The intrinsic line width of the Fe Ka line of AGN
aa r X i v : . [ a s t r o - ph . H E ] A ug Mon. Not. R. Astron. Soc. , 000–000 (0000) Printed 13 November 2018 (MN L A TEX style file v2.2)
The intrinsic line width of the Fe K α line of AGN Jiren Liu ⋆ National Astronomical Observatories, 20A Datun Road, Beijing 100012, China
ABSTRACT
X-ray fluorescent lines are unique features of the reflection spectrum of the cold toruswhen irradiated by the central AGN. Their intrinsic line widths can be used to probe the line-emitting region. The line widths of the Fe K α line measured from the first order Chandra
High Energy Grating (HEG) spectra are 3 − α line for Circinus, Mrk 3, and NGC 1068. Because the observed Si K α and Fe K α lines arenot necessarily coming from the same physical region, it is uncertain whether the line widthsof the Fe K α line are over-estimated or not. We measured the intrinsic line widths of the FeK α line of several nearby bright AGN using the second and third order Chandra
HEG spectra,whose spectral resolutions are better than the first order data. We found the measured widthsare all smaller than those from the first order data. The results clearly show that the widths ofthe Fe K α line measured from the first order HEG data are over-estimated. It indicates that theFe K α lines of the studied sources are originating from regions around the cold dusty torus. Key words: atomic processes – galaxies: Seyfert – galaxies: individual: (Circinus, Mrk 3,NGC 1068, NGC 3783, NGC 4151, NGC 4388, NGC 4507) – X-rays: galaxies
The cold dusty gas around active galactic nuclei (AGN) ob-scures and reprocesses the intrinsic radiation of AGN and is the keyingredient of understanding di ff erent types of AGN (e.g. Antonucci1993). The obscuring gas emits fluorescent lines when irradiatedby the central AGN. The most prominent one is the Fe K α line at6.4 keV, which is found to be ubiquitous in all types of AGN (e.g.Nandra & Pounds 1994; Shu et al. 2010; Fukazawa et al. 2011). Asa result, the fluorescent Fe K α line can be used to probe the prop-erties of the obscuring gas. For example, the stability of the FeK α line of most AGN indicates that the obscuring gas should beoutside the broad line region (BLR) (e.g. Bianchi 2012). The exactemitting region of the fluorescent line can be inferred from the in-trinsic width of the Fe K α line (e.g. Shu et al. 2011, and referencetherein).The measurement of the intrinsic line width, however, is lim-ited by the spectral resolution of currently available instruments.The first order data of Chandra
High Energy Transmission Grat-ing Spectrometer (HETGS, Canizares et al. 2005) provide a spec-tral resolution of 0.012 Å (full width half maximum, FWHM) withits High Energy Grating (HEG), which at 6.4 keV corresponds to ∼ − , very close to the measured mean FWHM of the FeK α line (2000 km s − ) by Shu et al. (2011).Because the HEG spectral resolution is higher at lower ener-gies, the measurement of the line width can be improved by using ⋆ E-mail: [email protected] other low-energy fluorescent lines. In a recent paper (Liu 2016),we measured the FWHM of the Si K α line (at 1.74 keV) for Circi-nus, Mrk 3, and NGC 1068, which are 570 ± ± ±
280 km s − , respectively. These values are 3 − α line previously, and the esti-mated line-emitting regions are outside the dust sublimation radiiof these AGN. It indicates that the intrinsic widths of the Fe K α lineare likely to be over-estimated. Nevertheless, because the Si K α lineis much more sensitive to absorption than the Fe K α line, the SiK α line may not come from the same physical region as the FeK α line. This makes the estimation of the emitting region of the FeK α line still uncertain.In principle, the uncertainty of the Fe K α emitting regioncan be solved by spectroscopic observations with resolution higherthan the first order HEG data. We noted that the second and thirdorder Chandra
HEG data have a resolution approximately 2 and3 times better than the first order data, but with a less e ff ectivearea (Canizares et al. 2005). We checked the second and third or-der Chandra
HEG data of Circinus, Mrk 3, and NGC 1068, andfound that indeed, the higher order HEG data provide more strin-gent constraints on the intrinsic widths of the Fe K α line than thefirst order data. In this letter we present these measurements. Be-sides Circinus, Mrk 3, and NGC 1068, we also include NGC 3783,NGC 4151, NGC 4388, and NGC 4151, the higher order HEG dataof which are deep enough to provide a meaningful measurement ofthe intrinsic width of the Fe K α line. The errors quoted are for 90%confidence level. c (cid:13) J. Liu et al.
Table 1.
