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Register a new userThe puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence
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Maria Cristina Baglio
Publication date
2017
fields
Physics
and research's language is
English
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We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran Telescopio Canarias (GTC) in August 2014. Despite the source being in quiescence at the time of our observations, it showed a strong optical flaring activity, more pronounced at higher frequencies (i.e. the g band). Once the flares were subtracted, we tentatively recovered a sinusoidal modulation at the system orbital period in all bands, even if a significant phase shift with respect to an irradiated star, typical of accreting millisecond X-ray pulsars is detected. We conclude that the observed flaring could be a manifestation of the presence of an accretion disc in the system. The observed light curve variability could be explained by the presence of a superhump, which might be another proof of the formation of an accretion disc. In particular, the disc at the time of our observations was probably preparing to the new outburst of the source, that happened just a few months later, in 2015.
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We report on optical and NIR observations obtained during and after the 2004 December discovery outburst of the X-ray transient and accretion-powered millisecond pulsar IGR J00291+5934. Our observations monitored the evolution of the brightness and the spectral properties of J00291 during the outburst decay towards quiescence. We also present optical, NIR and Chandra observations obtained during true quiescence. Photometry of the field during outburst reveals an optical and NIR counterpart that brightened from R~23 to R~17 and from K=19 to K~16. Spectral analysis of the RIJHK broadband photometry shows excess in the NIR bands that may be due to synchrotron emission. The Halpha emission line profile suggests the orbital inclination is ~22-32 degrees. The preferred range for the reddening towards the source is 0.7 < E(B-V) < 0.9, which is equivalent to 4.06E21 cm^-2 < NH < 5.22E21 cm^-2. The Chandra observations of the pulsar in its quiescent state gave an unabsorbed 0.5-10 keV flux for the best-fitting power-law model to the source spectrum of (7.0 +/- 0.9)E-14 ergs/cm^2/s (adopting a hydrogen column of 4.6E21 cm^-2. The fit resulted in a power-law photon index of 2.4 +/- 0.5. The (R-K)o color observed during quiescence supports an irradiated donor star and accretion disk. We estimate a distance of 2 to 4 kpc towards J00291 by using the outburst X-ray light curve and the estimated critical X-ray luminosity necessary to keep the outer parts of the accretion disk ionized. Using the quiescent X-ray luminosity and the spin period, we constrain the magnetic field of the neutron star to be < 3E8 Gauss.
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.
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 the spectral and timing properties of the accreting millisecond X-ray pulsar IGR J00291+5934 observed by XMM-Newton and NuSTAR during its 2015 outburst. The source is in a hard state dominated at high energies by a comptonization of soft photons ($sim0.9$ keV) by an electron population with kT$_esim30$ keV, and at lower energies by a blackbody component with kT$sim0.5$ keV. A moderately broad, neutral Fe emission line and four narrow absorption lines are also found. By investigating the pulse phase evolution, we derived the best-fitting orbital solution for the 2015 outburst. Comparing the updated ephemeris with those of the previous outbursts, we set a $3sigma$ confidence level interval $-6.6times 10^{-13}$ s/s $< dot{P}_{orb} < 6.5 times 10^{-13}$ s/s on the orbital period derivative. Moreover, we investigated the pulse profile dependence on energy finding a peculiar behaviour of the pulse fractional amplitude and lags as a function of energy. We performed a phase-resolved spectroscopy showing that the blackbody component tracks remarkably well the pulse-profile, indicating that this component resides at the neutron star surface (hot-spot).
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.
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