Category Archives: Public

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The Intrinsic Two-Dimensional Size of Sagittarius A*

The attached paper has been accepted for publication in ApJ. Abstract below:

We report the detection of the two-dimensional structure of the radio source associated with the Galactic Center black hole, Sagittarius A*, obtained from Very Long Baseline Array (VLBA) observations at a wavelength of 7mm. The intrinsic source is modeled as an elliptical Gaussian with major axis size 35.4 x 12.6 R_S in position angle 95 deg East of North. This morphology can be interpreted in the context of both jet and accretion disk models for the radio emission. There is supporting evidence in large angular-scale multi-wavelength observations for both source models for a preferred axis near 95 deg. We also place a maximum peak-to-peak change of 15% in the intrinsic major axis size over five different epochs. Three observations were triggered by detection of near infrared (NIR) flares and one was simultaneous with a large X-ray flare detected by NuSTAR. The absence of simultaneous and quasi-simultaneous flares indicates that not all high energy events produce variability at radio wavelengths. This supports the conclusion that NIR and X-ray flares are primarily due to electron excitation and not to an enhanced accretion rate onto the black hole.

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Sgr A* XVP Collaboration Policies

The slides in the attached talk summarize the major points of the Sgr A* XVP Collaboration Agreement that all members accepted when they registered for access to this website.  The purpose of these policies is to protect your rights regarding your own data while providing an environment in which all collaborators can feel secure enough to share their unpublished data products and ideas with the rest of the collaboration to produce the maximum science output as quickly as possible.  Read the Collaboration Agreement on this website for the full details.  Please respect your fellow Sgr A* XVP collaborators and abide by both the letter and spirit of these policies.  Do not discuss anything that you see or hear at the workshop or see posted on the password-protected section of the website with anyone outside the collaboration without express permission from the Science Steering Committee.  It is especially important that none of the NuSTAR results are discussed publicly prior to their publication by the NuSTAR Team.

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The Giant Flare of February 9, 2012

On February 9, 2012, Chandra-HETGS observed the highest peak 2-8 keV X-ray flux and greatest 2-8 keV X-ray fluence from a flare from Sgr A*.  The flare lasted for 5.6 ksec (out of a total observation duration of 60 ksec), and reached a peak flux approximately 130× the quiescent flux.

A comparably bright flare was observed by XMM-Newton in October 2002, but lasted half as long, i.e., 2.8 ksec. Assuming a distance of 8 kpc, this Chandra-HETGS observed flare had a 2-8 keV fluence of 1039 ergs, i.e., about equivalent to the entire output of our sun over 5.8 days.  Although this might sound like a huge amount of energy, and it is incredibly bright for Sgr A*, the stellar mass black hole Cygnus X-1 (which is only 14 times the mass of our Sun) puts out that much energy in under 2 minutes!

The published version of our paper can be found here on the Astrophysics Data Service, or here on the astro-ph  archive service.

sgra_movie

The above movie (click on image to see animation) was created from the actual Chandra-HETGS observation. Red sources are dominated by low energy X-rays (2-4 keV), while blue sources are dominated by high energy X-rays (6-8 keV). White sources have a more even blend of X-ray energies in the 2-8 keV range. The middle frame of the movie is the 5.6 ksec showing the flare from Sgr A*, the bright white source near the middle of the image.