-noise ratio for the aperiodic timing phenomena produced by accretion flows increases proportionally with count rate rather than as the square root of it. A 10 times larger instrument turns 1σ effects into 10σ effects (or does as well in 1% of the time). With the Rossi X-ray Timing Explorer (RXTE), using 0.6 m² collecting area, we have found several timing diagnostics from the accretion flow in the strong field region around neutron stars and black holes, as well as signals from neutron star surface hot spots. Combined work between RXTE and the new sensitive X-ray spectrographs onboard Chandra and XMM can already begin to clinch the geometry and physical mechanisms underlying these signals. Future instruments, larger in area by an order of magnitude and in some cases with enhanced spectral capabilities, are expected to turn these diagnostics of GR into true tests of GR. They are also expected to put strong constraints on neutron-star structure, and thereby on the EOS of supranuclear density matter.     

Progress of Solar Wind Magnetosphere Ionosphere Link Explorer(SMILE) Mission

SMILE (Solar wind Magnetosphere Ionosphere Link Explorer) mission is a joint ESA-CAS space science project. The working orbit is a 19 R_e 5000 km HEO with 4 scientific instruments:Soft X-ray Imager(SXI), Ultra-Violet Imager (UVI), Magnetometer (MAG) and Light Ion Analyzer (LIA). SMILE aims to understand the interaction between the solar wind and the Earth's magnetosphere through the images of SXI and UVI and in-situ measurement from LIA and MAG. After the kick-off in 2016, the SMILE project went to Phase A study. The mission adoption is scheduled for November 2018, with a target launch date in 2022-2023. In this paper, the background of the mission, scientific objectives, the design and characteristics of scientific instruments and the mission outline will be introduced in details.      

A broad spectral band Indian Astronomy satellite ‘Astrosat’

Astrosat will be the first full-fledged Indian Astronomy mission aimed at multiwavelength studies in the optical, near- and far-UV and a broad X-ray spectral band covering 0.5–100 keV. This mission will have the capability of high time-resolution X-ray studies (10 μs timing), low and medium energy-resolution spectral studies and high angular-resolution (about 2″) imaging observations in the UV and optical bands simultaneously. This is realized by using a set of three co-aligned X-ray astronomy instruments and one UV imaging telescope consisting of two similar instruments. Detection and timing studies of X-ray transients and persisting sources will be done by a Scanning Sky X-ray Monitor. This mission will enable studies of different classes of galactic and extragalactic sources in the frontier area of high energy astronomy. Scientific objectives of the mission are highlighted in this paper. A brief summary of the design and characteristics of the X-ray and UV instruments and their expected sensitivities are presented.     

The Scanning Sky Monitor (SSM) on ASTROSAT

The Scanning Sky Monitor is one of the experiments onboard the ASTROSAT, an Indian multiwavelength astronomy satellite mission. This experiment will detect and monitor X-ray transients in the energy band 2–10 keV. It is similar in design to the ASM on RXTE. It consists of position-sensitive proportional counters with one-dimensional mask. We describe the configuration of the experiment. We also discuss some of the results obtained using a detector which has already been fabricated and tested in our laboratory.     

WSO-UV project

During last three decades, astronomers have enjoyed continuous access to the 100–300 nm ultraviolet (UV) spectral range where the resonance transitions of the most abundant atoms and ions (at temperatures between 3000 and 300 000 K) reside. This UV range is not accessible from ground-based facilities. The successful International Ultraviolet Explorer (IUE) observatory, the Russian ASTRON mission and successor instruments such as the Galaxy Evolution Explorer (GALEX) mission or the COS and STIS spectrographs on-board the Hubble Space Telescope (HST) prove the major impact of observations in the UV wavelength range in modern astronomy. Future access to space-based observatories is expected to be very limited. For the next decade, the post-HST era, the World Space Observatory – Ultraviolet (WSO–UV) will be the only 2-m class UV telescope with capabilities similar to the HST. WSO–UV will be equipped with instruments for imaging and spectroscopy and it will be a facility dedicated, full-time, to UV astronomy. In this article, we briefly outline the current status of the WSO–UV mission and the science management plan.     

Orbital phase spectroscopy of X-ray pulsars to study the stellar wind of the companion

High Mass X-ray Binary Pulsars (HMXBP), in which the companion star is a source of supersonic stellar wind, provide a laboratory to probe the velocity and density profile of such winds. Here, we have measured the variation of the absorption column density along with other spectral parameters over the binary orbit for two HMXBP in elliptical orbits, as observed with the Rossi X-ray Timing Explorer (RXTE) and the BeppoSAX satellites. A spherically symmetric wind profile was used as a model to compare the observed column density variations. In 4U 1538-52, we find the model corroborating the observations; whereas in GX 301-2, the stellar wind appears to be very clumpy and a smooth symmetric wind model seems to be inadequate in explaining the variation in column density.     

