Indian X-ray Astronomy Experiment Research Articles

Journey through the birth, growth and maturity of X-ray astronomy

In this biographical account, I recall my scientific journey that began in 1961 when I joined as a Physics Trainee in the BARC Training School and after one year course was admitted in TIFR as a Research Associate. During 1962–1967, I worked on determination of the He3/He4 ratio in cosmic rays using a gas Cerenkov counter and also used a scintillation + solid Cerenkov detector to measure flux of protons and helium nuclei in primary cosmic rays. I also developed a large area scintillation-Cerenkov detector with a spark chamber between them for measuring charge composition of primary cosmic rays. I joined the X-ray astronomy programme in 1967 initiated by Prof. B. V. Sreekantan and played a leading role in developing a balloon borne hard X-ray instrument for studying time variability and energy spectra of cosmic X-ray sources. We carried out a series of balloon experiments and studied temporal intensity variation and energy spectra of X-ray objects like Sco X-1, Cyg X-1, etc. Results on rapid intensity changes and spectra appeared in a series of publications based on which I obtained a Ph.D. degree in 1972. Immediately afterwards, I joined Prof. Gordon Garmire at Caltech as a Research Fellow (a post-doctoral position) to work with him on the low energy component of HEAO A-2 experiment. The instrument was developed and successfully realised. Working on this satellite experiment was a thrilling experience which exposed me to the technical complexities of space instruments. The HEAO-1 satellite was launched in 1978. During 1978–1979, Guenter Riegler and I, working at JPL, reported detection of intense soft X-rays from several classes of extragalactic and Galactic objects like new BL Lacs, a new AM Her binary, coronal X-rays from active stars, etc. After returning to TIFR, I developed a rocket experiment with K. P. Singh to map the spatial and spectral distribution of the Soft Diffuse X-ray Background. At the same time, I also designed 15 cm deep Xenon filled Proportional Counters (XPCs) of ∼ 2500 cm2 area for balloon experiments which were done during 1983–1990 and produced new results on several X-ray binaries. During this period I also developed a proportional counter (PC) based satellite instrument and submitted a proposal jointly with the astronomy group at the Indian Space Research Organization (ISRO) for an Indian X-ray Astronomy Experiment (IXAE). After success of the Polar Satellite Launch Vehicle (PSLV), the IXAE instrument was included on the top deck of the IRS-P3 satellite that was launched in March 1996. Despite the severe limitation of a polar orbit, the IXAE observed a good number of X-ray binaries and produced interesting results, with the most notable being quasi-regular bursts from a new transient, GRS 1915+105, which turned out to be an enigmatic black hole source. Following success of the IXAE, a proposal for an ambitious multiwavelength astronomy satellite, named ASTROSAT, with broad spectral coverage in the optical, NUV, FUV, soft X-ray and hard X-ray by a suite of instruments, was submitted to ISRO and finally approved by the Govt. of India in 2004. Over the next 10 years, the development and fabrication of the five ASTROSAT instruments were accomplished and a PSLV launch on Sept. 28, 2015 placed ASTROSAT in a 650 km circular orbit with 6 degree inclination. The ASTROSAT instruments have been performing well for ∼ 4 years and have observed more than 500 cosmic sources. Results from ASTROSAT have appeared in a large number of journal publications. Successful realization and performance of ASTROSAT marks culmination of my research career. I formally retired as a Senior Professor from TIFR on 2006 April 30 on attaining the age of superannuation. However by agreement with TIFR, ISRO designated me ‘ISRO Chair Professor’, which was later renamed as ISRO Satish Dhawan Professor, and asked me to continue to lead ASTROSAT as PI. At the end of my tenure as ISRO Professor on 2011 April 30, I relinquished the PI position. I am at present affiliated with the MU-DAE Centre of Excellence for Basic Sciences, Mumbai University Campus at Mumbai as an Emeritus Professor.

Class Transitions in Black Holes

A black hole spectrum is known to change from the hard state to the soft state when the energy spectral index α (FE ∝ E-α) in, say, 2–20 keV range changes from α ~ 0.5 to ~ 1.5. However, this 'classical' definition which characterizes black holes like Cyg X-1, becomes less useful for many objects such as GRS 1915+105 in which the spectral slope is seen to vary from one to the other in a matter of seconds and depending on whether or not winds form, the spectral slope also changes. The light curves and the colour-colour diagrams may look completely different on different days depending on the frequency and mode of switching from one spectral state to the other. Though RXTE observations have yielded wealth of information on such 'variability classes' in GRS 1915+105, very rarely one has been able to observe how the object goes from one class to the other. In the present review, we discuss possible origins of the class transition and present several examples of such transitions. In this context, we use mostly the results of the Indian X-ray Astronomy Experiment (IXAE) which observed GRS 1915+105 more regularly.

