High resolution spectroscopy of X-ray emission from high mass X-ray binaries

This article briefly reviews first the progress of spectroscopy in X-ray astronomy from proportional counters, a major instrument in early phase of X-ray astronomy, to gas scintillation proportional counters, X-ray CCD cameras, transmission and reflection gratings, and finally to X-ray micro-calorimeters. As a typical example of spectral features observed from high mass X-ray binaries (HMXBs), the spectra observed from Vela X-1 with Chandra grating spectrometers are then presented and compared with computer simulations for high mass binary systems.     

Rapid short-term pulse-period variations in Vela X-1 (4U 0900-40)

Results are presented of an analysis of 83 days of 2–12 keV X-ray observations of Vela X-1 (4U 0900-40) obtained during three separate pointings with the ESA COS-B satellite. The pulsation period is shown to undergo very rapid intrinsic changes, at a rate of up to during intervals of a few days. The lower values of which were previously observed over longer intervals, appear to result from an averaging-out of these rapid changes. It is argued, that the transfer of angular momentum to the neutron star by the accreting matter is very unlikely to be sufficient to explain these pulsation-period changes. An alternative explanation is suggested.     

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.     

Broad-band spectral properties of accreting X-ray binary pulsars

Broad-band spectra of accreting X-ray binary pulsars can be fitted by a phenomenological model composed of a power law with a high energy rollover above 10 keV, plus a blackbody component with a temperature of few hundred eV. While, at least qualitatively, the hard tail can be explained in terms of (inverse) Compton scattering, the origin of the soft component cannot find a unique explanation. Recently, a qualitative picture able to explain the overall broad-band spectrum of luminous X-ray pulsars was carried out by taking into account the effect of bulk Comptonization in the accretion column. After a review of these recent theoretical developments, I will present a case study of how different modeling of the continuum affect broad features, in particular the cyclotron resonance features in Vela X-1.     

Hard X-ray focusing optics up to 80 keV for the future missions

X-ray telescopes have been providing high sensitivity X-ray observations in numerous missions. For X-ray telescopes in the future, one of the key technologies is to expand the energy band beyond 10 keV. We designed depth-graded multilayer, so-called supermirrors, for a hard X-ray telescope in the energy band up to 40 keV using lightweight thin-foil optics. They were successfully flown in a balloon flight and obtained a hard X-ray image of Cyg X-1 in the 20–40 keV band. Now supermirrors are promising to realize a hard X-ray telescope. We have estimated the performance of a hard X-ray telescope using a platinum–carbon supermirror for future satellite missions, such as NeXT (Japan) and XEUS (Europe). According to calculations, they will have a significant effective area up to 80 keV, and their effective areas will be more than 280 cm² even at 60 keV. Limiting sensitivity will be down to 1.7 × 10⁻¹³ erg cm⁻² s⁻¹ in the 10–80 keV band at a 100 ks observation. In this paper, we present the results of the balloon experiment with the first supermirror flown and projected effective areas of hard X-ray telescopes and action items for future missions.     

Very high energy gamma-ray sources

A brief review of very high energy gamma-ray astronomy achievements is presented. The results of observations of the Crab nebula, the Cygnus X-3, Vela pulsar, radiogalaxy Centaurus A and 2CG 195+4 show that all these objects are the sources of very high energy gamma-quanta. The most powerfull source is Cygnus X-3. Its gamma-ray luminosity is no less than 3·10³⁷ erg.s⁻¹. The upper limit of quanta energy is no less than 10¹⁶eV. All sources are variable in different time scales, from milliseconds up to years. The nature of all these sources is not known yet, but known ones are pulsars.     

Cyclotron features in X-ray spectra of accreting pulsars

The hard X-ray spectra of small subset of accreting pulsars show absorption-like line features in the range 10–100 keV. These lines, referred to as cyclotron lines or cyclotron resonance scattering features, are due to photons scattered out of the line of sight by electrons trapped in the 10¹² G pulsar polar cap magnetic field. In this paper we present a review of observations, from the discovery of a cyclotron line in Hercules X-1 to recent results with RXTE and INTEGRAL.     

Balloon altitude studies of southern hemisphere hard X-ray sources by the Imperial College-University of Tasmania-INPE collaboration

Hard X-ray balloon altitude measurements with a 1600 cm² phoswich array are described. Data from observations on Sco X-1, GX1+4, GX5−1, Nova Oph. 1977, SMC X-1, SS433, IC 4329A and MR 2251-178 are presented. The role of Comptonisation in X-ray production for Sco X-1 and GX1+4 is discussed.     

