The X-ray absorption in GRB afterglows

The X-ray absorption in GRB afterglows Darach Watson DARK Cosmology Centre Niels Bohr Institute University of Copenhagen

Overview Downturn at low energies deviating from a power-law Very similar to photoelectric absorption observed in the galaxy Fit well by photoelectric absorption by metals at host redshift Values well above Galactic Galama and Wijers, in average.; Watson et al. single afterglow; de Pasquale/Gendre/Stratta et al., Campana et al., Evans et al. samples

Overview Downturn at low energies deviating from a power-law Very similar to photoelectric absorption observed in the galaxy Fit well by photoelectric absorption by metals at host redshift N HI (cm 2 ) 10 22 10 21 10 20 Values well above Galactic 10 20 10 21 10 22 N HX (cm 2 ) Galama and Wijers, in average.; Watson et al. single afterglow; de Pasquale/Gendre/Stratta et al., Campana et al., Evans et al. samples

What causes the X- ray absorption? Photoelectric absorption Inner shells of metals dominate He, C, O, Fe, Si, S etc. Relatively insensitive to ionisation state or phase (i.e. in normal situations, X-rays see almost all metals) Use column density in hydrogen as a useful proxy, but actually, insensitive to hydrogen

What causes X-ray absorption in GRBs? H II regions helium

Redshift dependence Little redshift information in low-res X-ray spectra 100 z = 0 z = 2 Get redshifts from optical log N HX = 22.5 log N HX = 21.6 (z = 0 fit) But! Inferred absorption strongly redshift dependent: NH X (z) (1+z) 2.5 NH X (0) Flux (ph cm 2 s 1 kev 1 ) 10 1 log N HX = 22.5 0.1 1 10 Energy (kev)

The redshift distribution Oddity X-ray absorption rises with redshift. Why? Expect detectability threshold to rise with redshift [ NH X (z) (1+z) 2.5 NH X (0) ] But missing low redshift, high absorption GRBs Campana et al. 2010

Solution: Dust bias Watson & Jakkobson 2012 X-rays unbiased by dust But redshifts from optical Bias obtaining redshifts See also Campana et al. 2012

No NHX-AV correlation Watson & Jakobssen 2012 Evolving NH X /AV

NHX-AV correlation? Watson & Jakobssen 2012 Evolving NH X /AV

Evolving NHX-AV correlation Correlation between NH X and AV at z < 1, 1 < z < 2, and 2 < z < 4. N HX (cm 2 ) 10 23 10 22 10 21 z > 4 N HX (cm 2 ) 10 23 10 22 10 21 2 < z < 4 But mean ratio rises with redshift 10 23 0.1 1 10 A V (mag) 10 23 0.1 1 10 A V (mag) N HX (cm 2 ) 10 22 N HX (cm 2 ) 10 22 NH X AV correlation confirmed by Covino et al. 2013 10 21 1 < z < 2 10 21 z < 1 0.1 1 10 A V (mag) 0.1 1 10 A V (mag) Watson et al. 2013

(SUB-)Conclusion Dust produced more effectively from metals at lower redshifts? Unlikely Still do not understand: Where is the X-ray absorption? Its real column density distribution Ionisation state Abundances

Proposals Molecular cloud Intrinsic curvature Underestimated Galactic Intervening neutral absorbers Warm/hot IGM

Proposals Molecular cloud Intrinsic curvature No: Should see neutral hydrogen Underestimated Galactic Intervening neutral absorbers Warm/hot IGM

Proposals Molecular cloud Intrinsic curvature No: Should see neutral hydrogen No: objects with strong slope change, constant absorption Underestimated Galactic Intervening neutral absorbers Warm/hot IGM

Proposals Molecular cloud Intrinsic curvature No: Should see neutral hydrogen No: objects with strong slope change, constant absorption Underestimated Galactic No: consistent with other surveys (dust, galactic sources) Intervening neutral absorbers Warm/hot IGM

Proposals Molecular cloud Intrinsic curvature Underestimated Galactic Intervening neutral absorbers Warm/hot IGM No: Should see neutral hydrogen No: objects with strong slope change, constant absorption No: consistent with other surveys (dust, galactic sources) No: Not large enough. Metallicity decreases with redshift

Proposals Molecular cloud Intrinsic curvature Underestimated Galactic Intervening neutral absorbers Warm/hot IGM So what s Left? No: Should see neutral hydrogen No: objects with strong slope change, constant absorption No: consistent with other surveys (dust, galactic sources) No: Not large enough. Metallicity decreases with redshift No: No absorption seen in high-z AGN

What s left Progenitor wind? GRB destroys dust and strips gas out to large radii Mass is too large to be progenitor wind HII region? Previous problem not enough HI observed But if HI ionised by stars: He can absorb closer to the GRB than anything else log (N H X /cm 2 ) (H ionised by stars confirmed by Krongold & Prochaska 2013) 22.5 22 21.5 21 20.5 20 19.5 X-ray absorption HST Radio He absorber Dust O absorber 19 0.1 1 10 100 Radius (pc)

Signatures of He absorption No clear difference in low-res spectra between metal rich gas and He HII regions have higher column densities in dense environments => more likely to intersect a high density neutral cloud => approx. correlation between NHx and gas (not metals) NHx - NHI correlation (confirmed by Covino et al. 2013) normalized counts s 1 kev 1 0 0.05 0.1 0.15 0.2 0.4 0.6 0.8 1 1.2 1.4 Energy (kev) log N HI (cm 2 ) 23 22 21 20 19 NHx - AV correlation should change with redshift as mean cosmic metallicity drops. 18 17 21 21.5 22 22.5 23 log N HX (cm 2 )

Evolving NHX-AV correlation Correlation between NH X and AV at z < 1, 1 < z < 2, and 2 < z < 4. N HX (cm 2 ) 10 23 10 22 10 21 z > 4 N HX (cm 2 ) 10 23 10 22 10 21 2 < z < 4 But mean ratio rises with redshift 10 23 0.1 1 10 A V (mag) 10 23 0.1 1 10 A V (mag) N HX (cm 2 ) 10 22 N HX (cm 2 ) 10 22 10 21 1 < z < 2 10 21 z < 1 0.1 1 10 A V (mag) 0.1 1 10 A V (mag) Watson et al. 2013.

Conclusions The X-ray absorption in long GRB afterglows is primarily produced by He in the natal HII regions the stars that explode as GRBs. The GRB is powerful enough to destroy the dust and strip the metals associated with this gas The change in the NHx/AV ratio with redshift is largely due to the change in cosmic metallicity Using information on the luminosity of a GRB and its NHx it should be possible to place limits on the sizes and densities of the HII regions of massive starforming regions across a very large redshift range

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