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.     

Time dependent accretion onto a black hole in a disk plus corona system

Hard X-ray lightcurves exhibit delays of 1 s with respect to the soft X-ray lightcurves when the microquasar GRS 1915+105 is in the state of frequent, regular outbursts. Such outbursts are supposed to be driven by the radiation pressure instability of the inner disk parts. The hard X-ray delays are then caused by the time needed for the adjustment of the corona to changing conditions in the underlying disk. We support this claim by the computation of the time evolution of the disk, including a non-stationary evaporation of the disk and mass exchange with the corona.     

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.     

Hard X-ray component in Sco X-1 spectrum: Synchrotron emission from a nano quasar

Sco X-1 is a low mass X-ray binary system and with the recent observations of a resolved radio jet, the source has been included in the list of galactic microquasars. The observed spectral data in the 2–20 keV energy band fits a thermal emission. Above 20 keV, a hard tail has been reported on occasions. During our continuing balloon borne X-ray survey in the 20–200 keV region using high sensitivity Large Area Scintillation counter Experiment, Sco X-1 was observed on two different occasions. Even though the total X-ray luminosity of the source was different, the spectral nature of the source did not show any variation. The presence of hard X-ray flux is unmistakable. We present the spectral and temporal data in the hard X-ray band and discuss the results in terms of geometrical characteristics of X-ray source and its observed temporal properties. We note that the jet activity is similar to the microquasars, however, the absence of the large magnitude abrupt changes in X-ray light curve compared to GRS1915 + 105 suggest that the quasar-like behaviour is at a nano scale.     

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.     

Possible radio JETS in GX 1+4

GX 1+4 is an X-ray pulsar with a nominal period of 2 or 4 min. and has the highest of 0.22 yr⁻¹ among the X-ray pulsars. The source is identified with an optical counterpart V2116 Oph, however no binary period has been reported so far. There are several features of the source like , variable pulse shape, period history and the spectral changes which do not fit well with our present understanding of the source.

The radio observation of the galactic center region were made using Molonglo observatory Synthesis Telescope operating at a frequency of 843 MHz. A 12 hr observation was made with the field center at the position coordinates of the optical counterpart. No significant radio emission was seen corresponding to the center position.

The synthesis map however, showed the presence of two regions of radio enhancements almost equidistant from the optical source in the diametrically opposite directions/1/. The data is interpreted as due to synchrotron emission produced by the expanding jets of GX 1+4, similar to the radio jets observed in galaxies. Radio jets have been inferred for three galactic X-ray binary sources namely, Sco X-1, Cyg X-3, and SS 433. However, for all these sources no X-ray pulsation has been detected so far. The present observations if confirmed will be the first case of radio jets in a stellar system containing an X-ray pulsar.     

Hard X-ray observation of galactic X-ray sources

A large (1455 cm²) hard X-ray telescope was successfully launched aboard a stratospheric balloon on October 4, 1980. During this flight four galactic X-ray sources were observed, namely the transient recurrent X-ray pulsar A0535+26, the Crab Nebula, Cygnus X-1 and X Persei. Here we report the results on the latter two sources. From Cygnus X-1 we measured a photon flux in the band 30 to 200 keV, of 3.5 × 10^?2 photons cm^?2 which is 6.5 times lower than that recieved from the source in a “low” intensity state in the same energy band. In addition, the photon spectrum in the same energy band was very soft and consistent with a power law with photon index α = 2.71 ± 0.14. Even if a simultaneous observation of the source at lower energies was not available, our data strongly suggest that we observed the source during a “high” intensity state. We report also positive detection in the band 30 to 200 keV of the low luminosity X-ray pulsar X Persei. In its spectrum we confirm the presence of a hard X-ray tail consistent with a power law (photon index α = 2.17 ± 0.42).     

A serendipitous survey for galaxy clusters by the XMM-Newton Survey Science Center

We describe the initial results of a programme to detect and identify extended X-ray sources found serendipitously in XMM-Newton observations. We have analyzed 186 EPIC-PN images at high galactic latitude with a limiting flux of 1 × 10⁻¹⁴ erg cm⁻² s⁻¹ and found 62 cluster candidates. Thanks to the enhanced sensitivity of the XMM-Newton telescopes, the new clusters found in this pilot study are on the average fainter, more compact, and more distant than those found in previous X-ray surveys. At our survey limit the surface density of clusters is about 5 deg⁻². We also present the first results of an optical follow-up programme aiming at the redshift measurement of a large sample of clusters. The results of this pilot study give a first glimpse on the potential of serendipitous cluster science with XMM-Newton based on real data. The largest, yet to be fulfilled promise is the identification of a large number of high-redshift clusters for cosmological studies up to z = 1 or 1.5.     

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.     

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.     

XMM-Newton observations of low luminosity Be/X-ray candidates

A small number of early Be stars exhibit X-ray luminosities intermediate between those typical of early type stars and those radiated by Be/X-ray binaries in the quiescent state. We report on XMM-Newton observations of two such Be stars, HD 161103 and SAO 49725 which were originally discovered in a systematic cross-correlation between the ROSAT all-sky survey and SIMBAD. The new observations confirm the X-ray luminosity detected by ROSAT (L_X  10³² erg s⁻¹) and the hardness of their X-ray spectra (thin thermal with kT  8–10 keV or power law with photon index of 1.7) which are both unusual for normal early type stars. We discuss the possible origin of this excess X-ray emission in the light of the models proposed for γ-Cas, magnetic disc-star interaction or accretion onto a compact companion object, neutron star or white dwarf, and compare the properties of these two sources with those of the new massive systems discovered in the XMM- Newton/SSC survey of the Galactic plane.     