XMM-Newton observation of Vela shrapnel D

The ROSAT all-sky survey discovered several ‘shrapnels’, showing boomerang structures outside the Vela SNR. We observed the shrapnel D in the eastern limb of the Vela SNR with the XMM-Newton satellite. There is an X-ray bright ridge structure in our FOV running from north to south. There is also an optical bright ridge structure running parallel to that in X-ray about 3′ in the east. These suggest that the shrapnel D is now interacting with an interstellar cloud. Applying the VNEI model to X-ray spectra of various regions, we find that the plasma state of the eastern region of the X-ray ridge is significantly different from that of the western region. The X-ray spectra in the western region can be well fitted with a single temperature component. Abundances of heavy elements are almost uniform, whereas they are heavily overabundant (except Fe): the relative abundances to the solar values are O ∼5, Ne ∼8, Mg ∼8, Fe ∼1. It must have originated from the ejecta of the SN. We find that the plasma in the eastern part of the ridge consists of two temperature components: one component (hot plasma) representing the ejecta while the other (cold plasma) represents the interstellar cloud or swept-up ISM. These two components must have contacted to each other, indicating to form a contact discontinuity. Around northern part of the contact discontinuity, we found wavelike structures of which the typical scales are comparable with that of the Rayleigh–Taylor instability.     

Cyg X-2 — An accreting neutron star

HAKUCHO observation of Cyg X-2 over 40 days did not show a correlation between the hardness ratio and the intensity predicted for dwarf X-ray sources. The energy spectrum in the range 0.3 – 20 keV was found to deviate from the thermal bremsstrahlung spectrum below 2 keV. The X-ray spectrum can be accounted for in terms of the comptonization of blackbody radiation emitted from teh neutron star surface and the accretion disk.     

GMRT monitoring of Cyg X-1 and Cyg X-3

We present the results of low frequency radio observations of the X-ray binaries, Cygnus X-1 and Cygnus X-3, during different X-ray states. The low frequency observations were made for the first time for these sources at 0.61 and 1.28 GHz using the Giant Meter-wave Radio Telescope (GMRT) between 2003 and 2004. Both Cyg X-1 and Cyg X-3 are highly variable at low radio frequencies. We also compare our data with the observations at 15 GHz conducted by the Ryle telescope. Spectral turnover is seen for both the sources below 2 GHz. The data suggest that the change in the radio flux density in both the sources is correlated to the X-ray hardness ratio and follows a similar behavior pattern.     

Is cygnus X-3 a low-mass X-ray binary?

We present examples of the quasi-periodic variations in the X-ray flux of Cyg X-3 which we have recently found during observations of this source with EXOSAT. Amplitudes and periods of the variations range from 5% to 20% of the total flux and from 50 to 1500 s, respectively. Our tentative interpretation of these quasi-periodicities, the occurrence of quasi-periodic phenomena in an accretion disk which partially occults the X-ray source, points towards an analogy of Cyg X-3 with certain ‘dipping’ low-mass X-ray binaries such as 4U 1822-37, as suggested /1/ previously. We point out, however, that there are also fundamental differences between Cyg X-3 and this type of low-mass X-ray binary.     

In-orbit operation and performance of the CubeSat Soft X-ray polarimeter PolarLight

PolarLight is a compact soft X-ray polarimeter onboard a CubeSat, which was launched into a low-Earth orbit on October 29, 2018. In March 2019, PolarLight started full operation, and since then, regular observations with the Crab nebula, Sco X-1, and background regions have been conducted. Here we report the operation, calibration, and performance of PolarLight in the orbit. Based on these, we discuss how one can run a low-cost, shared CubeSat for space astronomy, and how CubeSats can play a role in modern space astronomy for technical demonstration, science observations, and student training.     

Radio continuum observations of the galactic supernova remnant Vela Z (G266.2-1.2)

We present multi frequency radio-continuum observations of Galactic SNR Vela Z (G266.2-1.2), associated with the prominent X-ray source RX J0852.0-4622 discovered in 1998. Our study of this Galactic supernova remnant (SNR) is based on low-resolution mosaic observations with the ATCA radio interferometer at 1384 and 2496 MHz, Parkes 4850 MHz and MOST 843 MHz survey data. We determine the radio spectral index for the most prominent features of this SNR and compare these features with X-ray observations. We note a sudden spectral turnover at higher then expected frequencies of ∼1384 MHz.     

