Enhanced Doppler Beaming for Dust-Enshrouded Objects and Pulsars in the Galactic Center

Abstract

Stars within the innermost part of the Nuclear Star Cluster can reach orbital velocities up to a few percent of the light speed. As analyzed by Rafikov (2020), Doppler boosting of stellar light may be of relevance at the pericenter of stellar orbits, especially with the upcoming high-precision photometry in the near- and mid-infrared bands. Here we analyze the previously neglected effect of infrared spectral index of monitored objects on the Doppler-boosted continuum emission in a narrow band. In contrast to main-sequences stars, the detected compact infrared-excess dust-enshrouded objects have an enhanced Doppler-boosting effect by as much as an order of magnitude, with the variability amplitude of the order of ten percent for the most eccentric orbits. In a similar way, pulsars dominated by non-thermal synchrotron emission are also expected to exhibit a stronger Doppler-boosted signal by a factor of at least four in comparison with canonical S stars. In case the stellar orbit is robustly determined, the relative flux variation can thus provide hints about the nature of the objects. For extended dust-enshrouded objects, such as G1, that are variable due to tidal, ellipsoidal, bow-shock, and irradiation effects, the subtraction of the expected Doppler-boosting variations will help to better comprehend their internal physics. In addition, the relative flux variability due to higher-order relativistic effects is also modified for different negative spectral indices in a way that it can obtain both positive and negative values with the relative variability of the order of one percent.