Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userIntermittent pulsations in an accretion-powered millisecond pulsar
75
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We describe observations of the seventh accretion-powered millisecond pulsar, HETE J1900.1-2455 made with the Rossi X-ray Timing Explorer during the year of activity that followed its discovery in 2005 June. We detected intermittent pulsations at a peak fractional amplitude of 3%, but only in the first two months of the outburst. On three occasions during this time we observed an abrupt increase in the pulse amplitude, approximately coincident with the time of a thermonuclear burst, followed by a steady decrease on a timescale of approx. 10 d. HETE J1900.1-2455 has shown the longest active period by far for any transient accretion-powered millisecond pulsar, comparable instead to the outburst cycles for other transient X-ray binaries. Since the last detection of pulsations, HETE J1900.1-2455 has been indistinguishable from a low-accretion rate, non-pulsing LMXB; we hypothesize that other, presently active LMXBs may have also been detectable initially as millisecond X-ray pulsars.
rate research
Read More
Accreting millisecond X-ray pulsars are an important subset of low-mass X-ray binaries in which coherent X-ray pulsations can be observed during occasional, bright outbursts (X-ray luminosity $L_Xsim 10^{36}$ erg s$^{-1}$). These pulsations show that matter is being channeled onto the neutron stars magnetic poles. However, such sources spend most of their time in a low-luminosity, quiescent state ($L_Xlesssim 10^{34}$ erg s$^{-1}$), where the nature of the accretion flow onto the neutron star (if any) is not well understood. Here we report that the millisecond pulsar/low-mass X-ray binary transition object PSR J1023+0038 intermittently shows coherent X-ray pulsations at luminosities nearly 100 times fainter than observed in any other accreting millisecond X-ray pulsar. We conclude that in spite of its low luminosity PSR J1023+0038 experiences episodes of channeled accretion, a discovery that challenges existing models for accretion onto magnetized neutron stars.
We report on observations of the sixth accretion-powered millisecond pulsar, IGR J00291+5934, with the Rossi X-Ray Timing Explorer. The source is a faint, recurrent X-ray transient initially identified by INTEGRAL. The 599 Hz (1.67 ms) pulsation had a fractional rms amplitude of 8% in the 2-20 keV range, and its shape was approximately sinusoidal. The pulses show an energy-dependent phase delay, with the 6-9 keV pulses arriving up to 85 us earlier than those at lower energies. No X-ray bursts, dips, or eclipses were detected. The neutron star is in a circular 2.46 hr orbit with a very low-mass donor, most likely a brown dwarf. The binary parameters of the system are similar to those of the first known accreting millisecond pulsar, SAX J1808.4-3658. Assuming that the mass transfer is driven by gravitational radiation and that the 2004 outburst fluence is typical, the 3-yr recurrence time implies a distance of at least 4 kpc.
Neutron Stars are among the most exotic objects in the Universe. A neutron star, with a mass of 1.4-2 Solar masses within a radius of about 10-15 km, is the most compact stable configuration of matter in which degeneracy pressure can still balance gravity, since further compression would lead to gravitational collapse and formation of a black hole. As gravity is extreme, rotation is extreme: neutron stars are the fastest rotating stars known, with periods as short as a millisecond. The presence of a magnetic field not aligned with the rotation axis of the star is the origin of pulsating emission from these sources, which for this reason are dubbed pulsars. The discovery in 1998 of the first Accreting Millisecond X-ray Pulsar, started an exciting season of continuing discoveries. In the last 20 years, thanks to the extraordinary performance of astronomical detectors in the radio, optical, X-ray, and Gamma-ray bands, astrophysicists had the opportunity to thoroughly investigate the so-called Recycling Scenario: the evolutionary path leading to the formation of a Millisecond-spinning Pulsar. In this chapter we review the general properties of Accreting Millisecond X-ray Pulsars, which provide the first evidence that neutron stars are spun up to millisecond periods by accretion of matter and angular momentum from a (low-mass) companion star. We describe the general characteristics of this class of systems with particular attention to their spin and orbital parameters, their short-term and long-term evolution, as well as the information that can be drawn from their X-ray spectra.
Weakly magnetic, millisecond spinning neutron stars attain their very fast rotation through a 1E8-1E9 yr long phase during which they undergo disk-accretion of matter from a low mass companion star. They can be detected as accretion-powered millisecond X-ray pulsars if towards the end of this phase their magnetic field is still strong enough to channel the accreting matter towards the magnetic poles. When mass transfer is much reduced or ceases altogether, pulsed emission generated by particle acceleration in the magnetosphere and powered by the rotation of the neutron star is observed, preferentially in the radio and gamma-ray bands. A few transitional millisecond pulsars that swing between an accretion-powered X-ray pulsar regime and a rotationally-powered radio pulsar regime in response to variations of the mass in-flow rate have been recently identified. Here we report the detection of optical pulsations from a transitional pulsar, the first ever from a millisecond spinning neutron star. The pulsations were observed when the pulsar was surrounded by an accretion disk and originated inside the magnetosphere or within a few hundreds of kilometres from it. Energy arguments rule out reprocessing of accretion-powered X-ray emission and argue against a process related to accretion onto the pulsar polar caps; synchrotron emission of electrons in a rotation-powered pulsar magnetosphere seems more likely.
We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1-2455. Our R-band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Halpha emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km/s, placing a lower limit of 0.05 Msun on the secondary mass. For a 1.4 Msun primary, this implies that the orbital inclination is low, < 20 degrees. Utilizing the observed relationship between the secondary mass and orbital period in short period cataclysmic variables, we estimate the secondary mass to be ~0.085 Msun, which implies an upper limit of ~2.4 Msun for the primary mass.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
