Ramanpreet Kaur

Quasi-periodic oscillations in XTE J0111.2-7317 : Highest frequency among the HMXB pulsars

We report the discovery of quasi-periodic oscillations (QPOs) in the high-mass X-ray binary (HMXB) pulsar XTE J0111.2-7317 during a transient outburst of the source in 1998 December. Using observations made with the Proportional Counter Array of the Rossi X-Ray Timing Explorer during the second peak and the declining phase of the outburst, we have discovered a QPO feature at a frequency of 1.27 Hz. We have ruled out the possibility that the observed feature could instead be from the neighboring bright X-ray pulsar SMC X-1. This is the highest frequency QPO ever detected in an HMXB pulsar. In the absence of a cyclotron absorption feature in the X-ray spectrum, the QPO, along with the pulse period and X-ray flux measurement, helps us to constrain the magnetic field strength of the neutron star.

A study of the long-term evolution of quasi-periodic oscillations in the accretion-powered X-ray pulsar 4U 1626-67

We report here a study of the long-term properties of quasiperiodic oscillations (QPOs) in an unusual accreting X-ray pulsar, 4U 1626–67. This is a unique accretion-powered X-ray pulsar in which we have found the QPOs to be present during all sufficiently long X-ray observations with a wide range of X-ray observatories. In the present spin-down era of this source, the QPO central frequency is found to be decreasing. In the earlier spin-up era of this source, there are only two reports of QPO detections, in 1983 with EXOSAT and 1988 with Ginga with an increasing trend. The QPO frequency evolution in 4U 1626–67 during the last 22 years changed from a positive to a negative trend, somewhat coincident with the torque reversal in this source. In the accretion-powered X-ray pulsars, the QPO frequency is directly related to the inner radius of the accretion disk, as per the Keplerian frequency model (KFM) and the beat frequency mdel (BFM). A gradual depletion of accretion disk is reported earlier from the X-ray spectral, flux, and pulse profile measurements. The present QPO frequency evolution study shows that X-ray flux and mass accretion rate may not change by the same factor; hence the simple KFM and BFM are not able to explain the QPO evolution in this source. This is the only X-ray pulsar to show persistent QPOs and is also the first accreting X-ray pulsar in which the QPO history is reported for a long timescale relating it with the long-term evolution of the accretion disk.

Swift detection of an intermediately long X-ray burst from the very faint X-ray binary XMMU J174716.1–281048

We report on the Swift detection of a thermonuclear X-ray burst from the very-faint quasi-persistent neutron star X-ray binary XMMU J174716.1-281048, which triggered the satellite’s Burst Alert Telescope (BAT) on 2010 August 13. Analysis of the BAT spectrum yields an observed bolometric peak flux of ≃4.5 × 10−8 erg cm−2 s−1, from which we infer a source distance of ≲8.4 kpc. Follow-up observations with Swift’s X-ray Telescope (XRT) suggest that the event had a duration of ≃3 h and a total radiated energy of ≃9 × 1040 erg, which classify it as an intermediately long X-ray burst. This is only the second X-ray burst ever reported from this source. Inspection of Swift/XRT observations performed between 2007 and 2010 suggests that the 2-10 keV accretion luminosity of the system is ≃5 × 1034 erg s−1 for an assumed distance of 8.4 kpc. Despite being transient, XMMU J174716.1-281048 appears to have been continuously active since its discovery in 2003.

Near-infrared/optical identification of five low-luminosity X-ray pulsators

We present the identification of the most likely near-infrared (NIR)/optical counterparts of five low-luminosity X-ray pulsators (AX J1700.1−4157, AX J1740.1−2847, AX J1749.2−2725, AX J1820.5−1434 and AX J1832.3−0840) which have long pulse periods (>150 s). The X-ray properties of these systems suggest that they are likely members of persistent high-mass X-ray binaries or intermediate polars (IPs). Using our Chandra observations, we detected the most likely counterparts of three sources (excluding AX J1820.5−1434 and AX J1832.3−0840) in their European Southern Observatory-New Technology Telescope (ESO-NTT) NIR observations, and a possible counterpart for AX J1820.5−1434 and AX J1832.3−0840 in the Two Micron All Sky Survey and Digitized Sky Survey observations, respectively. We also performed the X-ray timing and spectral analysis for all the sources using our XMM-Newton observations, which further helped us to constrain the nature of these systems. Our multiwavelength observations suggest that AX J1749.2−2725 and AX J1820.5−1434 most likely harbour accreting neutron stars, while AX J1700.1−4157, AX J1740.1−2847 and AX J1832.3−0840 could be IPs.

