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We present a comprehensive timing and spectral analysis of the HMXB 4U 1538-522 by using the Nuclear Spectroscopic Telescope Array (NuSTAR) observatory data. Using three archived observations made between 2019 and 2021, we have detected $\sim $ 526 s coherent pulsations up to 60 keV. We have found an instantaneous spin-down rate of $\dot{P} = 6.6_{-6.0}^{+2.4} \times 10^{-6}$ s s$^{-1}$ during the first observation. The pulse profiles had a double peaked structure consisting of a broad primary peak and an energy dependent, weak secondary peak. We have also analysed the long-term spin-period evolution of 4U 1538-522 from data spanning more than four decades, including the data from Fermi/GBM. Based on the recent spin trends, we have found that the third torque reversal in 4U 1538-522 happened around MJD 58800. The source is currently spinning up with $\dot{P} = -1.9(1) \times 10^{-9}$ s s$^{-1}$. We also report a periodic fluctuation in the spin-period of 4U 1538-522. The broad-band persistent spectra can be described with a blackbody component and either powerlaw or Comptonization component along with a Fe K$_{\alpha}$ line at 6.4 keV and a cyclotron absorption feature around 22 keV. We have also found a relatively weak absorption feature around 27 keV in the persistent spectra of 4U 1538-522 in all three observations. We have estimated a magnetic field strength of $1.84_{-0.06}^{+0.04} (1+z) \times 10^{12}$ and $2.33_{-0.24}^{+0.15} (1+z) \times 10^{12}$ G for the two features, respectively., Comment: Accepted for publication in MNRAS

We report the results of broadband timing and spectral analysis of data from an AstroSat observation of the High Mass X-ray binary LMC X$-$4. The Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) instruments on-board the AstroSat observed the source in August 2016. A complete X-ray eclipse was detected with the LAXPC. The 3$-$40 keV power density spectrum showed the presence of coherent pulsations along with a $\sim 26$ mHz quasi-periodic oscillation feature. The spectral properties of LMC X$-$4 were derived from a joint analysis of the SXT and LAXPC spectral data. The 0.5$-$25 keV persistent spectrum comprised of an absorbed high energy cutoff power law with photon index of $\Gamma \sim$ 0.8 and cutoff at $\sim$16 keV, a soft thermal component with kT$_{BB} \sim$ 0.14 keV and Gaussian components corresponding to Fe K$_\alpha$, Ne \textsc{ix} and Ne \textsc{x} emission lines. Assuming a source distance of 50 kpc, we determined 0.5--25 keV luminosity to be $\sim 2 \times 10^{38}$ erg s$^{-1}$., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society

We present the broad-band spectral analysis of the low-mass X-ray binary 2S 0921-63 by using the Suzaku archival data covering the orbital phase between 0.31 and 1.16 during four close observations. It is the first time that a broad-band spectral analysis of 2S 0921-63 has been done up to 25 keV. The 0.5-10 keV XIS count rate varied between $\sim$ 1 and $\sim$ 5 counts s$^{-1}$ during the observations. A partial X-ray eclipse and broad post-eclipse intensity dip were observed during the observations. The X-ray emission hardened marginally during the intensity dip. We have modelled the source spectra by simultaneously fitting the XIS and HXD-PIN spectra for each of the four observations. The broad-band spectra of the source can be described by a model comprising a very hot blackbody having temperature, $kT_{\rm BB} \approx$ 1.66 - 2.13 keV, a high-energy cutoff power law, and an Fe emission line at $E_{\rm line} \sim$ 6.7 keV. A second model, accounting for the Comptonization of the thermal emission from accretion disc along with an Fe emission line, describes the broad-band spectra of 2S 0921-63 equally well., 7 pages, 6 figures, 2 Tables, Accepted for publication in MNRAS

