2023ApJ...956...93H

We did 194 epochs of simultaneous 2.25/8.60 GHz observations of XTE J1810-197 in 926 days shortly following its reactivation in 2018 with Shanghai Tian Ma Radio Telescope (TMRT). Although its integrated profiles changed with both time and frequency during this period, they could be classified into 12 types according to phase areas of active radiation components. After MJD 59015, XTE J1810-197 turned from the normal emission state into the spiky emission state that was manifesting as a series of bright narrow sub-pulse bursts at both 2.25 and 8.60 GHz. Due to its variable integrated profiles and unsteady rotations, we got the spin frequency ν and spin-frequency derivatives $\dot{\nu }$ with the piecewise fitting method. In addition, its long-term declining trend of ν was also obtained as $\dot{\nu }=-3.2(1)\times {10}^{-13}$ s^–2 with the linear fitting method based on our observations spanning a comparatively longer period. We found its flux densities went through three stages: sharp decrease, stabilization, and day-to-day fluctuations in our dual-frequency observations. Although the flux density showed different stages, XTE J1810-197 showed a relatively flat spectrum (α > -1) in most observations. Assuming an ideal dipole magnetic field as some previous research works, we obtained the emission heights of XTE J1810-197 were about 7.5(9) × 10⁴ km and 2.38(7) × 10⁴ km at 2.25 and 8.60 GHz, respectively. We also found that its emission heights would decrease rapidly with the degree of magnetic field twisting.