Long-term periodicities in the flux from low mass X-ray binaries

Using data from the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE), we have searched for long term periodicities in the X-ray flux of GX 1+4, Sco X-2 (GX 349+2), and GX 339−4. For GX 1+4, we also used data from BATSE and Galactic Centre scans performed by RXTE. We find no evidence for X-ray modulations at the suggested ∼304 d orbital period of GX 1+4. However, we find tentative evidence for a periodicity at 420–460 d. An upper limit of 15% peak-to-peak is set on any sinusoidal modulation in the 1.5–3.0 keV flux of Sco X-2 for periods in the 30–100 d range. For GX 339−4, we confirm the Low State modulation and report the detection of significant low-frequency modulations in both the High State and Very High State. We fail to detect this modulation in the Off State. We show that if the reported orbital period of GX 339−4 lies in the range 0.5–1.7 d, then it is not present in the RXTE ASM light curve.     

X-ray timing explorer

The payload for the U.S. X-ray Timing Explorer is currently being selected by NASA. Some of the possible instrumental capabilities and scientific objectives of the mission are described.     

Orbital phase spectroscopy of four high mass X-ray binary pulsars to study the stellar wind of the companion

Our work focuses on a comprehensive orbital phase-dependent spectroscopy of the four High Mass X-ray Binary Pulsars (HMXBPs) 4U 1538-52, GX 301-2, OAO 1657-415 and Vela X-1. We hereby report the measurements of the variation of the absorption column density and iron-line flux along with other spectral parameters over the binary orbit for the above-mentioned HMXBPs in elliptical orbits, as observed with the Rossi X-ray Timing Explorer (RXTE) and the BeppoSAX satellites. A spherically symmetric wind profile was used as a model to compare the observed column density variations. Out of the four pulsars, only in 4U 1538-52, we find the model having a reasonable corroboration with the observations, whereas in the remaining three the stellar wind seems to be clumpy and a smooth symmetric stellar wind model appears to be quite inadequate in explaining the data. Moreover, in GX 301-2, neither the presence of a disk nor a gas stream from the companion was validated. Furthermore, the spectral results obtained in the case of OAO 1657-415 and Vela X-1 were more or less similar to that of GX 301-2.     

Two years of INTEGRAL result on X-ray binaries

INTEGRAL is the ESA lead International Gamma-Ray Astrophysical Laboratory, successfully launched the 17th October 2002 from Baikonur with a Proton vehicle. In view of the high sensitivity of the two γ-ray instruments IBIS and SPI and their capability to provide at the same time image, spectra and time profiles of all the sources in their wide field of view, a key project was approved as “Core Programme” to obtain deep observations of the Galactic Centre (GCDE) and to exploit regular scan of the whole Galaxy Plane since the beginning of the mission. This paper will briefly review the main astrophysical results obtained in the field of high energy Galactic sources with the INTEGRAL/IBIS γ-ray Imager onboard INTEGRAL, and make a comparison with the previous scenario depicted by the BeppoSAX and RXTE results.     

The Japanese lunar mission SELENE: Science goals and present status

The Japanese lunar mission SELENE (SELenological and ENgineering Explorer) has been in development to target launch scheduled 2007 summer by H-IIA rocket. The SELENE is starting final integration test after SAR (System Acceptance Review), SRR (System Reliability Review) and instrument environment test. The SELENE is a remote-sensing mission orbiting 100 km altitude of the Moon for nominal one year and extended some months to collect the data for studying the origin and evolution of the Moon. Fourteen instruments and experiment systems are preparing for studies of the Moon, in the Moon, and from the Moon; global element and mineral compositions, topological structure, gravity field of whole moon, and electromagnetic and particle environment of the Moon. The new data center SOAC (SELENE Operation and data Analysis Center) are completed to construct in JAXA Sagamihara campus, and end-to-end test will be carried out between SOAC and data downlink stations.     

Update on the ESA-CAS Joint Solar Wind Magnetosphere Ionosphere Link Explorer (SMILE) Mission

The SMILE (Solar wind Magnetosphere Ionosphere Link Explorer) mission aims at deepening our understanding of the interaction of the solar wind with the Earth magnetosphere. It is the first time that ESA and CAS jointly select, design, implement, launch, and operate a space mission. The mission was adopted by CAS in November 2016 and by ESA in March 2019 with a target launch date by the end of 2023.      

Orbital evolution studies of two HMXB pulsars

High mass X-ray binary (HMXB) pulsars are of two types, persistent and transient. 4U1538−52 is a persistent HMXB whose orbit was previously measured to be circular but the RXTE observations revealed an eccentric orbit. We observed this system with RXTE-PCA in August 2003 and our timing analysis supports the eccentric orbit of the system. However, we do not find any evidence for orbital evolution.

Rotational and tidal interactions between the stars of a closed binary system result in apsidal motion which can be measured in systems with eccentric orbit. 4U0115+63 is a Be-transient HMXB whose eccentric orbit was well-determined during its 1978 outburst. We report preliminary results from analysis of data obtained during the 1999 outburst of this source with the RXTE-PCA.     

