An extreme, blueshifted iron line profile in the Narrow Line Seyfert 1 PG 1402+261; an edge-on accretion disk or highly ionized absorption?

We report on a short XMM-Newton observation of the radio-quiet Narrow Line Seyfert 1 PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong excess of counts between 6-9 keV in the rest frame. This feature can be modeled by an unusually strong (equivalent width 2 keV) and very broad (FWHM velocity of 110000 km/s) iron K-shell emission line. The line centroid energy at 7.3 keV appears blue-shifted with respect to the iron Kalpha emission band between 6.4-6.97 keV, while the blue-wing of the line extends to 9 keV in the quasar rest frame. The line profile can be fitted by reflection from the inner accretion disk, but an inclination angle of >60 deg is required to model the extreme blue-wing of the line. Furthermore the extreme strength of the line requires a geometry whereby the hard X-ray emission from PG 1402+261 above 2 keV is dominated by the pure-reflection component from the disk, while little or none of the direct hard power-law is observed. Alternatively the spectrum above 2 keV may instead be explained by an ionized absorber, if the column density is sufficiently high (N_H > 3 x 10^23 cm^-2) and if the matter is ionized enough to produce a deep (tau~1) iron K-shell absorption edge at 9 keV. This absorber could originate in a large column density, high velocity outflow, perhaps similar to those which appear to be observed in several other high accretion rate AGN. Further observations, especially at higher spectral resolution, are required to distinguish between the accretion disk reflection or outflow scenarios.Comment: Accepted for publication in ApJ (18 pages, 5 figures, 1 table

EXITE2 Observation of the SIGMA Source GRS 1227+025

We report the EXITE2 hard X-ray imaging of the sky around 3C273. A 2h observation on May 8, 1997, shows a $\sim$260 mCrab source detected at $\sim4\sigma$ in each of two bands (50-70 and 70-93 keV) and located $\sim$30' from 3C273 and consistent in position with the SIGMA source GRS1227+025. The EXITE2 spectrum is consistent with a power law with photon index 3 and large low energy absorption, as indicated by the GRANAT/SIGMA results. No source was detected in more sensitive followup EXITE2 observations in 2000 and 2001 with 3$\sigma$ upper limits of 190 and 65 mCrab, respectively. Comparison with the flux detected by SIGMA shows the source to be highly variable, suggesting it may be non-thermal and beamed and thus the first example of a ``type 2'' (absorbed) Blazar. Alternatively it might be (an unprecedented) very highly absorbed binary system undergoing accretion disk instability outbursts, possibly either a magnetic CV, or a black hole X-ray nova.Comment: 12 pages, 4 figures, accepted for publication in Ap

Comparisons of various model fits to the Iron line profile in MCG-6-30-15

The broad Iron line in MCG-6-30-15 is fitted to the Comptonization model where line broadening occurs due to Compton down-scattering in a highly ionized optically thick cloud. These results are compared to the disk line model where the broadening is due to Gravitational/Doppler effects in the vicinity of a black hole. We find that both models fit the data well and it is not possible to differentiate between them by fitting only the ASCA data. The best fit temperature and optical depth of the cloud are found to be kT = 0.54 keV and $\tau = 4.0$ from the Comptonization model. This model further suggests that while the temperature can be assumed to be constant, the optical depth varies during the observation period. We emphasis an earlier conclusion that simultaneous broad band data ($3 - 50$ keV) can rule out (or confirm) the Comptonization model.Comment: 4 figures. uses aasms4.sty, accepted by ApJ, email: [email protected]

Occultation Mapping of the Central Engine in the Active Galaxy MCG -6-30-15

The colossal power output of active galactic nuclei (AGN) is believed to be fueled by the accretion of matter onto a supermassive black hole. This central accreting region of AGN has hitherto been spatially unresolved and its structure therefore unknown. Here we propose that a previously reported `deep minimum' in the X-ray intensity of the AGN MCG-6-30-15, was due to a unique X-ray occultation event and that it probes structure of the central engine on scales < 1e14 cm, or 1.4e-7 arcseconds. The data are consistent with a bright central source surrounded by a less intense ring, which we identify with the inner edge of an accretion disk. These may be the first direct measurements of the spatial structure and geometry of the accreting black-hole system in an active galaxy.If the ring of X-ray emission is identified with the inner edge of an accretion disk, upper limits on the BH mass can be derived. Our occultation interpretation is controversial in the sense that X-ray variability in AGNs is normally attributed to intrinsic physical changes in the X-ray emission region, such as disk or coronal instabilities.Comment: 15 pages, 2 Figures. Latex with separate postscript figure files. Accepted for publication in ApJ Letter

