Study of the Largest Multiwavelength Campaign of the Microquasar GRS 1915+105

We present the results from a multiwavelength campaign of GRS 1915+105 performed from 2000 April 16 to April 25. This is one of the largest coordinated set of observations ever performed for this source, covering the wide energy band in radio (13.3-0.3 cm), near-infrared (J, H, and K bands), X-rays, and gamma rays (from 1 keV to 10 MeV). During the campaign GRS 1915+105 was predominantly in the "plateau" (or low/hard) state but sometimes showed soft X-ray oscillations: before April 20.3, rapid, quasi-periodic (≃45 minutes) flare-dip cycles were observed. In the spectral energy distribution in the plateau state, optically thick synchrotron emission and Comptonization is dominant in the radio and X- to gamma-ray bands, respectively. The small luminosity in the radio band relative to that in X-rays indicates that GRS 1915+105 falls in the regime of "radio-quiet" microquasars. In three epochs we detected faint flares in the radio or infrared bands with amplitudes of 10-20 mJy. The radio flares observed on April 17 shows frequency-dependent peak delay, consistent with an expansion of synchrotron-emitting region starting at the transition from the hard-dip to the soft-flare states in X-rays. On the other hand, infrared flares on April 20 appear to follow (or precede) the beginning of X-ray oscillations with an inconstant time delay of ≃5-30 minutes. This implies that the infrared-emitting region is located far from the black hole by ≳1013 cm, while its size is ≲1012 cm constrained from the time variability. We find a good correlation between the quasi-steady flux level in the near-infrared band and in the X-ray band. From this we estimate that the reprocessing of X-rays, probably occurring in the outer parts of the accretion disk, accounts for about 20%-30% of the observed K magnitude in the plateau state. The OSSE spectrum in the 0.05-10 MeV band is represented by a single power law with a photon index of 3.1 extending to ~1 MeV with no cutoff. We can model the combined GIS-PCA-HEXTE spectra covering 1-200 keV by a sum of the multicolor disk model, a broken power law modified with a high-energy cutoff, and a reflection component. The power-law slope above ~30 keV is found to be very similar between different states in spite of large flux variations in soft X-rays, implying that the electron energy distribution is not affected by the change of the state in the accretion disk.