NICER detection of possibly periodic X-ray absorption dips in MAXI J1803-298

After the observations reported in ATel #14602, the Neutron Star Interior Composition Explorer (NICER) has continued observing the outburst of the recently discovered X-ray transient MAXI J1803-298 (ATel #14587, #14588, #14591, #14594, #14597, #14598, #14601). A light curve from the full NICER data set, which covers May 2 UTC 12:55 to May 5 23:06 UTC, reveals three strong dipping intervals in the 0.5-10 keV band (see link below). These intervals are centered around May 2 17:31 UTC, May 5 09:07 UTC, and May 5 16:48 UTC. The beginning or end of the dipping activity was not observed for any of the dipping intervals, indicating that the dipping intervals last longer than 500 s. The time between the last two dipping intervals is ~7.7 hrs. The time between the first and last dipping intervals is roughly nine times the interval between the second and third dips, suggesting that the dips may be periodic. All three dips showed a corresponding increase in spectral hardness (4-10 keV/0.5-2 keV). We analyzed the spectrum of the second dip and the persistent emission from the two good-time-intervals immediately following the dip. For the persistent emission we can fit a spectrum that is consistent with that reported in ATel #14602: an absorbed continuum of a power-law and a disk blackbody. We measure a neutral absorption column of ~3.2(1)e21cm^-2, a power-law index of 1.73(1) and a disk temperature of 0.27(1) keV. The dip spectrum can be fitted with the same continuum model, but requires an additional absorber. We find that a partial covering warm absorber (zxipcf in XSPEC) fits better than a (partial covering) neutral absorber. For the zxipcf component we find an absorption column of 15.0(7)e22 cm^-2, an ionization parameter (log(xi)) of 2.32(2), and a covering fraction of 0.42(1). We further note that we see hints of Iron absorption lines around 6.6 keV and 7.0 keV in the spectrum of the third dipping interval, which was significantly deeper than the second dipping interval. The repeating absorption dips in MAXI J1803-298 are likely of the same nature as the nearly-periodic absorption dips seen in the light curves of some other X-ray binaries that are viewed at high inclination angles (>60 degrees). These recur on periods that are in general very close to the orbital periods of those systems. Given the limited NICER coverage of the dipping intervals in MAXI J1803-298 we suggest that the orbital period of the system is likely between 7 and 8 hrs, although an integer fraction of that cannot be ruled out with our data. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.