On the lack of X-ray iron line reverberation in MCG-6-30-15: Implications for the black hole mass and accretion disk structure

We use the method of Press, Rybicki & Hewitt (1992) to search for time lags and time leads between different energy bands of the RXTE data for MCG-6-30-15. We tailor our search in order to probe any reverberation signatures of the fluorescent iron Kalpha line that is thought to arise from the inner regions of the black hole accretion disk. In essence, an optimal reconstruction algorithm is applied to the continuum band (2-4keV) light curve which smoothes out noise and interpolates across the data gaps. The reconstructed continuum band light curve can then be folded through trial transfer functions in an attempt to find lags or leads between the continuum band and the iron line band (5-7keV). We find reduced fractional variability in the line band. The spectral analysis of Lee et al. (1999) reveals this to be due to a combination of an apparently constant iron line flux (at least on timescales of few x 10^4s), and flux correlated changes in the photon index. We also find no evidence for iron line reverberation and exclude reverberation delays in the range 0.5-50ksec. This extends the conclusions of Lee et al. and suggests that the iron line flux remains constant on timescales as short as 0.5ksec. The large black hole mass (>10^8Msun) naively suggested by the constancy of the iron line flux is rejected on other grounds. We suggest that the black hole in MCG-6-30-15 has a mass of M_BH~10^6-10^7Msun and that changes in the ionization state of the disk may produce the puzzling spectral variability. Finally, it is found that the 8-15keV band lags the 2-4keV band by 50-100s. This result is used to place constraints on the size and geometry of the Comptonizing medium responsible for the hard X-ray power-law in this AGN.Comment: 11 pages, 13 postscript figures. Accepted for publication in Ap

Iron K-alpha Fluorescent Line Profiles from Spiral Accretion Flows in AGNs

We present 6.4 keV iron K-alpha fluorescent line profiles predicted for a relativistic black hole accretion disk in the presence of a spiral motion in Kerr geometry, the work extended from an earlier literature motivated by recent magnetohydrodynamic (MHD) simulations. The velocity field of the spiral motion, superposed on the background Keplerian flow, results in a complicated redshift distribution in the accretion disk. An X-ray source attributed to a localized flaring region on the black hole symmetry axis illuminates the iron in the disk. The emissivity form becomes very steep because of the light bending effect from the primary X-ray source to the disk. The predicted line profile is calculated for various spiral waves, and we found, regardless of the source height, that: (i) a multiple-peak along with a classical double-peak structure generally appears, (ii) such a multiple-peak can be categorized into two types, sharp sub-peaks and periodic spiky peaks, (iii) a tightly-packed spiral wave tends to produce more spiky multiple peaks, whereas (iv) a spiral wave with a larger amplitude seems to generate more sharp sub-peaks, (v) the effect seems to be less significant when the spiral wave is centrally concentrated, (vi) the line shape may show a drastic change (forming a double-peak, triple-peak or multiple-peak feature) as the spiral wave rotates with the disk. Our results emphasize that around a rapidly-rotating black hole an extremely redshifted iron line profile with a noticeable spike-like feature can be realized in the presence of the spiral wave. Future X-ray observations, from {\it Astro-E2} for example, will have sufficient spectral resolution for testing our spiral wave model which exhibits unique spike-like features.Comment: 30 pages, 10 figures, submitted to ApJ, will be presented at 204th Meeting of AAS in Denve

Highly magnetized region in pulsar wind nebulae and origin of the Crab gamma-ray flares