Evidence of Class Transitions in GRS 1915+105 from Indian X‐Ray Astronomy Experiment Data

GRS 1915+105 shows at least 12 distinct classes of light curves. By analyzing the data obtained from the Indian X-ray Astronomy Experiment (IXAE) instrument on board the IRS-P3 satellite, we show that on several days, transitions between one class to another were observed. On these days, the so-called κ class went to ρ class and the χ class went to ρ class. In the frequency-time plane, such transitions exhibited change in quasi-periodic oscillation (QPO) frequency. We could detect that low-frequency QPOs occurred in anticipation of a class transition several hundred minutes before the actual transition.

New measurements of orbital period change in Cygnus X-3

A nonlinear nature of the binary ephemeris of Cygnus X-3 indicates either a change in the orbital period or an apsidal motion of the orbit. We have made extended observations of Cygnus X-3 with the Pointed Proportional Counters (PPCs) of the Indian X-ray Astronomy Experiment (IXAE) during 1999 July 3-13 and October 11-14. Using the data from these observations and the archival data from ROSAT, ASCA, BeppoSAX and RXTE, we have extended the data base for this source. Adding these new arrival time measurements to the published results, we make a comparison between the various possibilities, (a) orbital decay due to mass loss from the system, (b) mass transfer between the stars, and (c) apsidal motion of the orbit due to gravitational interaction between the two components. Orbital decay due to mass loss from the companion star seems to be the most probable scenario.

X-ray properties of the microquasar GRS 1915105 during a variability class transition

We present a detailed X-ray study of the microquasar GRS 1915+105 during a variability class transition observed in 2000 June with the Pointed Proportional Counters (PPCs) of the Indian X-ray Astronomy Experiment (IXAE). We supplement this observation with data from the RXTE archives. The source made a transition from a steady low–hard state to a regular oscillatory behaviour in the light curve known as bursts or class ρ (variability classes from Belloni et al.) between 2000 May 11 and 17 and reverted back to the low–hard state on 2000 June 27. A gradual change in the burst recurrence time from about 75 s to about 40 s was observed, which then increased to about 120 s during the ∼40 d of class ρ. The regular bursts disappeared from the X-ray light curves and the class transition was observed to occur within 1.5 h on 2000 June 27 with the PPCs. A correlation is found between the observed quasi-periodic oscillation frequency at 5–8 Hz in the quiescent phase and the average X-ray intensity of the source during the class ρ. We notice a strong similarity between the properties of the source during the class ρ and those during the oscillatory phase of the observations of class α, which contains a long stretch (∼1000 s) of steady low–hard state in the light curve along with the regular periodic bursts. From the timing and spectral analysis, it is found that the observed properties of the source over tens of days during the class ρ are identical to those over a time-scale of a few hundreds of seconds in the class α. Examining the light curves from the beginning of the RXTE/PCA and RXTE/ASM observations, it is found that the change of state from radio-quiet low–hard state to high state occurs through the X-ray classes ρ and α, which appear together during the state transition. It is further inferred that the source switches from low–hard state to the class ρ through the intermediate class α.

IXAE observations of the X-ray pulsar XTE J1946+274

We report results from two observations of the transient binary X-ray pulsar XTE J1946+274 with the Indian X-ray Astronomy Experiment (IXAE) made in September 18-30 1999 and June 28-July 7 2000. The pulsar has a ~80 days orbital period during which the X-ray intensity varies almost sinusoidally between intensity levels of 5 and 50 mCrab. The mid-time of the two observations with the IXAE are separated by an orbital phase of 0.56 and were in the decaying and rising parts of its orbital modulation, respectively. During both the observations, strong pulsations with 15.8 s period are detected in the 2-6 and 6-18 keV energy bands of IXAE. The pulse profiles in both the observations are double peaked and identical with a similar pulse fractions of ~30% in the two energy bands. The pulse periods and its local derivatives are obtained from the IXAE observations. The known pulse period history shows small deviations from an otherwise nearly constant spin-up trend of the pulsar since its discovery. If the measured local period derivatives are a sum of a nearly constant spin-up of the pulsar and effect of its orbital motion, it suggests that the orbit of this Be X-ray binary is eccentric. The RXTE-ASM light curve shows a continuous detection of 10 orbital modulation of the source since its discovery.

