Long-term Multiwavelength Observations of RX J0440.9+4431: Unraveling the Mysteries of the Missing X-Ray Outburst and the Unique Giant Outburst

Abstract RX J0440.9+4431 is a Be/X-ray binary. Since its discovery, the H α equivalent width (EW) of this source has exhibited three peaks, which correspond to different X-ray activities: three type I X-ray outbursts, no outburst, and a combined type I + type II outburst. In this work, we present photometric and spectroscopic observations spanning 2008–2023 and 2011–2023, respectively, covering the three peaks of H α EW. Combined with related multiwavelength public data, we investigate the long-term evolution of RX J0440.9+4431. We propose that the longer disk growth time prior to 2010 led to a higher-density disk and triggered three type I outbursts. In contrast, the more rapid disk growth preceding 2018 resulted in lower disk density, and material accreted during the neutron star’s periastron passage was insufficient to trigger outburst. Later, the disk began to dissipate but ended with a substantial size and residual material. This prior density accumulation, combined with renewed growth, yielded a much higher-density disk at maximum extent during the subsequent phase, ultimately triggering the unique type II outburst.