NuSTAR Detec,on of High- Energy X- ray Emission and Rapid Variability from SagiAarius A* Flares

Transcription

1 NuSTAR Detec,on of High- Energy X- ray Emission and Rapid Variability from SagiAarius A* Flares Nicolas M. Barrière, UC Berkeley, Space Sciences Laboratory John A. Tomsick, Frederick K. Baganoff, Steven E. Boggs, Finn Christensen, William W. Craig, Jason Dexter, Brian GrefensteAe, Charles J. Hailey, Fiona Harrison, Kris,n Madsen, Kaya Mori, Daniel Stern, Shuo Zhang, William Zhang, Andreas Zoglauer IAU symposium 303, Santa Fe, October 2013

2 SagiAarius A* At the dynamic center of our Galaxy ~4 x 10 6 M sun Bright radio and submm source Very faint accre,ng BH L bol ~ 10-9 L edd Quiescent emission: - ADAF: Hot, geometrically thick, op,cally thin accre,on flow (Narayan et al. 1998) - Jet (Falcke & Markoff, 2000) Flares: - NIR: a few,mes per day: synchrotron emission - X- ray: ~once a day ( X quiescence), always associated with NIR flare: uncertain

3 Observa,on campaign NuSTAR: July, August, October 2012: 325 ks Chandra: XVP program, 3Ms in 2012 Keck along with the Chandra XVP

4 Observa,on campaign NuSTAR: July, August, October 2012: 325 ks Chandra: XVP program, 3Ms in 2012 Keck along with the Chandra XVP

5 NuSTAR s image of the flare 3-79 kev, 3700 s in each panel, not bkg subtracted ~1200 flare photons

6 Lightcurve (3-79 kev) FPM A & B combined 50 extrac,on region 100 s bins Flare detec,on done by Bayesian block analysis of the event list 2

7 Lightcurve (3-79 kev) FPM A & B combined 50 extrac,on region 100 s bins Flare detec,on done by Bayesian block analysis of the event list 2 Flare Dura)on (s) Significance (σ) J20 > J21_1 > J21_2 > O17 >

8 J21_2 and O17 lightcurves FPM A & B combined 50 extrac,on region 100 s bins Significant variability within 1 bin

9 J21_2 and O17 lightcurves FPM A & B combined 50 extrac,on region 100 s bins Significant variability within 1 bin Rise factor: >3.8 ± 1.1 Decay factor: 3.4 ± 1.8

10 Photon indices Fix N H = cm 2 (Nowak et al. 2012) Absorbed power- law model No spectral evolu,on during Flare J21_2 O17 J21_1 ( 98.5% significance)

11 Spectral analysis: J21_2 Model: TBabs*PEGpwrlw ( wilm abundances, vern cross sec,ons) N H =1.43 x cm - 2 (Nowak et al., 2012) Γ=2.19 ± 0.18 Peak luminosity: 3-79 kev: (2.7 ± 0.5) erg s kev: (1.9 ± 0.4) erg s 1

12 Spectral analysis: O17 Model: TBabs*PEGpwrlw ( wilm abundances, vern cross sec,ons) N H =1.43 x cm - 2 (Nowak et al., 2012) Γ= 1.97± 0.15 Cut off? - > no (F- test probability of 90.2%) Peak luminosity: 3-79 kev: (4.0 ± 0.5) erg s kev: (1.8 ± 0.2) erg s 1

13 Flare emission mechanism SED, corrected for interstellar absorp,on 3 single zone, sta,c models: - Synchrotron w cooling break - Synchrotron self Compton - External Compton J21_2 O17 => Not possible to iden,fy dominant mechanism based on the fit

14 EC, SSC and SB models parameters

15 EC, SSC and SB models parameters Flare J21_2: Synchrotron with cooling break slightly favored

16 EC, SSC and SB models parameters Flare O17: Clearly inconclusive

17 EC, SSC and SB models parameters SSC: Extremely high electron density required (10 7 cm - 3 typical)

18 EC, SSC and SB models parameters SSC: Extremely high electron density required (10 7 cm - 3 typical) EC: Very small sub- mm volume (an order of magnitude smaller than observed by VLBI, Fish et al., ApJ 727 (2011) )

19 EC, SSC and SB models parameters SSC: Extremely high electron density required (10 7 cm - 3 typical) EC: Very small sub- mm volume (an order of magnitude smaller than observed by VLBI, Fish et al., ApJ 727 (2011) ) SSC: Extremely high magne,c field required => Similar conclusion to Dodds- Eden et al., ApJ 698 (2009)

20 Loca,on of the flaring region Ra,o of magne,c energy density within the volume to the flare energy (5.3 x erg) Size of the emission region: 3 x cm (2.5 Rs) B field intensity R - 1 The flare emission region is located very close to the SMBH, within 9 Rs (15 Rs)

21 Summary 4 Flares detected (~12X sensi,vity): Up to kev No cutoff: absorbed power law Hint of differences photon indices Amongst the 3 model tested, the synchrotron emission model is favored, mostly because EC and SSC require unphysical parameters => Con,nuous par,cle accelera,on is required => γ max = 3x10 5 Significant variability with,mescale 100 s The flaring region radius is 2.5 R S in diameter Flaring region loca,on constrained within 10 Rs from the BH Barrière et al coming up, stay tuned!

22 Thank you for your aaen,on

23 Flare emission mechanism The NuSTAR spectrum is consistent with a synchrotron model with a cooling break in the op,cal or UV 2007 Flare (Dodds- Eden et al. 2009)

24 Size of the emi~ng region 100 s variability, propaga,on at the speed of light: Radius of a spherical emission region No rela,vis,c beaming: r c t 3 x cm Rela,vis,c beaming (jet scenario): Spin axis of the SMBH > 35º from our line of sight (Dexter et al. (2010), Broderick et al. (2011), Shcherbakov et al. (2012) Bulk speed in the nozzle ~ 0.4 c Falcke et al. (2000) => Doppler factor: D~1.4 => r c t D 4 x cm