Simultaneous observations of a pair of kilohertz QPOs and a plausible 1860Hz QPO from an accreting neutron star system

We report an indication （3.22 or） of ≈ 1860 Hz quasi-periodic oscillations from a neutron star low-mass X-ray binary 4U 1636-536. If confirmed, this will be by far the highest frequency feature observed from an accreting neutron star system, and hence could be very useful in understanding such systems. This plausible timing feature was observed simultaneously with lower （≈ 585 Hz） and upper （≈ 904 Hz） kilohertz quasi-periodic oscillations. The two kilohertz quasi-periodic oscillation frequencies had a ratio of ≈ 1.5, and the frequency of the alleged ≈ 1860 Hz feature was close to the triple and the double values of these frequencies. This can be useful for constraining the models of all the three features. In particular, the ≈ 1860Hz feature could be （1） from a new and heretofore unknown class of quasi-periodic oscillations, or （2） the first observed overtone of lower or upper kilohertz quasi-periodic oscillations. Finally, we note that, although the relatively low significance of the ≈ 1860 Hz feature argues for caution, even a 3.22 σ feature at such a uniquely high frequency should be interesting enough to spur a systematic search in the archival data, as well as to scientifically motivate sufficiently large timing instruments for the next generation X-ray missions.

Quark-novae in neutron star - white dwarf binaries: a model for luminous (spin-down powered) sub-Chandrasekhar-mass Type Ia supernovae?

We show that, by appealing to a Quark-Nova （QN） in a tight binary system containing a massive neutron star and a CO white dwarf （WD）, a Type Ia explosion could occur. The QN ejecta collides with the WD, driving a shock that triggers carbon burning under degenerate conditions （the QN-Ia）. The conditions in the compressed low-mass WD （MwD 〈 0.9 M） in our model mimic those of a Chandrasekhar mass WD. The spin-down luminosity from the QN compact remnant （the quark star） pro- vides additional power that makes the QN-Ia light-curve brighter and broader than a standard SN-Ia with similar 56Ni yield. In QNe-Ia, photometry and spectroscopy are not necessarily linked since the kinetic energy of the ejecta has a contribution from spin-down power and nuclear decay. Although QNe-Ia may not obey the Phillips relationship, their brightness and their relatively ＂normal looking＂ light-curves mean they could be included in the cosmological sample. Light-curve fitters would be con- fused by the discrepancy between spectroscopy at peak and photometry and would correct for it by effectively brightening or dimming the QNe-Ia apparent magnitudes, thus over- or under-estimating the true magnitude of these spin-down powered SNe-Ia. Contamination of QNe-Ia in samples of SNe-Ia used for cosmological analyses could systematically bias measurements of cosmological parameters if QNe-Ia are numerous enough at high-redshift. The strong mixing induced by spin-down wind combined with the low 56Ni yields in QNe-Ia means that these would lack a secondary maximum in the/-band despite their luminous nature. We discuss possible QNe-Ia progenitors.

Broad-band spectroscopy of the transient X-ray binary pulsar KS 1947+300 during 2013 giant outburst: Detection of pulsating soft X-ray excess component

We present the results obtained from detailed timing and spectral studies of the Be/X-ray binary pulsar KS 1947＋300 during its 2013 giant outburst. We used data from Suzaku observations of the pulsar at two epochs, i.e. on 2013 October 22 （close to the peak of the outburst） and 2013 November 22. X- ray pulsations at - 18.81 s were clearly detected in the light curves obtained from both observations. Pulse periods estimated during the outburst showed that the pulsar was spinning up. The pulse profile was found to be single-peaked up to -10 keV beyond which a sharp peak followed by a dip-like feature appeared at hard X-rays. The dip-like feature has been observed up to -70 keV. The 1-110 keV broad-band spectroscopy of both observations revealed that the best-fit model was comprised of a partially absorbed Negative and Positive power law with EXponential cutoff （NPEX） continuum model along with a blackbody component for the soft X-ray excess and two Gaussian functions at 6.4 and 6.7 keV for emission lines. Both the lines were identified as emission from neutral and He-like iron atoms. To fit the spectra, we included the previously reported cyclotron absorption line at 12.2 keV. From the spin-up rate, the magnetic field of the pulsar was estimated to be -1.2 x 10^12 G and found to be comparable to that obtained from the detection of the cyclotron absorption feature. Pulse-phase resolved spectroscopy revealed the pulsating nature of the soft X-ray excess component in phase with the continuum flux. This confirms that the accretion column and/or accretion stream are the most probable regions of the soft X-ray excess emission in KS1947＋300. The presence of the pulsating soft X-ray excess in phase with continuum emission may be the possible reason for not observing the dip at soft X-rays.

How will our knowledge of short gamma-ray bursts affect the distance measurement of binary neutron stars?