The line widths of the Fe K α line measured from the second andthird order Chandra
HEG data and the first order dataName σ ( ± ± σ ( ± v FWHM ( ± ± v FWHM (Si K α ) a eV eV km s − km s − Circinus 6 . ± . . ± . ±
150 570 ± . ± . . ± . ±
760 610 ± . ± . . ± . ±
570 -Mrk 3 4 . + . − . . ± . + − ± . + . − . . ± . + − ± . + . − . . ± . + − -NGC 4507 11 . + . − . . ± . + − -Note: For Circinus, NGC 4151, and NGC 3783, the second and thirdorder Chandra
HEG data are deep enough to allow the χ statistic, whilefor other sources, the Cash statistic is used. The χ statistic is used for thefirst order data of all sources. a The FWHM of the Si K α line is quoted fromLiu (2016), except for that of NGC 4151, which is measured in this paper,while for NGC 3783, NGC 4388 and NGC 4507, the Si K α line is too weakto provide meaningful constraints. Because the
Chandra
HEG e ff ective areas of the secondand third order are about 15 times less than that of the first or-der (Canizares et al. 2005), only for nearby bright sources withdeep exposures, the higher order HEG data can be used to mea-sure the intrinsic width of the Fe K α line. We searched Chan-dra
Transmission Grating Data Archive and Catalog (TGCat,Huenemoerder et al. 2011), and found that besides Circinus, Mrk 3,and NGC 1068, the second and third order HEG data of NGC 3783,NGC 4151, NGC 4388, and NGC 4151 are also usable. Amongthem, NGC 3783 and NGC 4151 are classified as type 1.5 AGN,while the others are type 2 AGN. All the spectra are extracted froma region with a 2 arcsec half-width in the cross-dispersion direc-tion. The instrumental responses are extracted using TGCat soft-ware with the calibration database (CALDB) 4.6.8. The HEG datafrom ± ± α line of NGC 4151 of di ff erent observations show vari-ations with a factor of 5, and only the low-state observations areused. The continuum variations of NGC 3783 between di ff erentobservations are within a factor of 2, and to improve the signal-to-noise (S / N) ratio, all the observations of NGC 3783 are used. Forall the other sources, no apparent continuum variations are noted.Some higher order HEG spectra of stellar-mass black holes havebeen published in Miller et al. (2015, 2016).
For Circinus, NGC 4151, and NGC 3783, the second and thirdorder
Chandra
HEG spectra are deep enough to allow a robust mea-surement of the width of the Fe K α line. Their spectra are plottedin Figure 1, which are rebinned with a minimum S / N ratio of 4. Forcomparison, the corresponding first order HEG spectra (reducedby a factor of 15) are over-plotted in Figure 1. As can be seen, thesecond and third order HEG spectra are narrower than the first or-der spectra, and can provide better constraints on the width of theFe K α line. It also shows that the Fe K α line is dominated by theneutral Fe atoms, and the contribution from low-ionized Fe + ions(located around 6.42 keV, Kaastra & Mewe 1993) is negligible.The neutral Fe K α line is composed of a doublet, K α at 6.404keV, and K α at 6.391 keV, with a flux ratio of 2:1 (Bearden 1967). × − × − × − × − C oun t s s − k e V − − χ Energy (keV)
Circinus × − × − × − × − C oun t s s − k e V − − χ Energy (keV) − × − × − × − C oun t s s − k e V − − χ Energy (keV)
NGC 4151 − × − × − × − C oun t s s − k e V − − χ Energy (keV) − × − × − C oun t s s − k e V − − χ Energy (keV)
NGC 3783 − × − × − C oun t s s − k e V − − χ Energy (keV)
Figure 1.
The second and third order
Chandra
HEG spectra of the FeK α line of Circinus, NGC 4151, and NGC 3783, corrected for redshift. Thedata are rebinned with a minimum S / N of 4. The fitted models of two Gaus-sian lines plus a linear continuum are plotted as the thick solid histograms.For comparison, the first order HEG spectra (reduced by a factor of 15) areover-plotted as triangles.
We model the observed spectra with two Gaussian lines plus a lin-ear continuum. The two Gaussian lines are centered at 6.404 and6.391 keV with their redshifts and widths fixed with each other, andthe intensity of the K α line is set to be half of the K α line. The fit-ting region is between 6 and 6.6 keV. The fitted results are plotted inFigure 1 and listed in Table 1. For comparison, the line widths ob-tained from the first order HEG data using the same model are alsolisted in Table 1. Note that the widths of the first order HEG datapresented here are a little smaller than those reported by Shu et al.(2010, 2011), because they used one Gaussian line to model the Fe c (cid:13) , 000–000 he intrinsic line width of the Fe K α line of AGN − × − × − C oun t s s − k e V − − ∆ C Energy (keV)
Mrk 3 − × − × − C oun t s s − k e V − − ∆ C Energy (keV)
NGC 1068 − × − × − C oun t s s − k e V − − ∆ C Energy (keV)
NGC 4388 − × − × − C oun t s s − k e V − − ∆ C Energy (keV)
NGC 4507
Figure 2.
