An Ultra Low Mass and Small Radius Compact Object in 4U 1746-37?

Photospheric radius expansion (PRE) bursts have already been used to constrain the masses and radii of neutron stars. RXTE observed three PRE bursts in 4U 1746-37, all with low touchdown fluxes. We discuss here the possibility of low mass neutron star in 4U 1746-37 because the Eddington luminosity depends on stellar mass. With typical values of hydrogen mass fraction and color correction factor, a Monte-Carlo simulation was applied to constrain the mass and radius of neutron star in 4U 1746-37. 4U 1746-37 has a high inclination angle. Two geometric effects, the reflection of the far side accretion disc and the obscuration of the near side accretion disc have also been included in the mass and radius constraints of 4U 1746-37. If the reflection of the far side accretion disc is accounted, a low mass compact object (mass of $0.41\pm0.14~M_{\odot}$ and radius of $8.73\pm1.54~\rm km$ at 68% confidence) exists in 4U 1746-37. If another effect operated, 4U 1746-37 may contain an ultra low mass and small radius object ($M=0.21\pm0.06~M_{\odot},~R=6.26\pm0.99~\rm km$ at 68% confidence). Combined all possibilities, the mass of 4U 1746-37 is $0.41^{+0.70}_{-0.30}~M_\odot$ at 99.7% confidence. For such low mass NS, it could be reproduced by a self-bound compact star, i.e., quark star or quark-cluster star.Comment: accepted by Ap

The NuSTAR View of a QPO Evolution of GRS 1915+105

We report a timing analysis of the black hole binary GRS 1915+105 with the NuSTAR observatory. A strong type-C QPO below 2 Hz appears in the power density spectrum during the whole observation, whose frequency is correlated with the 3-25 keV count rate. The QPO shows a sudden increase in frequency along with an increase in flux and a softening of the spectrum. We discuss the possible origin of the QPO and the reasons that lead to the QPO frequency variation. It is suggested that the reflection component has little influence on QPO frequency and the increase in QPO frequency could be associated with the inward motion of the outer part of the disk.Comment: 21 pages, 8 figures, published in A

Simultaneous constraints on the mass and radius of Aql X-1 from quiescence and X-ray burst observations

The measurement of neutron star mass and radius is one of the most direct way to distinguish between various dense matter equations of state. The mass and radius of accreting neutron stars hosted in low mass X-ray binaries can be constrained by several methods, including photospheric radius expansion from type-I X-ray bursts and from quiescent spectra. In this paper, we apply for the first time these two methods simultaneously to constrain the mass and radius of Aql X-1, as a reliable distance estimation, high signal-to-noise ratio quiescent spectra from Chandra and XMM-Newton, and photospheric radius expansion bursts from RXTE are available. This is also used to verify the consistency between the two methods, and to narrow down the uncertainties of the neutron star mass and radius. It is found that the distance to Aql X-1 should be in the range of $4.0-5.75$ kpc, based on the overlapping confidence regions between photospheric radius expansion burst and quiescent spectra methods. In addition, we show that the mass and radius determined for the compact star in Aql X-1 are compatible with strange star equations of state and conventional neutron star models.Comment: 16 pages, 9 figures, accepted by Ap

The accretion rate independence of horizontal branch oscillation in XTE J1701-462

We study the temporal and energy spectral properties of the unique neutron star low-mass X-ray binary XTE J1701-462. After assuming the HB/NB vertex as a reference position of accretion rate, the horizontal branch oscillation (HBO) of the HB/NB vertex is roughly 50 Hz. It indicates that the HBO is independent with the accretion rate or the source intensity. The spectral analysis shows $R_{\rm{in}}\propto\dot{M}_{\rm{Disk}}^{2.9\pm0.09}$ in the HB/NB vertex and $R_{\rm{in}}\propto\dot{M}_{\rm{Disk}}^{1.7\pm0.06}$ in the NB/FB vertex, which implies that different accretion rate may be produced in the HB/NB vertex and the NB/FB vertex. The Comptonization component could be fitted by constrained broken power law (CBPL) or nthComp. Different with GX 17+2, the frequencies of HBO positively correlate with the inner disk radius, which contradict with the prediction of Lense-Thirring precession model. XTE J1701-462, both in the Cyg-like phase and in the Sco-like phase, follows a positive correlation between the break frequency of broad band noise and the characteristic frequency of HBO, which is called the W-K relation. An anticorrelation between the frequency of HBO and photon energy is observed. Moreover, the rms of HBO increases with photon energy till ~10 keV. We discuss the possible origin of HBO from corona in XTE J1701-462.Comment: 45 pages, 18 figures, accepted by Ap

