Galaxy Clusters with Swift/BAT

Transcription

1 Galaxy Clusters with Swift/BAT Marco Ajello [KIPAC/SLAC] with Paola Rebusco (MIT), Nico Cappelluti(MPE), Olaf Reimer (UIBK), Hans Böhringer (MPE)

2 Outline Why studying clusters at high energies? The BAT survey: How BAT works, cluster detection, limitations Clusters in the BAT all-sky survey Spectral analysis Assessing the presence of hard X-ray excess A few cases Limits on the magnetic field The Case of Abell 3571 Conclusions M. Ajello 2

3 Reasons to study clusters at high Energy Merger Shocks Cosmic Rays in the ICM Can heat the ICM up to ~15 kev (eg Takizawa99, Markevitch03, Pfrommer07) Can accelerate particles on Mpc scales (eg Petrosian&Bykov08) Open Questions Was NT component ever detected (eg Fusco- Femiano+99,+03,+04, De Grandi&Molendi+99)? How much pressure do CRs supply to the ICM? In case of a DM annihilation signal it s important to assess the role of CRs Evidence for the existence of relativistic GeV e - from radio-halos/ relics (eg Wilson70, Kempner&Sarazin01,Brunetti+09) e - loose energy on short timescale (e.g. <Gyr): IC scattering off CMB photons expect power law at kev energ. Protons confinement time in the ICM Hubble time energy stored in p might be relevant expect γ-rays from p-p collision 3

4 The BAT survey: Deep, all-sky and Unbiased All-sky observed down to ~0.5 mcrab (~6e-12 erg/cm 2 /s) (see Ajello+08a,08b,08c,09a,09b etc..) No bias against N H up to Compton-thick regime Tueller+10 BAT more sensitive (at high-e.) than Suzaku/ HXD on long exposures M. Ajello 4

5 How does BAT work? asada If a source extension is much larger than the PSF (22 ) then part of the flux is lost in the background (J. Skinner) M. Ajello 5

6 How to constrain hard X-ray excesses? Extract a background-subtracted BAT spectrum Systematic uncertain is energy-dependent (4-20%) Extract an XMM spectrum Using a 10 region Evaluate bkg using blank-sky observations Adopt a 2% systematic uncertain Make joint fits to XMM-BAT data: Allow inter-calibration to vary (e.g. different calibration of XMM vs BAT, missed flux, etc.) Start with simplest model (e.g. thermal) and add more components until χ 2 ~1 All parameters are free to vary (including kt) M. Ajello 6

7 What to expect in BAT? Non-thermal excess as those reported in the past would produce a relevant deviation from the pure thermal spectrum above 50 kev M. Ajello 7

8 What to expect in BAT? Non-thermal excess as those reported in the past would produce a relevant deviation from the pure thermal spectrum above 50 kev BAT has the required sensitivity if HXR flux is in the mcrab range (e.g erg cm -2 s -1 ) Average spectrum of LMXB (Voss&Ajello10) M. Ajello 8

9 Some homework first Be sure to be able to detect what you expect to see: i.e. evaluate systematics using >160 source-free regions In empty fields expect signal compatible with zero (normal Gaussian) kev 1mCrab M. Ajello 9

10 20 clusters serendipitously detected above 15 kev: Ajello+09 (ApJ690) and Ajello+10, ready for submission The BAT clusters 2 years of data 5 years of data M. Ajello 10

11 Data Quality: some example spectra Perseus, >40σ Bullet, ~4.5σ Detection significance ranges from ~4 to >40 σ In most cases, a single thermal model is ok In just a few cases a more complex model is needed Ophiucus M. Ajello 11

12 Too Two Early! hard Only X-ray AGN.. excesses! Perseus Sanders+05 Abell 0754 M. Ajello 12

13 NGC 1275 Sander&Fabian07 NGC 1275 detected at γ-rays Fermi source is variable over ~1yr timescale, thus securely unassociated with the ICM Abdo+09,ApJ699,31 NB: a 2 nd source has been detected in Perseus M. Ajello 13

14 IC 310 detected by Fermi? New source detected at ~7σ by Fermi (Neronov+10) It s coincident with the head-tail radio galaxy IC 310 Tail would be produced by the fast motion of the galaxy in the ICM (eg Sarazin88) Detected at TeV energies by Magic (Atel #2510) However, interpretation not certain yet Eg most of the photons have E~100 GeV Neronov+10, arxiv: GeV GeV GeV M. Ajello 14