The X-ray properties of accreting black holes

Accreting black holes in binary systems exhibit two properties that distinguish them from the vast majority of binary X-ray sources: (1) rapid ≤ 1 s variability and (2) bi-modal spectral behaviour. These findings are based primarily on the observed properties of Cyg X-1 where an estimate of the X-ray source mass from radial velocity studies indicates a mass of 10 M. The recently suggested black hole candidacy of LMC X-3 is based on a similar mass estimate. The X-ray properties are similar to those of Cyg X-1 in its “high state”. The unique spectral properties of these systems are used to search for other similar systems, and new “possible” candidates are suggested, that include several transient sources. A possible spectral connection between these systems and their more massive counterparts in AGN is discussed.     

X-ray emission from supernova remnants observed with ROSAT

Spectrally resolved X-ray images of galactic supernova remnants have been obtained both from the ROSAT all-sky survey and a number of pointed observations. There is substantial evidence for significant spatial variation in temperature, density and pressure across the older, thermal remnants like the Vela SNR, the Cygnus Loop and the North Polar Spur. Both the brightness distribution and the pressure variations observed point towards recently shocked interstellar clouds and filaments, which dominate the X-ray emission pattern. Across the Puppis-A SNR an arc-shaped absorption structure has been detected, which is demonstrated to be produced by cold gas located close to the shock front of the Vela SNR. Across IC443 a similar absorption pattern has been observed, which is created by a cold shell associated with a previously unknown, ROSAT discovered SNR, which lies in front of IC443. Finally, a statistically overview of the SNRs detected in the ROSAT all-sky survey is presented. About half of the catalogued radio remnants have been observed in the survey and another 90 sources have been found which are considered to be candidates of new SNRs.     

Multiband echo tomography of Sco X-1

We present preliminary results of a simultaneous X-ray/optical campaign of the prototypical LMXB Sco X-1 at 1–10 Hz time resolution. Lightcurves of the high excitation Bowen/HeII emission lines and a red continuum at λ_c  6000 Å were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with simultaneous RXTE X-ray lightcurves. We find evidence for correlated variability, in particular when Sco X-1 enters the Flaring branch. The Bowen/HeII lightcurves lag the X-ray lightcurves with a light travel time which is consistent with reprocessing in the companion star while the continuum lightcurves have shorter delays consistent with reprocessing in the accretion disc.     

Optical and X-ray observations of 4U2129+47/V1727 CYG in a quiescent state

We observed the 5.2 h X-ray binary 4U2129+47 for more than one orbital cycle on 29 September and 4 October 1983 using the LE, ME and GSPC detectors of the ESA X-ray satellite EXOSAT. In neither detector an X-ray flux from the source could be detected. Quasi-simultaneous UBV observations using the 2.2 m telescop at the German-Spanish Astronomical Center, Calar Alto, failed to detect the large amplitude light curve reported in earlier observations but show the optical companion in a low intensity state. The large amplitude light curve has been interpreted as due to X-ray heating of the optical star by the X-ray source similar to the system Her X-1/HZ Her. Our optical observations indicate that the heating X-ray source has been shut off and nicely explain that EXOSAT failed to detect the source. 4U2129+47 seems to behave similar to Her X-1 in the optical quiescent state, seen for instance from 1949 to 1956.     

Hard X-ray observations of Cygnus X-1 with the MISO telescope

The black hole candidate Cygnus X-1 was observed in the hard X ray - soft γ ray energy range by the MISO telescope on two different occasions: in September 1979 and May 1980. We have measured two hard X-ray states of the source: in 1979 the observed spectrum confirms the superlow state measured in the same period by the HEAO-3 satellite, while in 1980 the MISO X-ray data are consistent with the so called low state of Cygnus X-1. In both occasions, no γ -ray excess has been observed above 200 KeV.     

SIGMA observations of hard X-ray and soft gamma-ray emission from X-ray binaries

After more than two years of operation, the imaging γ-ray SIGMA telescope has accumulated several days of observation toward well known X-ray binaries. Four bright sources falling in this category have been detected so far: The pulsar GX 1+4 near the center of our galaxy, the stellar wind accreting system 4U 1700-377, and the black hole candidates Cygnus X-1 and GX 339-4. Moreover, SIGMA have observed three transients sources, which turned out to be also hard X-ray sources : The burster KS 1731-260, Tra X-1, and the Musca Nova. The properties of these systems in the SIGMA domain will be reviewed and a spectral distinction between black holes and neutron stars will be sketched.