Pulse Characteristics of the X‐Ray Pulsar 4U 1907+09

The X-ray pulsar 4U 1907+09 was observed in the 2-18 keV energy band with the Indian X-Ray Astronomy Experiment (IXAE) during 1996 August and again in 1998 June. From the timing measurements, the spin-down rate of the neutron star is measured to be +0.23 ± 0.01 s yr-1. A straight-line fit to the historical pulse period data indicates that the pulsar has been in a monotonic phase of spin-down since its discovery. The day-to-day pulse profile indicates some variations during its 8.4 day binary period. The average profile obtained from these observations shows a double-pulse shape with a pronounced asymmetric primary separated by a dip from a relatively weaker but broad secondary. These profiles show a closer resemblance to the pulse shape obtained with the recent observations with the Rossi X-Ray Timing Explorer (RXTE) during 1996 February, as compared to the earlier observations carried out six to 13 years earlier. A secondary flare of 88 mcrab intensity was detected during the IXAE observations in 1996 August. A detailed analysis of the flare data shows the presence of transient 14.4 s oscillations, which may be quasi-periodic during the flaring activity but have a period different from the earlier reported oscillations of 18.2 s as detected by RXTE during the flare of 1996 February. These results, therefore, strengthen the evidence for the presence of a transient accretion disk around the neutron star during the flare, which may be responsible for the continuous slowing down of the pulsar. The studies of such transient quasi-periodic oscillations during flaring activities of 4U 1907+09 thus provide opportunities for understanding the transient behavior of the accretion disk and its physical characteristics.

Detection of a Series of X‐Ray Dips Associated with a Radio Flare in GRS 1915+105

We report the detection of a series of X-ray dips in the Galactic black hole candidate GRS 1915+105 during 1999 June 6-17 from observations carried out with the pointed proportional counters of the Indian X-ray Astronomy Experiment on board the Indian Remote Sensing Satellite (IRS-P3). The observations were made after the source made a transition from a steady low-hard state to a chaotic state, which occurred within a few hours. Dips of about 20-160 s in duration were observed on most of the days. The X-ray emission outside the dips shows a quasi-periodic oscillation (QPO) at ~4 Hz that has characteristics similar to the ubiquitous 0.5-10 Hz QPO seen during the low-hard state of the source. During the onset of dips this QPO is absent, and, also, the energy spectrum is soft and the variability is low compared to the nondip periods. These features gradually reappear as the dip recovers. The onset of the occurrence of a large number of such dips followed the start of a huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as the cause for mass ejection due to the evacuation of matter from an accretion disk around the black hole. We propose that a superposition of a large number of such dip events produces a huge radio jet in GRS 1915+105.

Peculiar x-ray dips in the superluminal source GRS 1915+105 observed in the soft state

This paper reports the detection of a series of X-ray dips in the superluminal black hole source GRS 1915+105 during 1999 June 6–17 from observations carried out with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment on board the Indian satellite IRS—P3. The observations made in the soft state after the source had made a transition from a low-hard state to a high-soft state, reveal large number of x-ray dips each lasting for about 100– 150 seconds on most of the days. Quasi periodic oscillation (QPOs) with characteristics similar to those of the 0.5–10 Hz QPOs, seen during the low-hard state of the source, were detected in the non-dip portions of the data. The QPOs are, however, not present during the onset or in the dip-period data. During the dips the energy spectrum is soft and the variability is low compared to the non-dip periods. These features re-appear as the dip gradually recovers. Coincident with the occurrence of a large number of X-ray dips, a huge radio flare of strength 0.48 Jy (at 2.25 GHz) was found from the NRAO archival data. It is suggested that the X-ray dips represent mass ejection due to the evacuation of matter from an accretion disk around the black hole and that a super-position of a large number of such dip events leads to production of huge radio jet in GRS 1915+105.

New measurements of orbital period changes in Cygnus X-3

A nonlinear nature of the arrival time of the 4.8 hr binary modulation in the X-ray light curve of Cygnus X-3 indicates either a change in the orbital period or an apsidal motion of the orbit. We have made extended observations of Cygnus X-3 with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment during 1999 July 3–13 and and 199 October 11–24. Using these and archival data from ROSAT, ASCA, and RXTE we have extended the data base for this source. Adding these new arrival time measurements to the published results, we make a comparison between the various possibilities, (a) orbital expansion due to mass loss from the system, (b) mass transfer between the components, and (c) apsidal motion of the orbit due to gravitational interaction between the two components. Orbital expansion due to mass loss from the companion star seems to be the most probable scenario. Most possibly, the compact star in this system has an accretion disk with an extended corona.

