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Register a new userDiscovery of a 205.89 Hz accreting-millisecond X-ray pulsar in the globular cluster NGC 6440
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We report the discovery of the second accreting millisecond X-ray pulsar (AMXP) in the globular cluster NGC 6440. Pulsations with a frequency of 205.89 Hz were detected with the Rossi X-Ray Timing Explorer on August 30th, October 1st and October 28th, 2009, during the decays of ~4 day outbursts of a newly X-ray transient source in NGC 6440. By studying the Doppler shift of the pulsation frequency, we find that the system is an ultra-compact binary with an orbital period of 57.3 minutes and a projected semi-major axis of 6.22 light-milliseconds. Based on the mass function, we estimate a lower limit to the mass of the companion to be 0.0067 M_sun (assuming a 1.4 M_sun neutron star). This new pulsar shows the shortest outburst recurrence time among AMXPs (~1 month). If this behavior does not cease, this AMXP has the potential to be one of the best sources in which to study how the binary system and the neutron star spin evolve. Furthermore, the characteristics of this new source indicate that there might exist a population of AMXPs undergoing weak outbursts which are undetected by current all-sky X-ray monitors. NGC 6440 is the only globular cluster to host two known AMXPs, while no AMXPs have been detected in any other globular cluster.
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We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of $sim339.97$ Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of $44.3$ minutes and a projected semi-major axis of $sim17.6$ lt-ms. Based on the mass function we estimate a minimum companion mass of 0.024 M$_{odot}$, which assumes a neutron star mass of 1.4 M$_{odot}$ and a maximum inclination angle of $75^{circ}$ (derived from the lack of eclipses and dips in the light-curve of the source). We find that the companion stars Roche-Lobe could either be filled by a hot ($5times 10^{6}$ K) pure helium white dwarf with a 0.028 M$_{odot}$ mass (implying $isimeq58^{circ}$) or an old (>5 Gyr) brown dwarf with metallicity abundances between solar/sub-solar and mass ranging in the interval 0.065$-$0.085 M$_{odot}$ (16 < $i$ < 21). During the outburst the broad-band energy spectra are well described by a superposition of a weak black-body component (kT$sim$ 0.5 keV) and a hard cutoff power-law with photon index $Gamma sim$ 1.7 and cut-off at a temperature kT$_esim$ 130 keV. Up to the latest Swift-XRT observation performed on 2016 July 19 the source has been observed in outburst for almost 150 days, which makes MAXI J0911-655 the second accreting millisecond X-ray pulsar with outburst duration longer than 100 days.
The dynamics of the plasma in the inner regions of an accretion disk around accreting millisecond X-ray pulsars is controlled by the magnetic field of the neutron star. The interaction between an accretion disk and a strong magnetic field is not well-understood, particularly at low accretion rates (the so-called ``propeller regime). This is due in part to the lack of clear observational diagnostics to constrain the physics of the disk-field interaction. Here we associate the strong ~1 Hz modulation seen in the accreting millisecond X-ray pulsar NGC 6440 X-2 with an instability that arises when the inner edge of the accretion disk is close to the corotation radius (where the stellar rotation rate matches the Keplerian speed in the disk). A similar modulation has previously been observed in another accreting millisecond X-ray pulsar (SAX J1808.4-3658) and we suggest that the two phenomena are related and that this may be a common phenomenon among other magnetized systems. Detailed comparisons with theoretical models suggest that when the instability is observed, the interaction region between the disk and the field is very narrow -- of the order of 1 km. Modelling further suggests that there is a transition region (~1-10 km) around the corotation radius where the disk-field torque changes sign from spin up to spin down. This is the first time that a direct observational constraint has been placed on the width of the disk-magnetosphere interaction region, in the frame of the trapped-disk instability model.
