Fast extragalactic x-ray transients from binary neutron star mergers

The observed light curves and other properties of the two extragalactic fast x-ray transients, CDF-S XT1 and CDF-S XT2, which were discovered recently in archival data of the Chandra Deep Field-South (CDF-S) observations, indicate that they belong to two different populations of x-ray transients. XT1 seems to be an x-ray flash, i.e., a narrowly beamed long duration gamma ray burst viewed from far off axis, while XT2 seems to be a nebular emission powered by a newly born millisecond pulsar in a binary neutron stars merger.

Figure 1

Comparison between the scaled 0.3–10 keV light curves of the well sampled x-ray afterglow of SGRBs during the first couple of days after burst and extended emission (when present) measured with the Swift XRT [20] and the 0.3–10 keV light curve of CDF-S XT2 [10]. The line is the expected universal behavior given by Eq. (6) of a PWN afterglow powered by a newly born millisecond pulsar with a braking index $n=3$.

Figure 2

The expected full sky rate of NSB mergers with redshift $\le z$, as a function of $z$. The calculated rate is based on the standard cosmological model and the assumption that the NSB merger rate as a function of redshift $z$ is proportional to the observed star formation rate, SFR(z), as parametrized in Eq. (12). The full and thin lines correspond to the estimated rate and its errors in a comoving ${\mathrm{Gpc}}^3$ volume reported in [1] by the Ligo-Virgo Collaboration. The inserted point is the full sky rate estimated in [18], after subtracting the contribution of CDF-S XT1.

Figure 3

Comparison between the observed pulse shape of CDF-S XT1 [9] in the 0.3–7 keV x-ray band and (a) the expected pulse shape of far off axis LGRB pulse as given by Eq. (13) (thick red line) with the best fit parameters listed in the text, which yield ${\chi }^2/df=0.50$, (b) the best fit universal shape of a nebular x-ray emission powered by the spin-down of a newly born MSP (thin black line), as given by Eq. (5).