X-ray observation of XTE J2012+381 during the 1998 outburst

The outburst of X-ray transient source XTE J2012+381 was detected by the RXTE All-Sky Monitor on 1998 May 24th. Following the outburst, X-ray observations of the source were made in the 2–18 keV energy band with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment (IXAE) on-board the Indian satellite IRS-P3 during 1998 June 2nd–10th. The X-ray flux of the source in the main outburst decreased exponentially during the period of observation. No large amplitude short-term variability in the intensity is detected from the source. The power density spectrum obtained from the timing analysis of the data shows no indication of any quasi-periodic oscillations in 0.002–0.5 Hz band. The hardness ratio i.e. the ratio of counts in 6–18 keV to 2–6 keV band, indicates that the X-ray spectrum is soft with spectral index >2. From the similarities of the X-ray properties with those of other black hole transients, we conclude that the X-ray transient XTE J2012+381 is likely to be a black hole.

Studies of binary sources with the Indian X-ray Astronomy Experiment

Studies of several x-ray binaries have been made with the Indian X-ray Astronomy Experiment on the IRS-P3 satellite. We present here results from timing analysis of four of the neutron star and black hole binaries; namely GRS 1915+105, the recently discovered x-ray transient XTE J1748-288 and the x-ray pulsars Cep X-4 and 4U1907+09. New results include detection of a variety of regular and irregular bursts from GRS 1915+105 and detection of irregular intensity variations in the transient source XTE J1748-288. The x-ray pulse profile of 4U1907+09 was found to have changed from a double-peak pulse observed in August 4–7, 1996 to a broad single-peak pulse in May 26 – June 3, 1998 period. Details of these results are presented.

Different types of X-ray bursts during high state of the superluminal source GRS 1915+105

We have observed the galactic superluminal X-ray transient source GRS 1915+105 in its high state from June 12 to August 10, 1997 with the pointed proportional counters (PPCs) of Indian X-ray Astronomy Experiment on board IRS-P3 satellite. The observed light curves have revealed the presence of four types of intense X-ray bursts: regular persistent bursts with two distinct time scales, quasi-regular bursts and irregular bursts. These bursts have unique characteristics of slow exponential rise and fast linear decay and are different from the classical bursts seen in Low Mass X-ray Binaries. GRS 1915+105 is the only black hole source in which such X-ray bursts are observed. During this period, the total 2–18 keV flux changed by a factor of 3 and quiescent time hardness ratio remained in the region 0.9-1.4. We discuss our results in the light of recent results from RXTE/PCA of other galactic superluminal source GRO J1655-40 during its high state and speculate why such bursts have not been observed so far from GRO J1655-40.

Different Types of X‐Ray Bursts from GRS 1915+105 and Their Origin

We report X-ray observations of the Galactic X-ray transient source GRS 1915+105 with the pointed proportional counters of the Indian X-ray Astronomy Experiment (IXAE) onboard the Indian satellite IRS-P3, which show remarkable richness in temporal variability. The observations were carried out on 1997 June 12-29 and August 7-10, in the energy range of 2-18 keV and revealed the presence of very intense X-ray bursts. All the observed bursts have a slow exponential rise, a sharp linear decay, and broadly can be put in two classes: irregular and quasi-regular bursts in one class, and regular bursts in the other. The regular bursts are found to have two distinct timescales and to persist over extended durations. There is a strong correlation between the preceding quiescent time and the burst duration for the quasi-regular and irregular bursts. No such correlation is found for the regular bursts. The ratio of average flux during the burst time to the average flux during the quiescent phase is high and variable for the quasi-regular and irregular bursts, while it is low and constant for the regular bursts. We present a comprehensive picture of the various types of bursts observed in GRS 1915+105 in the light of the recent theories of advective accretion disks. We suggest that the peculiar bursts that we have seen are characteristic of the change of state of the source. The source can switch back and forth between the low-hard state and the high-soft state near critical accretion rates in a very short timescale, giving rise to the irregular and quasi-regular bursts. The fast timescale for the transition of the state is explained by invoking the appearance and disappearance of the advective disk in its viscous timescale. The periodicity of the regular bursts is explained by matching the viscous timescale with the cooling timescale of the postshock region. A test of the model is presented using the publicly available 13-60 keV RXTE/PCA data for irregular and regular bursts concurrent with our observations. It is found that the 13-60 keV flux relative to the 2-13 keV flux shows clear evidence for state change between the quiescent phase and the burst phase. The value of this ratio during burst is consistent with the values observed during the high-soft state seen on 1997 August 19, while its value during quiescent phase is consistent with the values observed during the low-hard state seen on 1997 May 8.