Multiwavelength study of the transient X‐ray binary IGR J01583+6713

We have investigated multiband optical photometric variability and stability of the Hα line profile of the transient X-ray binary IGR J01583+6713. We set an upper limit of 0.05 mag on photometric variations in the V band over a time-scale of three months. The Ha line is found to consist of non-Gaussian profile and quite stable for a duration of two months. We have identified the spectral type of the companion star to be B2 IVe while the distance to the source is estimated to be ∼ 4.0 kpc. Along with the optical observations, we have also carried out analysis of X-ray data from three short observations of the source, two with the Swift-XRT and one with the RXTE-PCA. We have detected a variation in the absorption column density, from a value of 22.0 × 10 22 cm -2 immediately after the outburst down to 2.6 × 10 22 cm -2 four months afterwards. In the quiescent state, the X-ray absorption is consistent with the optical reddening measurement of E(B - V) = 1.46 mag. From one of the Swift observations, during which the X-ray intensity was higher, we have a possible pulse detection with a period of 469.2 s. For a Be X-ray binary, this indicates an orbital period in the range of 216-561 d for this binary system.

A search for near-infrared counterparts of two faint neutron star X-ray transients: XMMU J174716.1−281048 and SAX J1806.5−2215

We present our near-infrared (NIR) imaging observations of two neutron star low-mass X-ray binaries XMMU J174716.1−281048 and SAX J1806.5−2215 obtained using the PANIC instrument on the 6.5-metre Magellan telescope and the WIYN High-Resolution Infrared Camera instrument on the 3.5-metre WIYN telescope, respectively. Both sources are members of the class of faint to very faint X-ray binaries and undergo very long X-ray outburst, hence classified as ‘quasi-persistent X-ray binaries’. While XMMU J174716.1−281048 was active for almost 12 yr between 2003 and 2015, SAX J1806.5−2215 has been active for more than 5 yr now since 2011. From our observations, we identify two NIR stars consistent with the Chandra X-ray error circle of XMMU J174716.1−281048. The comparison of our observations with the United Kingdom Infrared Telescope (UKIRT) Galactic plane observations taken during the same outburst, colour–colour diagram analysis and spectral energy distribution suggest that both stars are probably a part of the field population and are likely high-mass stars. Hence possibly neither of the two stars is a true NIR counterpart. For the faint X-ray binary SAX J1806.5−2215 during its current outburst, we detected an NIR star in our K-band WIYN observations consistent with its Chandra error circle. The identified NIR star was not detected during the UKIRT observations taken during its quiescent state. The comparison of two observations suggests that there was an increase in flux by at least one magnitude of the detected star during our observations, and hence suggests the detection of the likely counterpart of SAX J1806.5−2215.

Revealing a new symbiotic X-ray binary with Gemini Near-infrared Integral Field Spectrograph

We use K-band spectroscopy of the counterpart to the rapidly variable X-ray transient XMMU J174445.5-295044 to identify it as a new symbiotic X-ray binary. XMMU J174445.5-295044 has shown a hard X-ray spectrum (we verify its association with an Integral/IBIS 18-40 keV detection in 2013 using a short Swift/XRT observation), high and varying NH , and rapid flares on timescales down to minutes, suggesting wind accretion onto a compact star. We observed its near-infrared counterpart using the Near-infrared Integral Field Spectrograph (NIFS) at Gemini-North, and classify the companion as ∼M2 III. We infer a distance of 3.1 −1.1 kpc (conservative 1σ errors), and therefore calculate that the observed X-ray luminosity (2-10 keV) has reached to at least 4×10 erg s. We therefore conclude that the source is a symbiotic X-ray binary containing a neutron star (or, less likely, black hole) accreting from the wind of a giant.

Chandra and XMM-Newton observations of the low-luminosity X-ray pulsators SAX J1324.4−6200 and SAX J1452.8−5949

We present results from our Chandra and XMM-Newton observations of two low-luminosity X-ray pulsators SAX J1324.4-6200 and SAX J1452.8-5949 which have spin periods of 172 and 437 s, respectively. The XMM-Newton spectra for both sources can be fitted well with a simple power-law model of photon index, Gamma similar to 1.0. A blackbody model can equally well fit the spectra with a temperature, kT similar to 2 keV, for both sources. During our XMM-Newton observations, SAX J1324.4-6200 is detected with coherent X-ray pulsations at a period of 172.86 +/- 0.02 s while no pulsations with a pulse fraction greater than 18 per cent (at 95 per cent confidence level) in 0.2-12 keV energy band are detected in SAX J1452.8-5949. The spin period of SAX J1324.4-6200 is found to be increasing on a time-scale of (P) over dot = (6.34 +/- 0.08) x 10(-9) s s(-1) which would suggest that the accretor is a neutron star and not a white dwarf. Using subarcsec spatial resolution of the Chandra telescope, possible counterparts are seen for both sources in the near-infrared images obtained with the son of infrared spectrometer and array camera (SOFI) instrument on the New Technology Telescope. The X-ray and near-infrared properties of SAX J1324.4-6200 suggest it to be a persistent high-mass accreting X-ray pulsar at a distance <= 8 kpc. We identify the near-infrared counterpart of SAX J1452.8-5949 to be a late-type main-sequence star at a distance <= 10 kpc, thus ruling out SAX J1452.8-5949 to be a high-mass X-ray binary. However, with the present X-ray and near-infrared observations, we cannot make any further conclusive conclusion about the nature of SAX J1452.8-5949.