The recently discovered gamma-ray flares from the Crab nebula are generally attributed to the magnetic energy release in a highly magnetized region within the nebula. I argue that such a region naturally arises in the polar region of the inner nebula. In pulsar winds, efficient dissipation of the Poynting flux into the plasma energy occur only in the equatorial belt where the energy is predominantly transferred by alternating fields. At high latitudes, the pulsar wind remains highly magnetized therefore the termination shock in the polar region is weak and the postshock flow remains relativistic. I study the structure of this flow and show that the flow at first expands and decelerates and then it converges and accelerates. In the converging part of the flow, the kink instability triggers the magnetic dissipation. The energy release zone occurs at the base of the observed jet. A specific turbulence of relativistically shrinking magnetic loops efficiently accelerates particles so that the synchrotron emission in the hundreds MeV band, both persistent and flaring, comes from this site.Comment: Submitted to MNRA

Thermal Instability and Photoionized X-ray Reflection in Accretion Disks

We study the X-ray illumination of an accretion disk. We relax the simplifying assumption of constant gas density used in most previous studies; instead we determine the density from hydrostatic balance. It is found that the thermal ionization instability prevents the illuminated gas from attaining temperatures at which the gas is unstable. In particular, the uppermost layers of the X-ray illuminated gas are found to be almost completely ionized and at the local Compton temperature ($\sim 10^7 - 10^8$ K); at larger depths, the gas temperature drops abruptly to form a thin layer with $T\sim 10^6$ K, while at yet larger depths it decreases sharply to the disk effective temperature. We find that most of the Fe K$\alpha$ line emission and absorption edge are produced in the coolest, deepest layers, while the Fe atoms in the hottest, uppermost layers are generally almost fully ionized, hence making a negligible contribution to reprocessing features in $\sim 6.4-10$ keV energy range. We provide a summary of how X-ray reprocessing features depend on parameters of the problem. The results of our self-consistent calculations are both quantitatively and qualitatively different from those obtained using the constant density assumption. Therefore, we conclude that X-ray reflection calculations should always utilize hydrostatic balance in order to provide a reliable theoretical interpretation of observed X-ray spectra of AGN and GBHCs.Comment: Submitted to ApJ; 16 pages plus 13 figure

A Medium Survey of the Hard X-Ray Sky with ASCA. II.: The Source's Broad Band X-Ray Spectral Properties

A complete sample of 60 serendipitous hard X-ray sources with flux in the range $\sim 1 \times 10^{-13}$ \ecs to $\sim 4 \times 10^{-12}$ \ecs (2 - 10 keV), detected in 87 ASCA GIS2 images, was recently presented in literature. Using this sample it was possible to extend the description of the 2-10 keV LogN(>S)-LogS down to a flux limit of $\sim 6\times 10^{-14}$ \ecs (the faintest detectable flux), resolving about a quarter of the Cosmic X-ray Background. In this paper we have combined the ASCA GIS2 and GIS3 data of these sources to investigate their X-ray spectral properties using the "hardness" ratios and the "stacked" spectra method. Because of the sample statistical representativeness, the results presented here, that refer to the faintest hard X-ray sources that can be studied with the current instrumentation, are relevant to the understanding of the CXB and of the AGN unification scheme.Comment: 28 pages plus 6 figures, LaTex manuscript, Accepted for publication in the Astrophysical Journal, Figure 5 can retrieved via anonymous ftp at ftp://ftp.brera.mi.astro.it/pub/ASCA/paper2/fig5.ps.g

On the Evidence for Extreme Gravity Effects in MCG-6-30-15

We examine the unusual Fe-Kalpha line profile in MCG-6-30-15 observed by ASCA during a deep minimum in the source intensity. The intense red wing and depressed blue wing of the line have been interpreted as evidence for extreme gravitational redshifts in terms of emission from within six gravitational radii of a black hole. We find that the data do not uniquely support this interpretation and can be equally well explained by occultation of the continuum source and the putative line-emitting accretion disk, which we offer as an alternative hypothesis. Two problems with previous modeling were that the equivalent width of the line during the deep minimum was required to be unusually large (> 1 keV) and the line intensity was thought to increase as the source became dim. The occultation model does not suffer from these problems. Our results serve to highlight the hazards of over-interpreting observational results which have low statistical significance, to the extent that theoretical implications can become generally accepted when the data do not provide a strong case for them.Comment: Accepted for publication in The Astrophysical Journal Letters. Latex and 5 postscript figures; 13 pages including figure