Gravitational waves from binary neutron stars associated with short gamma-ray bursts have drawn considerable attention due to their prospect in cosmology.For such events,the sky locations of sources can be pinpointed with techniques such as identifying the host galaxies.However,the cosmological applications of these events still suffer from the problem of degeneracy between luminosity distance and inclination angle.To address this issue,a technique was proposed in previous study,i.e.,using the collimation property of short gamma-ray bursts.Based on the observations,we assume that the cosine of inclination follows a Gaussian distribution,which may act as a prior in the Bayes analysis to break the degeneracy.This paper investigates the effects of different Gaussian priors and detector configurations on distance measurement and cosmological research.We first derive a simplified Fisher information matrix for demonstration,and then conduct quantitative analyses via simulation.By varying the number of third-generation detectors and the scale of prior,we generate four catalogs of 1000 events.It is shown that,in the same detecting period,a network of detectors can recognize more and farther events than a single detector.Besides,adopting tighter prior and employing multiple detectors both decrease the error of luminosity distance.Also considered is the performance of a widely adopted formula in the error budget,which turns out to be a conservative choice in each case.As for cosmological applications,for theΛCDM model,500,200,600,and 300 events are required for the four configurations to achieve 1%H;accuracy.With all 1000 events in each catalog,H;and?;can be constrained to(0.66%,0.37%,0.76%,0.49%),and(0.010,0.006,0.013,0.010),respectively.The results of the Gaussian process also show that the gravitational wave standard siren can serve as a probe of cosmology at high redshifts.

Search for pulsations in the LMXB EXO 0748-676

We present here results from our search for X-ray pulsations of the neu- tron star in the low mass X-ray binary EXO 0748-676 at a frequency near the burst- oscillation frequency of 44.7 Hz. Using the observations made with the Proportional Counter Array onboard the Rossi X-ray Timing Explorer, we did not find any pulsations in the frequency band of 44.4Hz to 45.0Hz and obtained a 3σ upper limit of 0.47% on the pulsed fraction for any possible underlying pulsation in this frequency band. We also discuss the importance of EXO 0748-676 as a promising source for the detection of Gravitational Waves.

Population synthesis of ultra-compact X-ray binaries

Ultra-compact X-ray binaries （UCXBs） are very interesting and impor- tant objects. By taking the population synthesis approach to the evolution of binaries, we carry out a detailed study of UCXBs. We estimate that there are - 5000-10000 UCXBs in the Galaxy, and their birthrates are - 2.6-7.5×10-4 yr-1. Most UCXBs are transient X-ray sources, but their X-ray luminosities are much lower than those of persistent sources. Therefore, the majority of observed UCXBs should be persistent sources. About 40% - 70% of neutron stars （NSs） in UCXBs form via an accretion- induced collapse from an accreting ONe white dwarf （WD）, 1%-10% of NSs in UCXBs form via core-collapse supernovae and others form via the evolution-induced collapse of a naked helium star. About 50% - 80% of UCXBs have naked helium star donors, 5% - 10% of UCXBs have HeWD donors, 15% - 40% of UCXBs have COWD donors and UCXBs with ONeWD indicates that the uncertainty mainly comes which develops in these systems. donors are negligible. Our investigation from evolution of the common-envelope

Orbital X-ray modulation study of three supergiant HMXBs

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 15-50keV 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）×10^-7d d-1 on the period derivative. We have also accurately determined an orbital period of 18.5482（88）d for the intermediate system IGR J 18483-0311, which displays an unusual behavior 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.

Evolution of low-mass X-ray binaries:dependence on the massof the compact object

We perform numerical calculations to simulate the evolution of low-mass X-ray binary systems. For the accreting compact object we consider the initial mass of 1.4, 10, 20, 100, 200, 500 and 1000 Mo, corresponding to neutron stars （NSs）, stellar- mass black holes （BHs） and intermediate-mass BHs. Mass transfer in these binaries is driven by nuclear evolution of the donors and/or orbital angular momentum loss due to magnetic braking and gravitational wave radiation. For the different systems, we determine their bifurcation periods Pbif that separate the formation of converging systems from the diverging ones, and show that Pbif changes from ~ 1 d to ≥ 3 d for a 1 Mo donor star, with increasing initial accretor mass from 1.4 to 1000 Mo. This means that the dominant mechanism of orbital angular momentum loss changes from magnetic braking to gravitational radiation. As an illustration we compare the evolution of binaries consisting of a secondary star of 1 Mo at a fixed initial period of 2 d. In the case of the NS or stellar-mass BH accretor, the system evolves to a well-detached He white dwarf-neutron star/black hole pair, but it evolves to an ultra- compact binary if the compact object is an intermediate-mass BH. Thus the binary evolution heavily depends upon the mass of the compact object. However, we show that the final orbital period-white dwarf mass relation found for NS low-mass X-ray binaries is fairly insensitive to the initial mass of the accreting star, even if it is an intermediate-mass BH.

Optical observations of LIGO source GW 170817 by the Antarctic Survey Telescopes at Dome A,Antarctica

The LIGO detection of gravitational waves(GW) from merging black holes in 2015 marked the beginning of a new era in observational astronomy. The detection of an electromagnetic signal from a GW source is the critical next step to explore in detail the physics involved. The Antarctic Survey Telescopes(AST3),located at Dome A, Antarctica, is uniquely situated for rapid response time-domain astronomy with its continuous night-time coverage during the austral winter. We report optical observations of the GW source(GW 170817) in the nearby galaxy NGC 4993 using AST3. The data show a rapidly fading transient at around 1 day after the GW trigger, with the i-band magnitude declining from 17:23 ± 0:13 magnitude to 17:72 ± 0:09 magnitude in ~1:8 h. The brightness and time evolution of the optical transient associated with GW 170817 are broadly consistent with the predictions of models involving merging binary neutron stars. We infer from our data that the merging process ejected about ~10^(-2) solar mass of radioactive material at a speed of up to 30% the speed of light.