This work presents timing and spectral analysis of 4U 2206+54 using data obtained from LAXPC instrument onboard India's AstroSat mission. This source was observed with AstroSat in September 2016 and October 2016. We report detection of $5648 (4)$ s pulsations at MJD 57669 in the latter observation of 4U 2206+54. The pulse profile is sinusoidal and the inherent shape is independent of energy up to 30 keV. The pulse fraction increases with energy from $\sim 0.5$% to $\sim 0.8$%. We report an updated spin down rate of $2.95 (14) \times 10^{-7}$ s s$^{-1}$. This is about 0.40 times smaller than the previously reported long term value. The energy spectrum is best modelled with an absorbed power-law with high energy exponential cut-off. We have detected presence of broad emission line in 4U 2206+54 at an energy of $7$ keV with equivalent width of $\sim 0.4$ keV., Accepted in Journal of Astrophysics and Astronomy

We report on timing and spectral analysis of transient Be X-ray pulsar GRO J1750-27 by using the Nuclear Spectroscopic Telescope Array (NuSTAR) observation from September 2021. This is the fourth outburst of the system since 1995. The NuSTAR observation was performed during the rising phase of the outburst. Pulsations at a period of 4.450710(1) s were observed in the 3-60 keV energy range. The average pulse profile comprised of a broad peak with a weak secondary peak which evolved with energy. We did not find any appreciable variation in the X-ray emission during this observation. The broad-band phase-averaged spectrum is described by a blackbody, a powerlaw or Comptonization component. We report discovery of Fe K$_{\alpha}$ line at 6.4 keV along with presence of two cyclotron resonant scattering features around 36 and 42 keV. These lines indicate a magnetic field strength of $3.7_{-0.3}^{+0.1} \times 10^{12}$ and $4.4 \pm 0.1 \times 10^{12}$ G for the neutron star. We have estimated a source distance of $\sim$ 13.6-16.4 kpc based on the accretion-disc torque models., Comment: 8 pages, 9 Figures, 1 Table, Updated and accepted version for publication in MNRAS

We present an updated measurement of orbital period evolution of LMXB XTE J1710-281 by using eclipse timing technique. Using data obtained with XMM-Newton, Suzaku, RXTE, Chandra and AstroSat observatories, we report 21 new measurements of X-ray mid-eclipse times. We have discovered a third orbital period glitch in XTE J1710-281 with an F-test false alarm probability of ~0.7% for occurrence of the third glitch and report detection of four distinct epochs of orbital period in this system. This work presents a more robust estimation of occurrence of the second orbital period glitch. However, the epoch of occurrence of the third glitch is poorly constrained, between MJD 55726 to 56402. We have put lower limits of 1.48 ms, 0.97 ms and 0.45 ms, on sudden changes in orbital period between the successive epochs. We discuss the implications of our findings in context of magnetic nature of the companion star and possible scattering events with circum-binary objects around this binary system., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

This work presents a comprehensive timing and spectral analysis of high-mass X-ray binary pulsar, OAO 1657-415 by using the observation made with Nuclear Spectroscopic Telescope Array (NuSTAR) on June 2019. During this observation, OAO 1657-415 exhibited X-ray variability by a factor of about 3. X-ray pulsations at 37.03322(14) s were observed up to 70 keV. OAO 1657-415 was undergoing a spin-down phase with $\dot{P} = 9(1) \times 10^{-8}$ s s$^{-1}$. This is an order of about 100 larger than the long-term spin-up rate. The pulse profile evolved marginally during the observation. We have discussed the long-term pulse period history of the source spanning a time-base of 43 years, including the latest Fermi/GBM data. The 3-70 keV source spectrum is described by a partially covered cutoff power-law, an Fe K$_{\alpha}$ line at 6.4 keV and a Compton shoulder at 6.3 keV. We report the presence of a cyclotron absorption feature around 40 keV, which is indicative of a surface magnetic field strength of $3.59 \pm 0.06 \ (1 + z)^{-1} \times 10^{12}$ and $3.29_{-0.22}^{+0.23} \ (1 + z)^{-1} \times 10^{12}$ G. This work shows the first robust presence of cyclotron absorption feature in the source., Comment: Accepted for publication in MNRAS