The second and third order
Chandra
HEG spectra of the Fe K α line of Mrk 3, NGC 1068, NGC 4388, and NGC 4507, corrected for redshift. Thedata are rebinned with a minimum S / N of 1.5. The fitted models of two Gaussian lines plus a linear continuum are plotted as the thick solid histograms. K α doublet. For Circinus, the one with the best data, the line widthmeasured from the second and third order spectra is about 2 / α line in Liu (2016). ForNGC 4151 and NGC 3783, the line widths of the second and thirdorder spectra are also about 2 / Chandra
HEG spectra of all theother sources are plotted in Figure 2. Because their signals are notas good as the above three sources, their spectra are rebinned with aminimum S / N ratio of 1.5, and the Cash statistic is used. The samedoublet model is adopted. The fitted results are plotted in Figure 2and listed in Table 1. As expected, the errors of the measured linewidths are larger, compared with those of Circinus, NGC 4151, andNGC 3783. Nevertheless, all the line widths of the second and thirdorder spectra are smaller than those of the first order spectra. Thecorresponding FWHM velocities of the Fe K α line of Mrk 3 andNGC 1068 are consistent with those obtained from the Si K α linein Liu (2016). X-ray fluorescent lines are unique features of the reflectionspectrum emitted by the torus when irradiated by the central AGN.The intrinsic line widths of the X-ray fluorescent lines can be usedto probe the location of the line-emitting region. The line widths ofthe Fe K α line measured from the first order Chandra
HEG spec-tra are 3 − α linefor Circinus, Mrk 3, and NGC 1068. Nevertheless, because the ob-served Si K α and Fe K α lines are not necessarily coming from the same physical region, it is still uncertain whether the line widths ofthe Fe K α line are over-estimated or not.In this letter we measured the line width of the Fe K α lineusing the second and third order Chandra
HEG spectra of sev-eral nearby bright AGN. For Circinus, NGC 4151, and NGC 3783,which have deep enough signals, the measured line widths areabout 2 / α line measured from the first order HEG spectra are over-estimated.In principle, the first order and higher order HEG data shouldprovide consistent results on the line widths. However, when theintrinsic line width is too narrow to be resolved by the first orderdata, we do not expect the same results, since there are observa-tional noises and the instrument responses are not ideal. To vali-date our measurements, we simulated 100 HEG observations of theFe K α line using Marx (Davis et al. 2012) for the two Gaussianmodel of Circinus with an intrinsic width of 5 eV (correspondingto the velocity of that measured from the Si K α line width), and theexposure time is set to be the same as that of the real observations.We note that Marx is used to generate the Chandra
HEG RMFcalibration products, and contains all known e ff ects about the in-strumental line profile (D. Huenemoerder, private communication).We then fitted the simulated HEG spectra with the same two Gaus-sian model. We found that 90% of the fitted Fe K α widths from http: // space.mit.edu / cxc / marxc (cid:13) , 000–000 J. Liu et al. the first order spectra are within 8 . ± . . ± . α lines of all the other sources are not well resolved, andthe measurements presented here are only upper limits of the truewidths. Deeper data with similar or higher spectral resolution areneeded to obtain the true widths.For the current measurements, the FWHM velocities of the FeK α line are around 1000 km / s, similar to those of narrow emissionlines. If assuming a virialized orbit, the Fe K α line-emitting regionsare close to the dust sublimation radii, as inferred from the widthsof Si K α line in Liu (2016). It indicates that the Fe K α lines of thestudied sources are originating from regions around the cold dustytorus. ACKNOWLEDGEMENTS
We thank our referee for valuable comments that improved thework and David Huenemoerder for assistance on the higher orderHEG calibrations. JL is supported by NSFC grant 11203032. Thisresearch is based on data obtained from the
Chandra
Data Archive.
REFERENCES
Antonucci, R. 1993, ARA&A, 31, 473Bearden J. A. 1967, Rev. Mod. Phys., 39, 78Bianchi, S., Maiolino, R., Risaliti, G. 2012, AdAst, 2012, 782030Canizares, C. R., Davis, J. E., Dewey, D., et al. 2005, PASP, 117,1144Davis, J. E.; Bautz, M. W.; Dewey, D.; Heilmann, R. K.; Houck, J.C.; Huenemoerder, D. P.; Marshall, H. L.; Nowak, M. A.; Schat-tenburg, M. L.; Schulz, N. S.; Smith, R. K. 2012, SPIE, 8443,84431AFukazawa, Y., Hiragi, K., Mizuno, M., Nishino, S., Hayashi, K.,Yamasaki, T., Shirai, H., Takahashi, H., Ohno, M. 2011, ApJ,727, 19Huenemoerder, D. P., Mitschang, A., Dewey, D., et al. 2011, AJ,141, 129Kaastra, J. S. & Mewe, R. 1993, A&AS, 97, 443Liu, J. 2016, MNRAS, 459, L105Miller, J. M., Fabian, A. C., Kaastra, J., Kallman, T., King, A. L.,Proga, D., Raymond, J., Reynolds, C. S. 2015, ApJ, 814, 87Miller, J. M., Raymond, J., Fabian, A. C., Gallo, E., Kaastra, J.,Kallman, T., King, A. L., Proga, D., Reynolds, C. S., Zoghbi, A.2016, ApJ, 821, L9Nandra, K.; Pounds, K. A. , 1994, MNRAS, 268, 405Shu, X. W., Yaqoob, T., Wang, J. X. 2010, ApJS, 187, 581Shu, X. W., Yaqoob, T., Wang, J. X. 2011, ApJ, 738, 147 c (cid:13)000