X-ray Timing Observations of PSR J1930+1852 in the Crab-like SNR G54.1+0.3

We present new X-ray timing and spectral observations of PSR J1930+1852, the young energetic pulsar at the center of the non-thermal supernova remnant G54.1+0.3. Using data obtained with the Rossi X-ray Timing Explorer and Chandra X-ray observatories we have derived an updated timing ephemeris of the 136 ms pulsar spanning 6 years. During this interval, however, the period evolution shows significant variability from the best fit constant spin-down rate of $\dot P = 7.5112(6) \times 10^{-13}$ s s$^{-1}$, suggesting strong timing noise and/or glitch activity. The X-ray emission is highly pulsed ($71\pm5$ modulation) and is characterized by an asymmetric, broad profile ($\sim 70$ duty cycle) which is nearly twice the radio width. The spectrum of the pulsed emission is well fitted with an absorbed power law of photon index $\Gamma = 1.2\pm0.2$; this is marginally harder than that of the unpulsed component. The total 2-10 keV flux of the pulsar is $1.7 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$. These results confirm PSR J1930+1852 as a typical Crab-like pulsar.Comment: 14 pages with 7 figures included, accepted to Ap

The Evolution of the Phase Lags Associated with the Type-C Quasi-periodic Oscillation in GX 339--4 during the 2006/2007 Outburst

We present the evolution of the phase lags associated with the type-C QPO in GX 339--4 during the rising phase of the 2006/2007 outburst. We find that the phase lags at the QPO frequency are always positive (hard), and show very different behavior between QPOs with frequencies below and above $\sim1.7$ Hz: when the QPO frequency is below $\sim1.7$ Hz, the phase lags increase both with QPO frequency and energy, while when the QPO frequency is above $\sim1.7$ Hz, the phase lags remain more or less constant. When the QPO frequency is higher than $\sim1.7$ Hz, a broad feature is always present in the lag-energy spectra at around 6.5 keV, suggesting that the reflection component may have a significant contribution to the phase lags. Below $\sim1.7$ Hz, the QPO rms first decreases with energy and then turns to almost flat, while above $\sim1.7$ Hz, the QPO rms increases with energy. During the transition from the low-hard state to the hard-intermediate state, the second harmonic and subharmonic of this QPO appear in the power density spectra. The second-harmonic and subharmonic phase lags show very similar evolution with their centroid frequencies. However, the energy dependence of the second-harmonic and subharmonic phase lags are quite different. Our results suggest that, at different phases of the outburst, different mechanisms may be responsible for the phase lags of the QPO. We briefly discuss the possible scenarios for producing the lags.Comment: 15 pages, 12 figures, accepted for publication in Ap

Cystic cavernous malformation of the cerebellopontine angle: Case report and literature review

<p>Abstract</p> <p>Background</p> <p>Cavernous malformations (CMs) in the cerebellopontine angle (CPA) are rare, and most of such CMs reported to date are solid and extend from the internal auditory canal into the CPA. In contrast, cystic CMs that arise in the CPA and do not involve the internal auditory canal and dura of the skull base are extremely rare.</p> <p>Case presentation</p> <p>A 50-year-old man presented with vertigo and progressive hearing loss in the right ear. MRI examination revealed a lesion in the CPA with solid and cystic components. Surgery was performed. Well-circumscribed adhesion to cranial nerves, the cerebellum, or the brain stem was noted during surgery. The lesion was totally resected. Pathological examination suggested the lesion to be a CM. At 1-year follow-up, the symptoms at presentation had resolved and no complications had occurred.</p> <p>Conclusion</p> <p>Although cystic CMs of the CPA have no established imaging features, a diagnosis of CMs may be suspected when a cystic lesion is present in the CPA and does not involve internal acoustic meatus or dura mater of the skull base. Skillful microsurgical techniques and monitoring of cranial nerves will secure good outcomes for patients with cystic CMs in the CPA.</p