CHANDRA/HETGS OBSERVATIONS OF THE BRIGHTEST FLARE SEEN FROM Sgr A*

Abstract

Starting in 2012, we began an unprecedented observational program focused on the supermassive black hole in the center of our Galaxy, Sgr A * , utilizing the High Energy Transmission Gratings Spectrometer (HETGS) instrument on the Chandra X-ray Observatory. These observations will allow us to measure the quiescent Xray spectra of Sgr A * for the first time at both high spatial and spectral resolution. The X-ray emission of Sgr A * , however, is known to flare roughly daily by factors of a few to ten times over quiescent emission levels, with rarer flares extending to factors of greater than 100 times quiescence. Here were report an observation performed on 2012 February 9 wherein we detected what is the highest peak flux and fluence flare ever observed from Sgr A * . The flare, which lasted for 5.6 ks and had a decidedly asymmetric profile with a faster decline than rise, achieved a mean absorbed 2-8 keV flux of (8.5 ± 0.9) × 10 −12 erg cm −2 s −1 . The peak flux was 2.5 times higher, and the total 2-10 keV emission of the event was approximately 10 39 erg. Only one other flare of comparable magnitude, but shorter duration, has been observed in Sgr A * by XMM-Newton in 2002 October. We perform spectral fits of this Chandra observed flare, and compare our results to the two brightest flares ever observed with XMM-Newton. We find good agreement among the fitted spectral slopes (Γ ∼ 2) and X-ray absorbing columns (N H ∼ 15 × 10 22 cm −2 ) for all three of these events, resolving prior differences (which are most likely due to the combined effects of pileup and spectral modeling) among Chandra and XMM-Newton observations of Sgr A * flares. We also discuss fits to the quiescent spectra of Sgr A * .

The Chandra Dust-scattering Halo of Galactic Center Transient Swift J174540.7–290015

“…9 This smoothed model spectrum was used to estimate the flux for PSF construction (described below). The N H column obtained from the power-law fits to the readout streaks are consistent with other GC sources, including SgrA * , strongly suggesting that Swift J174540.7-290015 is part of the population of GC compact objects (e.g., Baganoff et al 2003;Nowak et al 2012). Table 2 lists the absorbed and unabsorbed fluxes, along with the source luminosity, assuming it is in the GC.…”

GRO J1008−57: an (almost) predictable transient X-ray binary

“…The best-fit hydrogen column densities, N H , are consistent between both fits. They also agree A95, page 6 of 15 with the foreground absorption found in 21 cm surveys (N H = 1.35 +0.21 −0.09 × 10 22 cm −2 , Kalberla et al 2005, and N H = 1.51 +0.38 −0.02 × 10 22 cm −2 , Dickey & Lockman 1990), as well as with the hydrogen column density of 1.49 × 10 22 cm −2 obtained from the interstellar reddening of E is (B − V) = 1.79(5) mag toward GRO J1008−57 (Riquelme et al 2012, converted to N H as outlined by Nowak et al 2012). …”

A variable-density absorption event in NGC 3227 mapped withSuzakuandSwift

“…We choose R V = 3.1 and estimate the equivalent column density via the assumed Galactic gas-todust ratio N H = A V ·2.69×10 21 cm −2 mag −1 (Nowak et al 2012). Note that the standard Galactic reddening curve does not have to apply for the intrinsic absorber in NGC 3227, where dust grains might have a different composition and size distribution compared to the Galaxy (Crenshaw et al 2001).…”