Accretion Powered X-ray Pulsars
The accretion powered X-ray pulsars are laboratories for study of various phenomena involving matter in strong magnetic fields. The accreting matter imparts or draws out angular momentum from the neutron star. Both the accretion torque and the X-ray beaming from the neutron star are dependent on the mass accretion rate and structure of the accretion column on the magnetic poles of the neutron star. For a neutron star with a supergiant companion star, the mass accretion from the wind causes these sources to show random variation in accretion torque. However, these sources occasionally show rapid spin-up phases, indicating a change in the mode of accretion from wind to a disk. Study of various pulsar characteristics like pulse shape, pulse fraction etc. at different accretion torque level measured with Daksha will lead to better understanding of the relative importance of the two accretion mechanisms in the persistent accretion powered X-ray pulsars. The neutron stars with Be-Star companions undergo large outbursts during which the neutron star shows large spin-up followed by slow spin-down during the quiescence. Measurement of the accretion torque as a function of the accretion power and the pulse profiles of X-ray pulsars will therefore be very useful in study of the accretion processes in high magnetic field neutron stars over a large range of mass accretion rate. Isolated X-ray pulsars and magnetars can also undergo discontinuous changes in their rotation rate (glitches and anti-glitches), some times accompanied by large scale pulse profile and emission mechanism changes.
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Daksha + Accretion Powered Pulsars
As an all sky X-ray sensitive satellite, Daksha will be able to gather time-tagged photons from all sources in the sky. The absolute timing precision of Daksha of the time-tagging will be sub-ms. Since individual Daksha detectors lack directionality, photons from any specific source will be confused. However, by barycentering the arrival times and optimally combining photon counts from different detector facets of Daksha, we can search for periodic astrophysical signals in a frequency–frequency derivative–sky-position phase space. For known pulsars, we can use Daksha photons to continuously monitor the spin periods for variations with accretion rates, detect glitches, and refine their ephemeris. This monitoring can simultaneously cover all the X-ray bright pulsars in the sky, unlike the monitoring with radio telescopes. In addition to barycentering the data and combining the signals from different detectors, Daksha can also use Earth occultation as a method to distinguish photons from bright sources near the Earth limb.
We calculated the expected source count rates for these sources with Daksha ME detectors based on the NuSTAR 20–50 keV spectra of several accretion powered pulsars. We created lightcurves for these pulsars based on the NuSTAR pulse profiles in the same energy band, scaled appropriately and a background rate of ∼ 7400 cts s−1 (for seven surfaces). We simulated observations of these pulsars with an integration time of 2.5 days with a duty cycle appropriate for the low-earth orbit of Daksha. A Lomb-Scargle periodogram search was used to determine the pulsed flux level required to detect the pulsation above a 3- and 5-σ threshold.
The simulations show that accretion powered pulsars with a flux level above 2.6 × 10−10 (3.1 × 10−10) ergs s−1 cm−2 in the 20–50 keV band will be detected with Daksha-ME at 3-σ (5-σ) level in every 2.5 day interval. Figure 6 shows these limits in comparison with the pulsed flux measurements of GX 301–2 from Fermi-GBM. The detection threshold depends partly on the pulsed fraction and the pulse shape of each source. Daksha-ME will therefore be able to carry out continuous monitoring of the pulsar frequency and pulsed flux history of about 10 persistent X-ray pulsars. A large number of transient pulsars (about six transient pulsars every year) will also be monitored during their outbursts. About two new transient pulsars are expected to be discovered every year. For study of accretion powered X-ray pulsars, Daksha-ME is at par with CGRO-BATSE and Fermi-GBM which have been extremely useful for study of this type of sources in the last three decades.