Density contours of a disk with an increasing mass planet. The planet excites spiral arms in the disk. The planet mass (shown on the upper right corner) is increasing gradually. When the planet mass increases, spiral arms are more open, a secondary inner arm appears and the arms are more prominent. The dotted curve is the spiral position predicted from the linear density wave theory. Thus, companion-induced spiral arms not only pinpoint the companion’s position but also provide three independent ways (pitch angle, separation between two arms, and contrast of arms) to constrain the companion’s mass.