Arunava Mukherjee

Observational constraints on spinning, relativistic Bose-Einstein condensate stars

Bose-Einstein condensates (BECs) have been proposed as candidate states of matter for the interior of neutron stars. Specifically, Chavanis and Harko obtained the mass-radius relation for a BEC star and proposed that the recently discovered neutron stars with masses around 2M⊙ are BEC stars. They employed a barotropic equation of state (EOS), with one free parameter, that was first found by Colpi, Wasserman, and Shapiro (CSW), to describe them and derive stable equilibrium configurations of spinning BEC stars in General Relativity. In this work we show that while it is true that BECs allow for compact object masses as heavy as the heaviest observed ones, such stars cannot simultaneously have radii that are small enough to be consistent with the latest observations, in spite of the flexibility available in the EOS in the form of the free parameter. In fact, our conclusion applies to any spinning relativistic boson star that obeys the CSW EOS.

FRACTIONAL AMPLITUDE OF KILOHERTZ QUASI-PERIODIC OSCILLATION FROM 4U 1728-34: EVIDENCE OF DECLINE AT HIGHER ENERGIES

A kilohertz quasi-periodic oscillation (kHz QPO) is an observationally robust high-frequency timing feature detected from neutron star low-mass X-ray binaries (LMXBs). This feature can be very useful to probe the superdense core matter of neutron stars and the strong gravity regime. Although many models exist in the literature, the physical origin of kHz QPO is not known, and hence this feature cannot be used as a tool yet. The energy dependence of kHz QPO fractional rms amplitude is an important piece of the jigsaw puzzle to understand the physical origin of this timing feature. It is known that the fractional rms amplitude increases with energy at lower energies. At higher energies, the amplitude is usually believed to saturate, although this is not established. We combine tens of lower kHz QPOs from a neutron star LMXB 4U 1728-34 in order to improve the signal-to-noise ratio. Consequently, we, for the first time to the best of our knowledge, find a significant and systematic decrease of the fractional rms amplitude with energy at higher photon energies. Assuming an energy spectrum model, blackbody+powerlaw, we explore if the sinusoidal variation of a single spectral parameter can reproduce the above-mentioned fractional rms amplitude behavior. Our analysis suggests that the oscillation of any single blackbody parameter is favored over the oscillation of any single power-law parameter, in order to explain the measured amplitude behavior. We also find that the quality factor of a lower kHz QPO does not plausibly depend on photon energy.

HIGHLY COHERENT KILOHERTZ QUASI-PERIODIC OSCILLATIONS FROM THE NEUTRON STAR X-RAY BINARY EXO 1745-248

We report the discovery (

Timing and spectral study of XB 1254−690 using new RXTE PCA data

20\sigma

Terzan 5 transient IGR J17480−2446: variation of burst and spectral properties with spectral states

) of kilohertz quasi-periodic oscillations (kHz QPOs) at ~ 690 Hz from the transient neutron star low-mass X-ray binary EXO 1745-248. We find that this is a lower kHz QPO, and systematically study the time variation of its properties using smaller data segments with and without the shift-and-add technique. The quality (Q) factor occasionally significantly varies within short ranges of frequency and time. A high Q-factor (264.5 +- 38.5) of the QPO is found for a 200 s time segment, which might be the largest value reported in the literature. We argue that an effective way to rule out kHz QPO models is to observationally find such high Q-factors, even for a short duration, as many models cannot explain a high coherence. However, as we demonstrate, the shift-and-add technique cannot find a very high Q-factor which appears for a short period of time. This shows that the coherences of kHz QPOs can be higher than the already high values reported using this technique, implying further constraints on models. We also discuss the energy dependence of fractional rms amplitude and Q-factor of the kHz QPO.

Hysteresis in the spectral states of the neutron star low-mass X-ray binary EXO 1745-248

We have analysed the new Rossi X-ray Timing Explorer Proportional Counter Array data of the atoll neutron star (NS) low-mass X-ray binary (LMXB) system XB 1254-690. The colour-colour diagram shows that the source was in the high-intensity banana state. We have found two low-frequency candidate peaks with single trial significances of ≈2.65 x 10 ―8 and ≈7.39 x 10 ―8 in the power spectra. After taking into account the number of trials, the joint probability of appearance of these two peaks in the data set only by chance is ∼ 4.5 × 10 ―4 , and hence a low-frequency quasi-periodic oscillation can be considered to be detected with a significance of ∼ 4.5 x 10- 4 , or ∼ 3.5σ for the first time from this source. We have also done the first systematic X-ray spectral study of XB 1254-690, and found that, while one-component models are inadequate, three-component models are not required by the data. We have concluded that a combined broken-power-law and Comptonization model best describes the source continuum spectrum among 19 two-component models. The plasma temperature (∼3 keV) and the optical depth (∼7) of the Comptonization component are consistent with the previously reported values for other sources. However, the use of a broken-power-law component to describe the NS LMXB spectra has recently been started, and we have used this component for XB 1254-690 for the first time. We have attempted to determine the relative energy budgets of the accretion disc and the boundary layer using the best-fitting spectral model, and concluded that a reliable estimation of these budgets requires correlations among time variations of spectral properties in different wavelengths.