This work presents the broad-band time-averaged spectral analysis of neutron star low-mass X-ray binary, XTE J1710-281 by using the Suzaku archival data. The source was in a hard or an intermediate spectral state during this observation. This is the first time that a detailed spectral analysis of the persistent emission spectra of XTE J1710-281 has been done up to 30 keV with improved constraints on its spectral parameters. By simultaneously fitting the XIS (0.6-9.0 keV) and the HXD-PIN (15.0-30.0 keV) data, we have modelled the persistent spectrum of the source with models comprising a soft component from accretion disc and/or neutron star surface/boundary layer and a hard Comptonizing component. The 0.6-30 keV continuum with neutral absorber can be described by a multi-colour disc blackbody with an inner disc temperature of $kT_{\rm disc} = 0.28$ keV, which is significantly Comptonized by the hot electron cloud with electron temperature of $kT_{\rm e} \approx 5$ keV and described by photon index $\Gamma = 1.86$. A more complex three-component model comprising a multi-colour disc blackbody $\approx 0.30$ keV, single temperature blackbody $\approx 0.65$ keV and Comptonization from the disc, partially absorbed (about 38 per cent) by an ionized absorber (log($\xi$) $\approx$ 4) describes the broad-band spectrum equally well., Comment: 9 pages and 6 figures

We present here the results of broadband spectral analysis of low-mass X-ray binary and a black hole candidate 4U 1957+11. The source was observed nine times with the Nuclear Spectroscopic Telescope Array (NuSTAR) between 2018 September and 2019 November. During these observations, the spectral state of 4U 1957+11 evolved marginally. The disc dominant spectra are well described with a hot, multicolour disc blackbody with disc temperature varying in the range $kT_{\rm in} \sim$ 1.35-1.86 keV and a non-thermal component having a steep slope ($\Gamma =$ 2-3). A broad Fe emission line feature (5-8 keV) was observed in the spectra of all the observations. The relativistic disc model was used to study the effect of distance, inclination, and the black hole mass on its spin. Simulations indicate a higher spin for smaller distances and lower black hole mass. At smaller distances and higher mass, spin is maximum and almost independent of the distance. An inverse correlation exists between the spin and the spectral hardening factor for all the cases. The system prefers a moderate spin of about 0.85 for black hole masses between 4-6 M_sun for a 7 kpc distance., Comment: 15 pages, 14 figures, Accepted for publication in Research in Astronomy and Astrophysics

We present an X-ray timing analysis of the transient X-ray binary MXB 1658-298, using data obtained from the RXTE and XMM-Newton observatories. We have made 27 new mid eclipse time measurements from observations made during the two outbursts of the source. These new measurements have been combined with the previously known values to study long term changes in orbital period of the binary system. We have found that the mid-eclipse timing record of MXB 1658-298 is quite unusual. The long term evolution of mid-eclipse times indicates an overall orbital period decay with a time scale of -- 6.5(7) x 10^7 year. Over and above this orbital period decay, the O-C residual curve also shows a periodic residual on shorter timescales. This sinusoidal variation has an amplitude of ~9 lt-sec and a period of ~760 d. This is indicative of presence of a third body around the compact X-ray binary. The mass and orbital radius of the third body are estimated to lie in the range, 20.5-26.9 Jupiter mass and 750-860 lt-sec, respectively. If true, then it will be the most massive circumbinary planet and also the smallest period binary known to host a planet., Comment: 5 pages, 3 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society Letters

We report the spectral analysis of accretion powered millisecond X-ray pulsar SAX J1748.9-2021 from the NuSTAR observation made during its 2015 outburst. The spectra displayed a broad emission line at $\sim 6.5$ keV with line width of $\sim 0.5$ keV and excess above $\sim 20$ keV due to Compton hump. The persistent emission of SAX J1748.9-2021 is described by a combination of soft thermal component with $kT =0.62^{+0.03}_{-0.05}$ keV and thermally Comptonized component with $kT_e=2.50^{+0.06}_{-0.03}$ keV reflected from the disc with reflection fraction of $0.30^{+0.08}_{-0.04}$. We have modeled the reflection spectrum with self-consistent model relxillCP and have found the inclination of the accretion disc to be $32.3^{^\circ +4.8}_{-4.7}$ and log $\xi$ is equal to $3.05^{+0.40}_{-0.34}$. We have obtained an upper limit of 57 km for inner disc radius; and the estimated magnetic field strength at the poles is less than $3.4 \times 10^9$ G. This kind of detailed investigation of reflection spectrum of SAX J1748.9-2021, especially above 10 keV, has been achieved only because of high statistics NuSTAR data of the source., Comment: 6 pages, 6 figures. Accepted for publication in MNRAS

