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Register a new userThe discovery of the 401 Hz accreting millisecond pulsar IGR J17498-2921 in a 3.8 hr orbit
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A. Papitto
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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.
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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.
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.
The accreting millisecond pulsars IGR J00291+5934 and SAX J1808.4-3658 are two compact binaries with very similar orbital parameters. The latter has been observed to evolve on a very short timescale of ~70 Myr which is more than an order of magnitude shorter than expected. There is an ongoing debate on the possibility that the pulsar spin-down power ablates the companion generating large amount of mass-loss in the system. It is interesting therefore to study whether IGR J00291+5934 does show a similar behaviour as its twin system SAX J1808.4-3658. In this work we present the first measurement of the orbital period derivative of IGR J00291+5934. By using XMM-Newton data recorded during the 2015 outburst and adding the previous results of the 2004 and 2008 outbursts, we are able to measure a 90% confidence level upper limit for the orbital period derivative of -5x10^-13<Pb_dot<6x10^-13. This implies that the binary is evolving on a timescale longer than ~0.5 Gyr, which is compatible with the expected timescale of mass transfer driven by angular momentum loss via gravitational radiation. We discuss the scenario in which the power loss from magnetic dipole radiation of the neutron star is hitting the companion star. If this model is applied to SAX J1808.4-3658 then the difference in orbital behavior can be ascribed to a different efficiency for the conversion of the spin-down power into energetic relativistic pulsar wind and X-ray/gamma-ray radiation for the two pulsars, with IGR J00291+5934 requiring an extraordinarily low efficiency of less than 5% to explain the observations. Alternatively, the donor in IGR J00291+5934 is weakly/not magnetized which would suppress the possibility of generating mass-quadrupole variations.
We report on a coherent timing analysis of the 163 Hz accreting millisecond X-ray pulsar IGR J17062-6143. Using data collected with the Neutron Star Interior Composition Explorer and XMM-Newton, we investigated the pulsar evolution over a timespan of four years. We obtained a unique phase-coherent timing solution for the stellar spin, finding the source to be spinning up at a rate of $(3.77pm0.09)times 10^{-15}$ Hz/s. We further find that the $0.4-6$ keV pulse fraction varies gradually between 0.5% and 2.5% following a sinusoidal oscillation with a $1210pm40$ day period. Finally, we supplemented this analysis with an archival Rossi X-ray Timing Explorer observation, and obtained a phase coherent model for the binary orbit spanning 12 years, yielding an orbital period derivative measurement of $(8.4pm2.0) times 10^{-12}$ s/s. This large orbital period derivative is inconsistent with a binary evolution that is dominated by gravitational wave emission, and is suggestive of highly non-conservative mass transfer in the binary system.
We present the discovery of a variable optical counterpart to the unidentified gamma-ray source 3FGL J0212.1+5320, and argue this is a new compact binary millisecond pulsar (MSP) candidate. We show 3FGL J0212.1+5320 hosts a semi-detached binary with a 0.86955$pm$0.00015 d orbital period and a F6-type companion star at an estimated distance of D=1.1$pm$0.2 kpc, with a radial velocity curve semi-amplitude K$_2$=214.1$pm$5.0 km s$^{-1}$ and a projected rotational velocity of Vsin(i)=73.2$pm$1.6 km s$^{-1}$. We find a hard X-ray source at the same location with a 0.5$-$10 keV luminosity L$_mathrm{X}$=2.6$times$10$^{32}$ (D/1.1 kpc)$^2$ erg s$^{-1}$, which strengthens the MSP identification. Our results imply a mass ratio q=M$_2$/M$_1$=0.26$^{+0.02}_{-0.03}$ if the companion star fills its Roche lobe, and q$gtrsim$0.26 in any case. This classifies 3FGL J0212.1+5320 as a redback binary MSP; if its MSP nature is confirmed, this will be the brightest compact binary MSP in the optical band (r$simeq$14.3 mag) and will have the longest orbital period among Galactic field systems (nearly 21 hr). Based on the light curve peak-to-peak amplitude ($Delta$r=0.19 mag), we further suggest that the orbital inclination is high and the putative pulsar mass is close to canonical (M$_1$$simeq$1.3$-$1.6 M$_odot$). Finally, we discuss the lack of heating signatures and asymmetric optical light curves in the context of other redback MSPs.
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