Her X-1的食变现象和伴星HZ Her的大气结构

在密近双星系统Her X-1/HZ Her中,Her X-1被HZ Her掩食时的X射线光度变化,为研究HZ Her的大气结构,提供了一个直接的观测证据。本文对这个系统的二次食变过程资料进行了拟合分析,推断了HZ Her的低层大气结构,面对X射线源一面的大气,由于X射线的加热作用,其低部存在着一层温度约为8×104K的色球层。与已有的HZ Her大气结构理论相比较,大致符合,但有一定差异。本文简单讨论了这种差异。      

Energy spectra of non-pulsating low mass X-ray binaries

Detailed analyses of data obtained with gas scintillation proportional counter on Tenma huave been done for four low mass X-ray binaries (LMXB's); that is, Sco X-1, 4U1608-52, GX 349+2, and GX 5-1. This paper presents a new scenario concerning the spectra of LMXB's based on the observational fact from Tenma. The energy spectra of these sources can be expressed by a sum of two spectral components; that is, a hard component with a blackbody of temperature, kT ? 2 keV and soft component with a multi-color blackbody of maximum temperature kT ? 1.4 keV which is expected from the optically thick accretion disk.     

Non-LTE analysis of a new pulsating PG 1159-type central star detected in the ROSAT XRT all sky survey

We summarize the analysis of a new PG 1159 star, i.e., a hydrogen deficient pre-white dwarf detected in the ROSAT XRT all sky survey /1/. The X-ray source RX J2117.1+3412 is relatively faint (≈0.33 cnt s⁻¹) and was selected for optical identification on the basis of its extreme X-ray softness. With V = 13.2, the counterpart of the X-ray source is in the optical the second brightest member of the PG1159 class. CCD imagery reveals that the star is surrounded by an old highly excited planetary nebula of faint surface brightness. Optical line profiles of HeII, CIV, and OVI and the overall optical and ROSAT PSPC (0.1–2.4 keV) energy distributions are compared to predictions of NLTE model atmospheres. We find an excellent agreement between the atmospheric parameters derived from optical and soft X-ray data. The effective temperature of the star is extremely high (150,000 K). Abundances of He, C and O are found to be consistent with those derived in other PG1159 stars whereas the surface gravity is significantly smaller in RXJ2117.1+3412. New optical spectra presented here show the presence of ultra-high excitation lines of O VIII.     

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.     

Can a double component outflow explain the X-ray and optical lightcurves of Swift Gamma-Ray Bursts?

An increasing sample of Gamma-Ray Bursts (GRBs) observed by Swift show evidence of ‘chromatic breaks’, i.e. breaks that are present in the X-ray but not in the optical. We find that in a significant fraction of these GRB afterglows the X-ray and the optical emission cannot be produced by the same component. We propose that these afterglow lightcurves are the result of a two-component jet, in which both components undergo energy injection for the whole observation and the X-ray break is due to a jet break in the narrow outflow. Bursts with chromatic breaks also explain another surprising finding, the paucity of late achromatic breaks. We propose a model that may explain the behaviour of GRB emission in both X-ray and optical bands. This model can be a radical and noteworthy alternative to the current interpretation for the ‘canonical’ XRT and UVOT lightcurves, and it bears fundamental implications for GRB physics.     

LMXB luminosity function and connection to globular cluster in early type galaxies

We derive bias-corrected X-ray luminosity functions (XLFs) of LMXBs detected in 14 E and S0 galaxies observed with Chandra. After correcting for incompleteness, the individual XLFs are statistically consistent with a single power-law. A break at or near L_X,Eddington , as previously reported, is not required in any individual case. The combined XLF with a reduced error, however, suggests a possible break at L_X = 5 × 10³⁸ erg s⁻¹, which may be consistent with the Eddington luminosity of neutron stars with the largest possible mass (3 M), or of He-enriched neutron star binaries. We confirm that the total X-ray luminosity of LMXBs is correlated with the the near-IR luminosities, but the scatter exceeds that expected from measurement errors. The scatter in L_X(LMXB)/L_K appears to be correlated with the specific frequency of globular clusters, as reported earlier.

We cross-correlate X-ray binaries with globular clusters determined by ground-based optical and HST observations in 6 giant elliptical galaxies. With the largest sample reported so far (300 GC LMXBs with a 5:2 ratio between red and blue GCs), we compare their X-ray properties, such as X-ray hardness, XLF and L_X/L_B and find no statistically significance difference between different groups of LMXBs. Regardless of their association with GCs, both GC and field LMXBs appear to follow the radial profile of the optical halo light, rather than that of more extended GCs. This suggests that while metallicity is a primary factor in the formation of LMXBs in GCs, there may be a secondary factor (e.g., encounter rate) playing a non-negligible role.     

Improved orbital parameters of accreting millisecond pulsar SAX J1808.4-3658

We analyze the three outbursts of the X-ray millisecond pulsar SAX J1808.4-3658 that occurred in 1998, 2000, and 2002 observed with RXTE. With a technique based on epoch folding search we find an unique orbital solution valid over the five years of high temporal resolution data available. We revise the estimate of the orbital period, P_orb = 7249.1569(1) s and of its error, which we decrease by one order of magnitude. We also give the first constraint on the orbital period derivative, . We find that in 2002 the pulse profile shape is clearly asymmetric, showing a secondary peak at about 145° from the main pulse, which is different from the sinusoidal shape reported at the beginning of the 1998 outburst.     

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.