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Register a new userDiscovery of an accreting millisecond pulsar in the eclipsing binary system Swift J1749.4-2807
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We report on the discovery and the timing analysis of the first eclipsing accretion-powered millisecond X-ray pulsar (AMXP): SWIFT J1749.4-2807. The neutron star rotates at a frequency of ~517.9 Hz and is in a binary system with an orbital period of 8.8 hrs and a projected semi-major axis of ~1.90 lt-s. Assuming a neutron star between 0.8 and 2.2 M_o and using the mass function of the system and the eclipse half-angle, we constrain the mass of the companion and the inclination of the system to be in the ~0.46-0.81 M_o and $sim74.4^o-77.3^o range, respectively. To date, this is the tightest constraint on the orbital inclination of any AMXP. As in other AMXPs, the pulse profile shows harmonic content up to the 3rd overtone. However, this is the first AMXP to show a 1st overtone with rms amplitudes between ~6% and ~23%, which is the strongest ever seen, and which can be more than two times stronger than the fundamental. The fact that SWIFT J1749.4-2807 is an eclipsing system which shows uncommonly strong harmonic content suggests that it might be the best source to date to set constraints on neutron star properties including compactness and geometry.
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We present an analysis of the Swift BAT and XRT data of GRB060602B, which is most likely an accreting neutron star in a binary system and not a gamma-ray burst. Our analysis shows that the BAT burst spectrum is consistent with a thermonuclear flash (type-I X-ray burst) from the surface of an accreting neutron star in a binary system. The X-ray binary nature is further confirmed by the report of a detection of a faint point source at the position of the XRT counterpart of the burst in archival XMM-Newton data approximately 6 years before the burst and in more recent XMM-Newton data obtained at the end of September 2006 (nearly 4 months after the burst). Since the source is very likely not a gamma-ray burst, we rename the source Swift J1749.4-2807, based on the Swift/BAT discovery coordinates. Using the BAT data of the type-I X-ray burst we determined that the source is at most at a distance of 6.7+-1.3 kpc. For a transiently accreting X-ray binary its soft X-ray behaviour is atypical: its 2-10 keV X-ray luminosity (as measured using the Swift/XRT data) decreased by nearly 3 orders of magnitude in about 1 day, much faster than what is usually seen for X-ray transients. If the earlier phases of the outburst also evolved this rapidly, then many similar systems might remain undiscovered because the X-rays are difficult to detect and the type-I X-ray bursts might be missed by all sky surveying instruments. This source might be part of a class of very-fast transient low-mass X-ray binary systems of which there may be a significant population in our Galaxy.
We report multi-wavelength observations of the unidentified Fermi object 2FGL J1653.6-0159. With the help of high-resolution X-ray observation, we have identified an X-ray and optical counterpart of 2FGL J1653.6-0159. The source exhibits a periodic modulation of 75 min in optical and possibly also in X-ray. We suggest that 2FGL J1653.6-0159 is a compact binary system with an orbital period of 75 min. Combining the gamma-ray and X-ray properties, 2FGL J1653.6-0159 is potentially a black widow/redback type gamma-ray millisecond pulsar (MSP). The optical and X-ray lightcurve profiles show that the companion is mildly heated by the high-energy emission and the X-rays are from intrabinary shock. Although no radio pulsation has been detected yet, we estimated that the spin period of the MSP is ~2ms based on a theoretical model. If pulsation can be confirmed in the future, 2FGL J1653.6-0159 will become the first ultracompact rotation-powered MSP.
We present a study of PSR J1723-2837, an eclipsing, 1.86 ms millisecond binary radio pulsar discovered in the Parkes Multibeam survey. Radio timing indicates that the pulsar has a circular orbit with a 15 hr orbital period, a low-mass companion, and a measurable orbital period derivative. The eclipse fraction of ~15% during the pulsars orbit is twice the Roche lobe size inferred for the companion. The timing behavior is significantly affected by unmodeled systematics of astrophysical origin, and higher-order orbital period derivatives are needed in the timing solution to account for these variations. We have identified the pulsars (non-degenerate) companion using archival ultraviolet, optical, and infrared survey data and new optical photometry. Doppler shifts from optical spectroscopy confirm the stars association with the pulsar and indicate a pulsar-to-companion mass ratio of 3.3 +/- 0.5, corresponding to a companion mass range of 0.4 to 0.7 Msun and an orbital inclination angle range of between 30 and 41 degrees, assuming a pulsar mass range of 1.4-2.0 Msun. Spectroscopy indicates a spectral type of G for the companion and an inferred Roche-lobe-filling distance that is consistent with the distance estimated from radio dispersion. The features of PSR J1723-2837 indicate that it is likely a redback system. Unlike the five other Galactic redbacks discovered to date, PSR J1723-2837 has not been detected as a gamma-ray source with Fermi. This may be due to an intrinsic spin-down luminosity that is much smaller than the measured value if the unmeasured contribution from proper motion is large.
We report the discovery of the millisecond pulsar (MSP) PSR J1950+2414 ($P=4.3$ ms) in a binary system with an eccentric ($e=0.08$) 22-day orbit in Pulsar ALFA survey observations with the Arecibo telescope. Its companion star has a median mass of 0.3 $M_odot$ and is most likely a white dwarf. Fully recycled MSPs like this one are thought to be old neutron stars spun-up by mass transfer from a companion star. This process should circularize the orbit, as is observed for the vast majority of binary MSPs, which predominantly have orbital eccentricities $e < 0.001$. However, four recently discovered binary MSPs have orbits with $0.027 < e < 0.44$; PSR J1950+2414 is the fifth such system to be discovered. The upper limits for its intrinsic spin period derivative and inferred surface magnetic field strength are comparable to those of the general MSP population. The large eccentricities are incompatible with the predictions of the standard recycling scenario: something unusual happened during their evolution. Proposed scenarios are a) initial evolution of the pulsar in a triple system which became dynamically unstable, b) origin in an exchange encounter in an environment with high stellar density, c) rotationally delayed accretion-induced collapse of a super-Chandrasekhar white dwarf, and d) dynamical interaction of the binary with a circumbinary disk. We compare the properties of all five known eccentric MSPs with the predictions of these formation channels. Future measurements of the masses and proper motion might allow us to firmly exclude some of the proposed formation scenarios.
We present a timing analysis of the 2009 outburst of the accreting millisecond X-ray pulsar Swift J1756.9-2508, and a re-analysis of the 2007 outburst. The source shows a short recurrence time of only ~2 years between outbursts. Thanks to the approximately 2 year long baseline of data, we can constrain the magnetic field of the neutron star to be 0.4x10^8 G < B < 9x10^8 G, which is within the range of typical accreting millisecond pulsars. The 2009 timing analysis allows us to put constraints on the accretion torque: the spin frequency derivative within the outburst has an upper limit of $|dot{ u}| < 3x10^-13 Hz/s at the 95% confidence level. A study of pulse profiles and their evolution during the outburst is analyzed, suggesting a systematic change of shape that depends on the outburst phase.
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