The processing electronics system for the pointed mode operation of IXAE on the Indian satellite IRS-P3

Indian Remote Sensing Satellite-P3 (IRS-P3: 1996-017A) was launched into a near-Earth, polar, Sun-synchronous orbit by India's Polar Satellite Launch Vehicle (PSLV-D3) on March 21, 1996. The Indian X-ray Astronomy Experiment (IXAE) is one of the prime payloads onboard IRS-P3. IXAE has two sets of detectors: three pointed-mode proportional counters (PPCs) and one X-ray Sky Monitor (XSM). IRS-P3 is configured to operate in two modes, namely, Earth-pointing mode and stellar-pointing mode. In Earth-pointing mode the remote sensing payloads are in operation; in addition, XSM maps the sky for bright X-ray sources and X-ray transients. In stellar mode, the PPCs observe several Galactic, bright X-ray, and extragalactic sources. The microprocessor-based processing electronics system was designed and developed for PPC data-handling and telemetering the stored data to the ground station. The system has been working well from the day the IXAE was commissioned. The processing electronics system for the PPCs, its interfaces, and the in-orbit instrument performance are described. Some results of the pointed mode observations are also presented.

X-ray variability of GRS 1915+105 during the low-hard state observed with the Indian X-ray astronomy experiment (IXAE)

The galactic superluminal transient X-ray source GRS 1915+105 was observed with the pointed proportional counters (PPCs) onboard the Indian satellite IRS-P3 during 1996 July 23-27. We report here details of the behavior of this source during the relatively quiet and low luminosity state. Large intensity variations by a factor of 2 to 3, generally seen in black-hole candidates, are observed at a time scale of 100 ms to few seconds. No significant variation is detected over larger time scale of minute or more. The intensity variations are described as sum of shots in the light curve, and the number distribution of the shots are found to be exponential function of the fluence and duration of the shots. The cross correlation spectrum between 6-18 keV and 2-6 keV X-rays is found to have asymmetry signifying a delay of the hard X-rays by about 0.2 to 0.4 sec. This supports the idea of hard X-rays being generated by Compton up-scattering from high energy clouds near the source of soft X-rays. Very strong and narrow quasi periodic oscillations in the frequency range 0.62 to 0.82 Hz are observed. We discuss about a model which explains a gradual change in the QPO frequencies with corresponding changes in the mass accretion rate of the disk.

Rapid X-Ray Variability of Cyg X-1

We have made observations of the black hole binary Cyg X-1 with the Indian X-ray Astronomy Experiment (IXAE). Observations made with time resolution ranging from 0.4 ms to 1 s showed variations and flaring activity on sub-sec and longer time scales. Results on time variability on different time scales and flaring characteristics in the two states of Cyg X-1 are presented.

Quasi-regular X-Ray Bursts from GRS 1915+105 Observed with the IXAE: Possible Evidence for Matter Disappearing into the Event Horizon of the Black Hole

Three different types of very intense, quasi-regular X-ray bursts have been observed from the Galactic superluminal X-ray transient source GRS 1915+105 with the pointed proportional counters of the Indian X-Ray Astronomy Experiment on board the Indian satellite IRS-P3. The observations were carried out from 1997 June 12 to 29 in the energy range of 2-18 keV and revealed the presence of persistent quasi-regular bursts with different structures. Only one of the three types of bursts is regular in occurrence, revealing a stable profile over extended durations. The regular bursts have an exponential rise with a timescale of about 7-10 s and a sharp linear decay in 2-3 s. The X-ray spectrum becomes progressively harder as the burst evolves, and it is the hardest near the end of the burst decay. The profile and energetics of the bursts in this black hole candidate source are distinct from both the type I and type II X-ray bursts observed in neutron star sources. We propose that the sharp decay in the observed burst pattern is a signature of the disappearance of matter through the black hole horizon. The regular pattern of the bursts can be produced by material influx into the inner disk that is due to oscillations in a shock front far away from the compact object.