15 Abell 0754 BAT confirms BeppoSAX results (Fusco-Femiano+03) Henry+04 z=0.3 Credits : A. Rau M. Ajello 15

16 Ophiucus Profumo08 Eckert+07 Excess detected at 2σ with BeppoSAX (Nevalainen+01) INTEGRAL detects an excess at 4-6.4σ Suzaku results in agreement with BAT(Fujita+09) M. Ajello 16

17 Ophiucus Profumo08 From BAT. Eckert+07 Eckart+07 Excess detected at 2σ with BeppoSAX (Nevalainen+01) INTEGRAL detects an excess at 4-6.4σ Suzaku results in agreement with BAT(Fujita+09) M. Ajello 17

18 Ophiucus: an update (5years) BAT 5years Nevalainen+09 Nevalainen+09 confirms Eckart+07 results: HXR excess with a 2.2 power law and F erg/ cm 2 /s Compatible with the BAT spectrum at 3.0σ, but not at 2.9σ.. The BAT spectrum is perfectly fit by 1kT = 9.5±0.8 kev M. Ajello 18

19 Abell 3667 A3667 hosts the brightest radio relic (Johnston-Hollit 2004) Marginal (2.6σ) HXR detected with BeppoSAX (Fusco-Femiano+01) Suzaku confirmed the existence of an HXR which can be modeled as a power-law with Γ=1.4 or as a thermal comp. with kt~20 kev (Nakazawa+09) M. Ajello 19

20 Abell 3667 A3667 hosts the brightest radio relic (Johnston-Hollit 2004) Marginal (2.6σ) HXR detected with BeppoSAX (Fusco-Femiano+01) Suzaku confirmed the existence of an HXR which can be modeled as a power-law with Γ=1.4 or as a thermal comp. with kt~20 kev (Nakazawa+09) BAT confirms the presence of the HXRE at ~4.6σ The 10-40keV power-law flux is a factor 10 lower than Nakazawa+09 1kT+PL or 1kT+2kT produce the same result in terms of GOF 1kT 1kT + 2kT 1kT + PL Difficult to say what s physical M. Ajello 20

21 Norma Norma detected at ~5.2σ in BAT BAT Spectrum compatible with kt=19.6 (+13.2/-8.5) kev A joint XMM/BAT fit with 1 thermal model yields χ 2 /dof=677/586 Adding a PL yields Δχ 2 ~12 (e.g. 2.3σ), alternatively 2kT~60 kev also ok ROSAT-PSPC kt=5.1 kev Γ=1.8 kt=5.2 kev M. Ajello 21

22 Stacking analysis Average of 18 spectra Preliminary Sensitivity pushed a factor 4 (18x exposure) Still no NT emission F <1.1 x cgs Best fit temperature of kt=10.5(±1.2) kev in agreement with the average temperature (arithmetic mean) of 10.6 kev M. Ajello 22

23 Stacking analysis Average of 18 spectra Maximum PL compatible with BAT data (99%CL) Preliminary Sensitivity pushed a factor 4 (18x exposure) Still no NT emission F <1.1 x cgs Still compatible with the averaged non-thermal emission detected in the stacked spectrum of 14 BeppoSAX clusters (Nevalainen+04), but redshift is larger M. Ajello 23

24 What do we learn? Relativistic (GeV) e - can upscatter CMB photons via Inverse Compton (e.g. Rephaeli+79, Sarazin+99) At the current fluxes, very low B a factor 10 lower than RM F IC ~ 1 F Sync B α +1 (e.g. Harris&Romanishin 74) M. Ajello 24

25 Abell 3571: tidal disruption event? Cappelluti, Ajello, Rebusco+09 Huge X-ray flare: 4 decades, decline as t -5/3 (not consistent with ULX, SN, Novae, SS sources) The host is a normal S0 gal., no AGN activity Sun-like star disrupted by a ~10 7 M o M. Ajello 25

26 Abell 3571: tidal disruption event? Cappelluti, Ajello, Rebusco+09 Huge X-ray flare: 4 decades, decline as t -5/3 (not consistent with ULX, SN, Novae, SS sources) The host is a normal S0 gal., no AGN activity Sun-like star disrupted by a ~10 7 M o M. Ajello 26

27 Conclusions BAT has detected 20 clusters in 5 years Satisfying prediction that the final cluster sample will probably count ~30objects Search for hard X-ray excesses: For Perseus and Abell 0754 is due to an AGN For Abell 3667, the hot component interpretation is preferred For Norma: a marginal ~2.3σ excess has been found (or kt>60 kev), still we cannot exclude this is due to a point source Advertisement: BAT is surveying the entire sky for fluxes >10-12 erg cm -2 s -1. >1000 objects have been detected (and many more will be!). Your favorite one might be there. Drop an if interested! M. Ajello 27