We report on the serendipitous discovery of a 442-Hz pulsar during a Rossi X-ray Timing Explorer (RXTE) observation of the globular cluster NGC 6440. The oscillation is detected following a burst-like event which was decaying at the beginning of the observation. The time scale of the decay suggests we may have seen the tail-end of a long-duration burst. Low-mass X-ray binaries (LMXBs) are known to emit thermonuclear X-ray bursts that are sometimes modulated by the spin frequency of the star, the so called burst oscillations. The pulsations reported here are peculiar if interpreted as canonical burst oscillations. In particular, the pulse train lasted for ~500 s, much longer than in standard burst oscillations. The signal was highly coherent and drifted down by ~2x10^-3 Hz, much smaller than the ~Hz drifts typically observed during normal bursts. The pulsations are reminiscent of those observed during the much more energetic ``superbursts, however, the temporal profile and the energetics of the burst suggest that it was not the tail end nor the precursor feature of a superburst. It is possible that we caught the tail end of an outburst from a new `intermittent accreting X-ray millisecond pulsar, a phenomenon which until now has only been seen in HETE J1900.1$-$2455 (Galloway et al. 2007). We note that (Kaaret et al. 2003) reported the discovery of a 409.7 Hz burst oscillation from SAX J1748.9-2021, also located in NGC 6440. However, Chandra X-ray Observatory imaging indicates it contains several point-like X-ray sources, thus the 442 Hz object is likely a different source.
We report on the detection of a 400.99018734(1) Hz coherent signal in the Rossi X-ray Timing Explorer light curves of the recently discovered X-ray transient, IGR J17498-2921. By analysing the frequency modulation caused by the orbital motion observed between August 13 and September 8, 2011, we derive an orbital solution for the binary system with a period of 3.8432275(3) hr. The measured mass function, f(M_2, M_1, i)=0.00203807(8) Msun, allows to set a lower limit of 0.17 Msun on the mass of the companion star, while an upper limit of 0.48 Msun is set by imposing that the companion star does not overfill its Roche lobe. We observe a marginally significant evolution of the signal frequency at an average rate of -(6.3 +/- 1.9)E-14 Hz/s. The low statistical significance of this measurement and the possible presence of timing noise hampers a firm detection of any evolution of the neutron star spin. We also present an analysis of the spectral properties of IGR J17498-2921 based on the observations performed by the Swift-X-ray Telescope and the RXTE-Proportional Counter Array between August 12 and September 22, 2011. During most of the outburst, the spectra are modeled by a power-law with an index Gamma~1.7-2, while values of ~3 are observed as the source fades into quiescence.
We present results of targeted searches for signatures of non-radial oscillation modes (such as r- and g-modes) in neutron stars using {it RXTE} data from several accreting millisecond X-ray pulsars (AMXPs). We search for potentially coherent signals in the neutron star rest frame by first removing the phase delays associated with the stars binary motion and computing FFT power spectra of continuous light curves with up to $2^{30}$ time bins. We search a range of frequencies in which both r- and g-modes are theoretically expected to reside. Using data from the discovery outburst of the 435 Hz pulsar XTE J1751$-$305 we find a single candidate, coherent oscillation with a frequency of $0.5727597 times u_{spin} = 249.332609$ Hz, and a fractional Fourier amplitude of $7.46 times 10^{-4}$. We estimate the significance of this feature at the $1.6 times 10^{-3}$ level, slightly better than a $3sigma$ detection. We argue that possible mode identifications include rotationally-modified g-modes associated with either a helium-rich surface layer or a density discontinuity due to electron captures on hydrogen in the accreted ocean. Alternatively, the frequency could be identified with that of an inertial mode or an r-mode modified by the presence of a solid crust, however, the r-mode amplitude required to account for the observed modulation amplitude would induce a large spin-down rate inconsistent with the observed pulse timing measurements. For the AMXPs XTE J1814$-$338 and NGC 6440 X-2 we do not find any candidate oscillation signals, and we place upper limits on the fractional Fourier amplitude of any coherent oscillations in our frequency search range of $7.8times 10^{-4}$ and $5.6 times 10^{-3}$, respectively. We briefly discuss the prospects and sensitivity for similar searches with future, larger X-ray collecting area missions.
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