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Register a new userSpectral and timing properties of the X-ray emission from the millisecond pulsar PSR B1821-24
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We report results on the timing and spectral analysis of observations of the millisecond pulsar PSR B1821-24 with RXTE, BeppoSAX and Chandra. The X-ray light curve is characterized by two narrow peaks at a phase distance of 0.452+/-0.002. The average pulsed emission, over the range 1.6-20 keV, is well represented by a single power law with a photon index alpha=1.30 +0.05 -0.02 and unabsorbed (2-10 keV) pulsed X-ray flux of 3.9x10^(-13) erg cm^(-2) s^(-1). We searched for a possible bunching of X-ray photons to verify if the X ray emission has a time structure similar to that of giant pulses and found a negative result.
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We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). 46 X-ray sources are detected, of which 12 lie within one core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources for which we determine the X-ray luminosity function down to a limit of about 6xE30 erg/s. For the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 is measured and found to be best described by a power law with photon index ~ 1.2. Marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum is found, which could be interpreted as cyclotron emission from a corona above the pulsars polar cap if the the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0 keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightest unidentified sources is presented. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T_eff^infty = 90^{+30}_{-10} eV and the radius R_NS^infty = 14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core of M28. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.
We report a 5.4sigma detection of pulsed gamma rays from PSR B1821-24 in the globular cluster M28 using ~44 months of Fermi Large Area Telescope (LAT) data that have been reprocessed with improved instrument calibration constants. We constructed a phase-coherent ephemeris, with post-fit residual RMS of 3 mu s, using radio data spanning ~23.2 years, enabling measurements of the multi-wavelength light curve properties of PSR B1821-24 at the milliperiod level. We fold RXTE observations of PSR B1821-24 from 1996 to 2007 and discuss implications on the emission zones. The gamma-ray light curve consists of two peaks, separated by 0.41$pm$0.02 in phase, with the first gamma-ray peak lagging the first radio peak by 0.05$pm$0.02 in phase, consistent with the phase of giant radio pulses. We observe significant emission in the off-peak interval of PSR B1821-24 with a best-fit LAT position inconsistent with the core of M28. We do not detect significant gamma-ray pulsations at the spin or orbital periods from any other known pulsar in M28, and we place limits on the number of energetic pulsars in the cluster. The derived gamma-ray efficiency, ~2%, is typical of other gamma-ray pulsars with comparable spin-down power, suggesting that the measured spin-down rate ($2.2times10^{36}$ erg s$^{-1}$) is not appreciably distorted by acceleration in the cluster potential. This confirms PSR B1821-24 as the second very energetic millisecond pulsar in a globular cluster and raises the question of whether these represent a separate class of objects that only form in regions of very high stellar density
IGR J17511-3057 is the second X-ray transient accreting millisecond pulsar discovered by INTEGRAL. It was in outburst for about a month from September 13, 2009. The broad-band average spectrum is well described by thermal Comptonization with an electron temperature of kT_e ~ 25 keV, soft seed photons of kT_bb ~ 0.6 keV, and Thomson optical depth tau_T ~ 2 in a slab geometry. During the outburst the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth being constant within errors. We fitted the outburst profile with the exponential model, and using the disk instability model we inferred the outer disk radius to be (4.8 - 5.4) times 1010 cm. The INTEGRAL and RXTE data reveal the X-ray pulsation at a period of 4.08 milliseconds up to ~ 120 keV. The pulsed fraction is shown to decrease from ~22% at 3 keV to a constant pulsed fraction of ~17-18% between 7-30 keV, and then to decrease again down to ~13% at 60 keV. The nearly sinusoidal pulses show soft lags monotonically increasing with energy to about 0.2 ms at 10-20 keV similar to those observed in other accreting pulsars. The short burst profiles indicate hydrogen-poor material at ignition, which suggests either that the accreted material is hydrogen-deficient, or that the CNO metallicity is up to a factor of 2 times solar. However, the variation of burst recurrence time as a function of m (inferred from the X-ray flux) is much smaller than predicted by helium-ignition models.
We analyze the spectral and timing properties of IGR J17498-2921 and the characteristics of X-ray bursts to constrain the physical processes responsible for the X-ray production in this class of sources. The broad-band average spectrum is well-described by thermal Comptonization with an electron temperature of kT_e ~ 50 keV, soft seed photons of kT_bb ~ 1 keV, and Thomson optical depth taut ~ 1 in a slab geometry. The slab area corresponds to a black body radius of R_bb ~9 km. During the outburst, the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth that are constant within the errors. This behavior has been interpreted as indicating that the X-ray emission originates above the neutron star (NS) surface in a hot slab (either the heated NS surface or the accretion shock). The INTEGRAL, RXTE, and Swift data reveal the X-ray pulsation at a period of 2.5 milliseconds up to ~65 keV. The pulsed fraction is consistent with being constant, i.e. energy independent and has a typical value of 6-7%. The nearly sinusoidal pulses show soft lags that seem to saturate near 10 keV at a rather small value of ~ -60mu s with those observed in other accreting pulsars. The short burst profiles indicate that there is a hydrogen-poor material at ignition, which suggests either that the accreted material is hydrogen-deficient, or that the CNO metallicity is up to a factor of about two times solar. However, the variation in the burst recurrence time as a function of dot{m} (inferred from the X-ray flux) is much smaller than predicted by helium-ignition models.
Pulsating thermal X-ray emission from millisecond pulsars can be used to obtain constraints on the neutron star equation of state, but to date only five such sources have been identified. Of these five millisecond pulsars, only two have well constrained neutron star masses, which improve the determination of the radius via modelling of the X-ray waveform. We aim to find other millisecond pulsars that already have well constrained mass and distance measurements that show pulsed thermal X-ray emission in order to obtain tight constraints on the neutron star equation of state. The millisecond pulsar PSR~J1909--3744 has an accurately determined mass, M = 1.54$pm$0.03 M$_odot$ (1 $sigma$ error) and distance, D = 1.07$pm$0.04 kpc. We analysed {em XMM-Newton} data of this 2.95 ms pulsar to identify the nature of the X-ray emission. We show that the X-ray emission from PSR~J1909--3744 appears to be dominated by thermal emission from the polar cap. Only a single component model is required to fit the data. The black-body temperature of this emission is kT=0.26ud{0.03}{0.02} keV and we find a 0.2--10 keV un-absorbed flux of 1.1 $times$ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$ or an un-absorbed luminosity of 1.5 $times$ 10$^{30}$ erg s$^{-1}$. Thanks to the previously determined mass and distance constraints of the neutron star PSR~J1909--3744, and its predominantly thermal emission, deep observations of this object with future X-ray facilities should provide useful constraints on the neutron star equation of state.
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