We report results from a broadband spectral analysis of the low mass X-ray binary MXB 1658-298 with the Swift/XRT and NuSTAR observations made during its 2015-2017 outburst. The source showed different spectral states and accretion rates during this outburst. The source was in low/hard state during 2015; and it was in high/soft state during the 2016 NuSTAR observations. This is the first time that a comparison of the soft and hard spectral states during an outburst is being reported in MXB 1658-298. Compared with the observation of 2015, the X-ray luminosity was about four times higher during 2016. The hard state spectrum can be well described with models consisting of a single-temperature blackbody component along with Comptonized disc emission, or three component model comprising of multi-colour disc, single-temperature blackbody and thermal Comptonization components. The soft state spectrum can be described with blackbody or disc blackbody and Comptonization component, where the neutron star surface (or boundary layer) is the dominant source for the Comptonization seed photons. We have also found a link between the spectral state of the source and the Fe K absorber. The absorption features due to highly ionized Fe were observed only in the soft state. This suggests a probable connection between accretion disc wind (or atmosphere) and spectral state (or accretion state) of MXB 1658-298., 11 pages, 8 figures. Accepted for publication in MNRAS

Introduction: Although red cell transfusions are lifesavers for patients with thalassemia, they are responsible for a series of complications and expose the patients to a variety of risks. Thalassemia major is the most common genetic disease caused by defective gene which produces abnormal haemoglobin. Material and Methods: We have investigated 100 patients of Thalassemia coming to blood bank for regular blood transfusion. Level of hemoglobin,TLC,DLC and Liver enzymes i.e. SGOT, SGPT, Alkaline Phosphatase along with S. Bilirubin were also estimated. The age group of children suffering from Thalassemia major was between one year to fifteen years. Results: Serum Ferritin Level was 2251 + 315 ng/ml(mean+S.D ), Haemoglobin level was 7.82 + 0.83 gm/dl,TLC was 7200 + 1000 cells/cmm,DLC was 68 + 12 Neutrophils,29 + 08 Lymphocytes,02 + 01 Eosinophils,01 + 00Monocytes,00 + 0.1 Basophils , SGOT (AST) value was 117 + 35.5 IU/L and SGPT(ALT) concentration 151 + 47.2 IU/L and Alkaline Phosphatase level was found to be 288 + 20.80 IU/L with mean + S.D. Bilirubin was 2.55 +1.12 mg/dl (mean + S.D). Conclusion: In Present study Liver Enzymes increased and haemoglobin level was reduced but no changes were found in TLC and DLC.

MXB 1658-298 is a transient Low-Mass X-ray Binary (LMXB), which shows eclipses, dips and bursts in its light curve. This source has undergone three active periods separated by long quiescent phases. The latest phase of enhanced X-ray emission was observed during 2015–2016. We have analysed broadband data from Swift/XRT and NuSTAR observations carried out in 2015. During NuSTAR observation, one thermonuclear X-ray burst took place. The X-ray emission during the burst was brighter by a factor of $$\sim 200$$ , compared to the pre-burst emission. This work focuses on the spectral analysis of MXB 1658-298 during the persistent and the burst phases using NuSTAR observation of 2015. We have also determined the temperature and radius evolution during the burst using the time-resolved spectroscopy. The burst phase shows mild Photospheric Radius Expansion (PRE).

