Pulsars : Problems & Progress ASP Conference Series, Vol. 105, 1996 S. Johnston, M. A. Walker and M. Bailes, eds. Macho Detection by Pulsar Timing Observation Kouji Ohnishi Kansai Advanced Research Center, Communications Research Laboratory, Kobe 651-24, Japan Mizuhiko Hosokawa Communications Research Laboratory, Koganei, Tokyo 184, Japan Toshio Fukushima National Astronomical Observatory, Mitaka, Tokyo 181, Japan Mine Takeuti Astronomical Institute, Tohoku University, Sendai 980-77, Japan 1. Mass measurement by pulsar timing observation The magnitudes of some general relativistic effects depend on the mass of gravi- tating objects; therefore, these effects can be used for the stellar mass measure- ment. Several methods have been proposed; for example, the microlensing effect (Paczyriski 1986), and the parallactic variation of gravitational deflection of the light from the source (Hosokawa et al. 1993). Recently some candidates for MACHOs - a kind of dark matter candidates in the Galactic halo - have been discovered by their microlensing effect, and their masses were estimated to be of the order of 0.1M©, under many assumptions. Actual measurement of MACHO masses are important in specifying what kind of object the MACHO is. We proposed a new method: the variation of the gravitational time delay of pulses from a pulsar caused by the motion of foreground stars relative to the pulsar in the pulsar timing observation (Ohnishi et al. 1995). Consider that the observer, the foreground star and the pulsar are well aligned. In this situation, the gravitational time delay due to the proper motion of the star relative to the pulsar is expressed as GM, At(t) = constant - 2—-j- In , t-t 0 ^ 2 1 + I V (1) where G is the gravitational constant, M is the mass of the foreground star, c is the speed of light, to is the epoch of the closest approach and £<j is the ratio of the minimum separation angle to the relative proper motion \i that characterizes the time scale of this effect. This gravitational delay as a function of time has two inflection points and will make a positive hump in the residuals of the time of arrival (TOA) of pulses 125 terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0252921100041233 Downloaded from https://www.cambridge.org/core. IP address: 168.151.178.27, on 07 May 2019 at 14:25:58, subject to the Cambridge Core