Large-scale Jets in GRS 1915+105

ATel #503; M. P. Rupen (NRAO/GSFC), V. Dhawan, A. J. Mioduszewski (NRAO)
on 27 May 2005; 18:29 UT
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Credential Certification: Michael P. Rupen (mrupen@nrao.edu)

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Observations with the Very Large Array (VLA), originally intended to support the HERO balloon flight, show multiple arcsecond-scale components moving out from the core of the black hole X-ray binary GRS 1915+105. Data taken at 8.46 GHz on May 11 and May 23 show extended emission along the known jet axis of this microquasar (e.g., Mirabel & Rodriguez 1994 [MR94], Nature, 371, 46-48). The beam was roughly round in both observations, with a full-width at half-maximum of about 0.67 arcseconds. Although poor weather on May 11 precludes an accurate position determination, fitting two Gaussians to the image yields a reasonable fit with a separation of 0.54 arcseconds along a position angle (PA) 149 degrees east of north, where the formal error is less than 10 milliarcseconds (mas). A similar fit to the May 23 data gives a separation of 0.87 arcseconds at PA=148 degrees, implying an increase of 27 mas/day. This is comparable to the range of proper motions previously observed in this source (e.g., MR94; Fender et al. 1999 [F99], MNRAS 304, 865-876), and implies a rough ejection date of April 21 (MJD 53481). The total flux densities at 8.46 GHz were 51.4 and 37.9 mJy on May 11 and 23, respectively, with the corresponding south:north flux density ratios being 5.8:1 and 4.2:1.

Higher-resolution images made at 15 and 22 GHz on May 23 (beam sizes 0.39 and 0.25 arcseconds, respectively) reveal a more complex picture. Good fits require three components: an extended Gaussian plus a point source to the southeast, and a point source to the northwest. There is no detectable emission from the core after subtracting these components; the rms noise levels are 0.24 and 0.16 mJy/beam at 15 and 22 GHz, respectively. The 22 GHz fits give the following:

• SE-1: 1.4 mJy, 0.44 arcsec from the core at PA= 151 degrees
• SE-2: 12.1 mJy, 0.70 arcsec from the core at PA= 148 degrees; FWHM= 0.11 +/- 0.02 x 0.07 +/- 0.03 arcsec at PA= 118+/-20 degrees
• NW-1: 2.7 mJy, 0.22 arcsec from the core at PA= -31 degrees

Here the core position is based on VLBI measurements (Dhawan et al., in prep.). There are several interesting implications.

• Previous observations suggest southern components move out with proper motions roughly a factor two faster than northern ones. Assuming ballistic motions, this would associate NW-1 with SE-1, with no northern counterpart to SE-2.
• Taking the proper motions measured with the VLA in MR94 as a guide (17.6+/-0.4 mas/day to the south, and 9.0+/-0.1 mas/day to the north) suggests corresponding explosion dates of April 28 (MJD 53488) for NW/SE-1, and April 13 (MJD 53473) for SE-2. The extent of the latter implies an ejection lasting about a week.
• In previous outbursts, the southern components have been much stronger than those to the north (factor 6-10 at similar separation, or a factor of a few at similar observing epochs: MR94, F99). Here NW-1 is actually stronger than SE-1, while the peak of SE-2 is at least 60 times stronger than any corresponding northern component. Note however that the ratio of integrated flux densities, (SE-1 + SE-2)/NW-1, 5:1, is closer to previous determinations. One interpretation is that the flux density ratios here are dominated by local interactions with the circum/interstellar medium, rather than simple relativistic beaming.

Contour plots, total flux densities, and other information may be found at

http://www.aoc.nrao.edu/~mrupen/XRT/GRS1915+105/grs1915+105.shtml which will be updated as further observations are taken, probably weekly.

Observations at other wavelengths, particularly infrared and (if possible) X-ray imaging, are strongly encouraged.

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.