We review the pulse profile evolution of the unique accretion powered X-ray pulsar 4U 1626-67 over the last 40 years since its discovery. This pulsar showed two distinct eras of steady spin-up separated by a steady spin-down episode for about 18 years. In the present work, using data from different observatories active during each phase of spin-up and spin-down we establish a clear correlation between the accretion torque acting on this pulsar and its pulse profile. The energy resolved pulse profiles are identical in both the spin-up eras and quite different in the spin-down era, especially in the low energy band. This correlation, along with the already known feature of strong Quasi Periodic Oscillations (QPO) that was present only in the spin-down era, clearly establish two different accretion modes onto the neutron star which produce different pulse profiles and only one of which produces the QPOs., 17 pages

We present an X-ray timing analysis of the Crab pulsar, PSR B0531+21, using the archival RXTE data. We have investigated the stability of the Crab pulse profile, in soft (2-20 keV) and hard (30-100 keV) X-ray energies, over the last decade of RXTE operation. The analysis includes measurement of the separation between the two pulse peaks; and intensity and the widths of the two peaks. We did not find any significant time dependency in the pulse shape. The two peaks are stable in phase, intensity and widths, for the last ten years. The first pulse is relatively stronger at soft X-rays. The first pulse peak is narrower than the second peak, in both, soft- and hard X-ray energies. Both the peaks show a slow rise and a steeper fall. The ratio of the pulsed photons in the two peaks is also constant in time., 9 pages, 5 figures, 1 table; accepted for publication in "Research In Astronomy and Astrophysics"

We present an X-ray eclipse timing analysis of the transient low mass X-ray binary XTE J1710-281. We report observations of 57 complete X-ray eclipses, made with the proportional counter array detectors aboard the RXTE satellite. Using the eclipse timing technique, we have derived a constant orbital period of 0.1367109674 (3) d, during the period from MJD 52132 upto MJD 54410; and 1{\sigma} limits of -1.6 $\times$ 10^ -12 d d-1 and 0.2 $\times$ 10^ -12 d d-1 , on the period derivative. This puts constraints on the minimum timescale of secular orbital period evolution of 2.34 $\times$ 10^8 yr for a period decay and 18.7 $\times$ 10^8 yr for a period increase. We also report detection of two instances of discontinuity in the mid-eclipse time, one before and one after the above MJD range. These results are interpreted as three distinct epochs of orbital period in XTE J1710-281. We have put lower limits of 1.4 ms and 0.9 ms on orbital period change at successive epochs.

4U 1822-37 is a low mass X-ray binary (LMXB) system with an accretion disc corona. It is one of the very few LMXBs that shows narrow X-ray eclipses and small amplitude pulsations of the neutron star. The X-ray eclipse is an excellent reference for measurement of orbital evolution of the binary, and we have obtained 16 new mid-eclipse time measurements of this source during the last 13 years using X-ray observations made with the RXTE-PCA, RXTE-ASM, Swift-XRT, XMM-Newton and Chandra observatories. These, along with the earlier known mid-eclipse times have been used to accurately determine the time-scale for a change in the orbital period of 4U 1822-37. We have derived an orbital period P orb = 0.23210887(15) d, which is changing at the rate of P orb = 1.3(3) × 10 -10 d d -1 (at To = MJD 45614). The time-scale for a change in the orbital period is P orb /P orb of 4.9(1.1) x 10 6 yr. We also report the detection of 0.59290132(11) s (at T 0 = MJD 51975) X-ray pulsations from the source with a long-term average P spin of -2.481(4) x 10 -12 s s -1 , i.e. a spin-up time-scale (P spin /P spin ) of 7578(13) yr. In view of these results, we have discussed various mechanisms that could be responsible for the orbital evolution in this system. Assuming the extreme case of conservative mass transfer, we have found that the measured P orb requires a large mass transfer rate of (4.2-5.2) x 10- 8 M ⊙ yr -1 which together with the spin-up rate implies a magnetic field strength in the range of ( 1-3) x 10 8 G. Using the long-term RXTE-ASM light curve, we have found that the X-ray intensity of the source has decreased over the last 13 yr by ~40 per cent and there are long-term fluctuations at time-scales of about a year. In addition to the long-term intensity variation, we have also observed significant variation in the intensity during the eclipse. Variation was also seen in the pulse profile, which could be due to changes in the accretion geometry. We briefly discuss the implications of these results on our understanding of the properties of the neutron star and the accretion geometry in this source.

