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2014 Chandra X-ray Monitoring of Sgr A*/G2 and SGR J1745-29

ATel #6242; Daryl Haggard (Northwestern/CIERA), Frederick K. Baganoff (MIT), Nanda Rea (CSIC-IEEC/U. Amsterdam), Francesco Coti Zelati (U. Insubria/INAF/U. Amsterdam), Gabriele Ponti (MPE), Craig Heinke (U. Alberta), Sergio Campana (INAF), Gian Luca Israel (INAF), Farhad Yusef-Zadeh (Northwestern/CIERA), Doug Roberts (Northwestern/CIERA)
on 17 Jun 2014; 14:35 UT
Credential Certification: Daryl Haggard (dhaggard@northwestern.edu)

Subjects: X-ray, AGN, Black Hole, Neutron Star, Soft Gamma-ray Repeater, Transient, Pulsar

We report ongoing Chandra monitoring of the object "G2" during its predicted close encounter with Sgr A* (~120 AU; Meyer et al. 2014, IAUS, 303, 264). We present six Chandra observations covering a period starting one month prior to and continuing about two months post pericenter (late March 2014, Phifer et al. 2013, ApJ, 773, 1; Gillessen et al. 2013, ApJ, 774, 44). To date, we find no clear changes in Sgr A*'s 2-8 keV X-ray emission.

Our instrumental configuration consists of the ACIS-S3 chip with a 1/8 subarray to minimize pile up from the nearby magnetar, SGR J1745-29, which is 2.4 arcsec from Sgr A* and remains bright throughout this period. Using a 1.25 arcsec region at the position of Sgr A* (2.0 arcsec for the magnetar), we extract lightcurves and spectra, mask Sgr A* flares, and determine X-ray fluxes for each epoch. We create model PSFs using CIAO's ChaRT/MARX ray-tracing simulations, which indicate that about 0.6% of the magnetar's flux falls within Sgr A*'s 1.25 arcsec extraction region. However, since the PSF is likely set by dust scattering (Tan & Draine 2004, ApJ, 606, 296), these simulations may not accurately describe the point source PSF in the Galactic Center. Hence, we check the magnetar's flux contribution in three additional extraction regions, also offset by 2.4 asec, and find an average contribution from the magnetar to Sgr A*'s flux of 0.9%. We use this more conservative estimate in the table below. Adopting an absorbed MEKAL model for Sgr A* (kT=2.7 keV), an absorbed blackbody for the magnetar, a neutral hydrogen column density of N_H=1.18(1)x10^23 cm^-2, and dust scattering, we find the following unabsorbed 2-8 keV fluxes (units: erg/cm^2/s; 90% confidence):

Date(UT)     Exp(ks)  flux_SgrA*   flux_mag    corr_flux_SgrA*
2014.02.21  41.78    5.1x10^-13   1.1x10^-11   4.1x10^-13
2014.03.14  41.78    4.9x10^-13   1.0x10^-11   4.0x10^-13
2014.04.04  45.39    4.6x10^-13   9.2x10^-12   3.7x10^-13
2014.04.28  44.96    4.6x10^-13   8.5x10^-12   3.8x10^-13
2014.05.20  45.41    4.2x10^-13   8.0x10^-12   3.5x10^-13
2014.06.03  17.02    4.2x10^-13   7.4x10^-12   3.6x10^-13

The final column is Sgr A*'s quiescent flux, corrected for the magnetar's contribution. Typical statistical uncertainties on Sgr A*'s fluxes are +/-0.6x10^-13 erg/cm^2/s for the long exposures and +/-0.8x10^-13 erg/cm^2/s for the short June exposure (for the magnetar, these are +/-0.2x10^-12 and +/-0.3x10^-12 erg/cm^2/s, respectively).

Sgr A*'s average quiescent 2-8 keV unabsorbed flux from 3 Ms of Chandra data collected during the Galactic Center XVP in 2012 (http://www.sgra-star.com) is 4.5x10^-13 erg/cm^2/s (Nowak et al. 2012, ApJ, 759, 95; Neilsen et al. 2013, ApJ, 774, 42). There is little evidence in our new data for the large rise predicted by some models during G2's close passage by Sgr A*.

Within the Chandra monitoring of the Galactic Center region, we also regularly observe the new magnetar SGR J1745-29 (Kennea et al. 2013, ApJ, 770, L24; Mori et al. 2013, ApJ, 770, L23; Rea et al. 2013, ApJ, 775, L34). The magnetar is still bright, with an ACIS-S count rate of ~0.11 cts/s. On 2014, June 3rd, its spectrum was consistent with a 0.82(3)keV blackbody, and the 1-10keV absorbed flux was 2.6(1)x10^-12 erg/s/cm^2, only marginally decayed in the past months (Rea et al. 2014, ATel \# 5922).

We thank Scott Wolk and the Science and Flight Operations teams of the Chandra X-ray Observatory for their dedicated work scheduling these observations.