Future X-ray astronomy missions of Japan

Japanese future space programs for high energy astrophysics are presented. The Astro-E2 mission which is the recovery mission of the lost Astro-E has been approved and now scheduled to be put in orbit in early 2005. The design of the whole spacecraft remains the same as that of Astro-E, except for some improvements in the scientific instruments. In spite of the five years of delay, Astro-E2 is still powerful and timely X-ray mission, because of the high energy resolution spectroscopy (FWHM 6 eV in 0.3–10 keV) and high-sensitivity wide-band spectroscopy (0.3–600 keV). The NeXT (New X-ray Telescope) mission, which we propose to have around 2010, succeeds and extends the science which Astro-E2 will open. It will carry five or six sets of X-ray telescopes which utilize super-mirror technology to enable hard X-ray imaging up to 60–80 keV. In mid-2010s, we would participate in the European XEUS mission, which explores the early (z>5) “hot” universe.     

Two years of INTEGRAL observation of the BHC IGR J17464-3213

On March 2003, IBIS, the γ-ray imager on board the INTEGRAL satellite, detected an outburst from a new source, IGR J17464-3213, that turned out to be an HEAO-1 transient, namely H1743-322. The spectral and temporal evolutions of the source were observed by INTEGRAL in different periods. Also RXTE observed the source for the first time on 2003 March 29 during a PCA Galactic bulge scan. The source flux decayed below the RXTE PCA sensitivity limit in November 2003, then in April 2004 it was again detected by INTEGRAL. On July 3, 2004 the source was again detected by RXTE/PCA at a 2–10 keV intensity of 16 mCrab and on July 7, reached 69 mCrab. Recently, a new outburst was observed on August 2005. We briefly summarise here the behaviour of the source observed by INTEGRAL from March 2003 to August 2005. The new outbursts of the source and the analysis of all the data collected (now public) give a global view of the spectral and time behaviour of this X-ray transient.     

ASTROSAT

The ASTROSAT satellite is an Indian National Space Observatory under development in India. Due for launch in 2010, ASTROSAT will carry a complement of five scientific instruments enabling simultaneous observations from the optical through to the hard X-ray energy band. This capability will enable broad-band spectroscopy and high time-resolution monitoring of both galactic and extra-galactic targets, such as X-ray binaries and AGN. One of the instruments is being built in collaboration with the Canadian Space Agency and another in collaboration with the University of Leicester. ASTROSAT also carries a scanning sky monitor to observe the variable X-ray sky. After an initial period of science verification and guaranteed time, a certain fraction of ASTROSAT observing time will also be made available to the community via a call for proposals. Here I summarise the instrument complement and principle scientific objectives of the mission.     

Preliminary observing achievements of supersoft X-ray detector and γ-ray detector onboard Shenzhou2

Preliminary observing achievements by the Super Soft X-ray Detector and the γ-ray Detector in the fields of cosmic gamma-ray bursts, solar X-ray, bursts and cosmic X-ray/γ-ray background radiation are summarized. The detectors are aboard the spacecraft Shenzhou-2 that was launched on 2001 January 10. The scientific mission and general situation of the instruments are briefly described.     

Progress of Double Star Program

The Double Star Programme (DSP) is the first joint space mission between China and ESA. The mission, which is made of two spacecraft, is designed to investigate the magnetospheric global processes and their response to the interplanetary disturbances in conjunction with the Cluster mission. The first spacecraft, TC-1 (Tan Ce means "Explorer"), was launched on 30 December 2003, and the second one, TC-2, on 25 July 2004 on board two Chinese Long March 2C. Due to the importance of and success of DSP, both CNSA and ESA approved the extension of DSP. This paper presents DSP mission and some important scientific results made based on the data of DSP.     

The coronal dynamics imagers for the KuaFu mission

The Space Weather Explorer – KuaFu mission will provide simultaneous, long-term, and synoptic observations of the complete chain of disturbances from the solar atmosphere to the geospace. KuaFu-A (located at the L1 liberation point) includes Coronal Dynamics Imagers composed of a Lyman-α coronagraph (from 1.15 to 2.7 solar radii) and a white light coronagraph (out to 15 solar radii), in order to identify the initial sources of Coronal Mass Ejections (CMEs) and their acceleration profiles. The difficulty of observing the lower corona should not be underestimated since instrumental stray light remains a critical issue in the visible because of the low contrast of the corona with respect to the Sun. Observing the corona in the Lyman-α line is a valid alternative to white light observations. This approach takes advantage of both the intrinsic higher contrast of the corona with respect to the solar disk in this line compared to the visible, and the absence of F-corona at 121.6 nm. Furthermore, it has been convincingly shown that the coronal structures seen in Lyman-α correspond to those seen in the visible and which result from Thomson scattering of the coronal ionized gas. This is because the plasma is still collisional in the lower corona so that the hydrogen neutral atoms are coupled to the protons. A classical, all-reflecting internally-occulted Lyot coronagraph is required so as to preserve the image quality down to the inner limit of the field-of-view. A narrow band interference filter located in a collimated beam allows isolating the Lyman-α line. The visible coronagraph will adopt the approach of a single instrument having a large field-of-view extending from 2.5 to 15 solar radii. Such a design is based on refractive externally-occulted coronagraphs built for recent past missions, essentially the LASCO-C2 and C3 instruments and the SECCHI/COR 2 of the STEREO mission, which is itself a combination of the C2 and C3 instruments.