4U 1626-67 is an accretion powered X-ray pulsar that shows remarkably stable X-ray luminosity above hours timescale and gradual intensity variation on a few years timescale. Here we report a significant increase in the X-ray intensity in the long term RXTE-ASM light curve of 4U 1626-67. Similar enhancement in the X-ray flux has also been detected in the Swift-BAT light curve. The increase in the X-ray flux took place over a long period of about 100 days and there appears to be two episodes of flux enhancement. We have investigated the spectral and timing features of 4U 1626-67 during its current state of enhanced flux emission with data obtained from the Proportional Counter Array and the High-Energy X-ray Timing Explorer on board the Rossi X-ray Timing Explorer. The source has entered a new spin-up phase with a spin-up rate of 4.02(5) $\times$ 10$^{-13}$ Hz s$^{-1}$. The present spin-up rate is almost half of the earlier spin-up and spin-down trends. A significant excess in soft X-ray photon emission is observed during the enhanced flux state, which is similar to the energy spectrum obtained during the spin-up era of the pulsar before 1990. We also report detection of a significant broadening in the wings of the 130 mHz peak and a change in the shape of the continuum of the power spectrum.

We report here results from a study of X-ray bursts from 3 magnetar candidates (SGR 1806-20, SGR 1900+14 and AXP 1E 2259+586). We have searched for a pulse phase dependence of the X-ray burst rate from these sources. X-ray light curves were obtained with the Proportional Counter Array on-board the Rossi X-ray Timing Explorer during the periods of intense burst activity in these sources. On detailed analysis of the three sources, we found a very significant burst rate for all pulsar phases. However, some locations appear to produce bursts slightly more often, rendering the non-isotropic distribution. Only in the case of SGR 1900+14, there is a clear pulse phase dependence of burst rate.

We present results from a pulse timing analysis of the accretion-powered millisecond X-ray pulsar SAX J1808.4-3658 using X-ray data obtained during four outbursts of this source. Extensive observations were made with the proportional counter array of the Rossi X-ray Timing Explorer (RXTE) during the four outbursts that occurred in 1998, 2000, 2002 and 2005. Instead of measuring the arrival times of individual pulses or the pulse arrival time delay measurement that is commonly used to determine the orbital parameters of binary pulsars, we have determined the orbital ephemeris during each observation by optimizing the pulse detection against a range of trial ephemeris values. The source exhibits a significant pulse shape variability during the outbursts. The technique used by us does not depend on the pulse profile evolution, and is therefore, different from the standard pulse timing analysis. Using 27 measurements of orbital ephemerides during the four outbursts spread over more than 7 years and more than 31,000 binary orbits, we have derived an accurate value of the orbital period of 7249.156862(5) s (MJD = 50915) and detected an orbital period derivative of (3.14 ± 0.21) × 10−12 s s−1. We have included a table of the 27 mid-eclipse time measurements of this source that will be valuable for further studies of the orbital evolution of the source, especially with ASTROSAT. We point out that the measured rate of orbital period evolution is considerably faster than the most commonly discussed mechanisms of orbital period evolution like mass transfer, mass loss from the companion star and gravitational wave radiation. The present time scale of orbital period change, 73 Myr is therefore likely to be a transient high value of period evolution and similar measurements during subsequent outbursts of SAX J1808.4-3658 will help us to resolve this.

We present here results from our search for X-ray pulsations of the neutron star in the low mass X-ray binary EXO 0748-676 at a frequency near a burst-oscillation frequency of 44.7 Hz. Using the observations made with the Proportional Counter Array on board the Rossi X-ray Timing Explorer we did not find any pulsations in the frequency band of 44.4 Hz to 45.0 Hz and obtained a 3\sigma upper limit of 0.47 % on the pulsed fraction for any possible underlying pulsation in this frequency band., Comment: Accepted for publication in "Research in Astronomy and Astrophysics", 7 pages, 3 figures

We report a study of the X-ray emission from the nuclear region of the low surface brightness (LSB) galaxy UGC 6614. Very little is known about the central objects in LSB galaxies especially their X-ray properties and X-ray spectra. In this study we have used XMM-Newton archival data to study the characteristics of the X-ray spectrum and the X-ray flux variability of the AGN in the LSB galaxy UGC 6614. The nucleus of UGC 6614 is very bright in X-ray emission with an absorption corrected 0.2-10.0 keV luminosity of ~1.1 x 10^{42} erg s^{-1}. The X-ray spectrum is found to be power-law type with a moderate column density. A short time scale of intensity variation and large X-ray flux is indicative of the presence of a black hole at the centre of this galaxy. Using the method of excess variance, we have determined the black hole mass to be ~0.12 x 10^{6} solar mass. The X-ray spectral properties are similar to that of the Seyfert I type AGNs. Our study thus demonstrates that although LSB galaxies are poor in star formation, they may harbour AGNs with X-ray properties comparable to that seen in more luminous spiral galaxies., Comment: 5 pages, 4 figures, Accepted for publication in MNRAS

We present the orbital X-ray modulation study of three high mass X- ray binary systems, IGR J18027-2016, IGR J18483-0311 and IGR J16318-4848 using data obtained with RXTE-ASM, Swift-BAT and INTEGRAL-ISGRI. Using the long term light curves of the eclipsing HMXB IGR J18027-2016, obtained with Swift-BAT in the energy range 15-50 keV and INTEGRAL-ISGRI in the energy range 22-40 keV, we have determined three new mid eclipse times. The newly determined mid eclipse times together with the known values were used to derive an accurate value of the orbital period of 4.5693(4) d at MJD 52168 and an upper limit of 3.9(1.2)x10^-7 d d^-1 on the period derivative. We have also accurately determined an orbital period of 18.5482(88) d for the intermediate system IGR J18483-0311, which displays an unusual behaviour and shares many properties with the known SFXTs and persistent supergiant systems. This is a transient source and the outbursts occur intermittently at intervals of 18.55 d. Similarly, in the third supergiant system, IGR J16318-4848, we have found that the outbursts are separated by intervals of 80 d or its multiples, suggesting a possible orbital period., 17 pages, 14 figures, accepted for publication in Research in Astronomy and Astrophysics

We report here discovery of a 3.32 day orbital period in the Supergiant Fast X-ray Transient (SFXT) source IGR J16479-4514. Using the long term light curve of this source obtained with Swift-BAT in the energy range of 15-50 keV, we have clearly detected an orbital modulation including a full eclipse of duration ~0.6 day. In the hard X-ray band of the BAT instrument, the eclipse ingress and egress are rapid. We have also used the long term light curve obtained with the RXTE -ASM in the energy range of 1.5-12 keV. Taken independently, the detection of orbital modulation in the RXTE -ASM light curve is not significant. However, considering a clear detection of orbital modulation in the BAT light curve, we have used the ASM light curve for a more precise determination of the orbital period. IGR J16479-4514 has the shortest orbital period among the three SFXTs with measured/known orbital period. We discuss the implication of a short orbital period with the various mechanisms proposed to explain the transient nature of this class of sources., Comment: Accepted for publication in MNRAS Letters; 5 pages,3 figures and 0 tables

We report here results from a new search for orbital motion of the accretion powered X-ray pulsar 4U 1626-67 using two different analysis techniques. X-ray light curve obtained with the Proportional Counter Array of the Rossi X-ray Timing Explorer during a long observation carried out in February 1996, was used in this work. The spin period and the local period derivative were first determined from the broad 2-60 keV energy band light curve and these were used for all subsequent timing analysis. In the first technique, the orbital phase dependent pulse arrival times were determined for different trial orbital periods in the range of 500 to 10,000 s. We have determined a 3 sigma upper limit of 13 It-ms on the projected semi-major axis of the orbit of the neutron star for most of the orbital period range, while in some narrow orbital period ranges, covering about 10% of the total orbital period range, it is 20 It-ms. In the second method, we have measured the pulse arrival times at intervals of 100 s over the entire duration of the observation. The pulse arrival time data were used to put an upper limit on any periodic arrival time delay using the Lomb-Scargle periodogram. We have obtained a similar upper limit of 10 It-ms using the second method over the orbital period range of 500-10,000 s. This puts very stringent upper limits for the mass of the compact object except for the unlikely case of a complete face-on orientation of the binary system with respect to our line-of-sight. In the light of this measurement and the earlier reports, we discuss the possibility of this system being a neutron star with a supernovae fall-back accretion disk.