Astronomy

How to shift an AGN X-ray spectrum to rest frame

How to shift an AGN X-ray spectrum to rest frame


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I have limited information to shift a spectrum (in the X-ray 0.5-10 keV bandpass) from redshift $z=2$ to the rest frame. I have a plot of normalized (photon) counts s$^{−1}$ keV$^{−1}$ as a function of energy (keV). Is there a way to shift the spectrum I have to rest frame? Or do I really need luminosity distance, etc.?


The frequency is related to the redshift by $$frac{ u_{ m obs}}{ u_{ m emit}} = frac{1}{1+z}$$

Another useful relation is the fact that, for any redshift,

$$frac{I_{ u}}{ u^3} = { m constant}$$

where $I_{ u}$ is the specific radiative intensity.

The specific count rate (counts per area per time per energy interval) is proportional to $I_{ u}/ u$. (Do you see why?)

That should give you enough information to derive how the measured count rate changes with $z.$ Of course, to know the actual value, not just the function of $z$, you'd need to know the actual count rate measured, not just the normalized value.


How to shift an AGN X-ray spectrum to rest frame - Astronomy


Aims: We discuss the broad band X-ray properties of one of the largest samples of X-ray selected type-1 AGN to date (487 objects in total), drawn from the XMM-Newton Wide Angle Survey (XWAS). The objects presented in this work cover 2-10 keV (rest-frame) luminosities from 10 42 -10 45 erg s -1 and are detected up to redshift 4. We constrain the overall properties of the broad band continuum, soft excess and X-ray absorption, along with their dependence on the X-ray luminosity and redshift. We discuss the implications for models of AGN emission.
Methods: We fitted the observed 0.2-12 keV broad band spectra with various models to search for X-ray absorption and soft excess. The F-test was used with a significance threshold of 99% to statistically accept the detection of additional spectral components.
Results: We constrained the mean spectral index of the broad band X-ray continuum to <Γ> = 1.96 ± 0.02 with intrinsic dispersion <σ <="" γ="" >="">= 0.27 -0.02 +0.01 . The continuum becomes harder at faint fluxes and at higher redshifts and hard (2-10 keV) luminosities. The dependence of Γ with flux is likely due to undetected absorption rather than to spectral variation. We found a strong dependence of the detection efficiency of objects on the spectral shape. We expect this effect to have an impact on the measured mean continuum shapes of sources at different redshifts and luminosities. We detected excess absorption in ⪆3% of our objects, with rest-frame column densities a few ×10 22 cm -2 . The apparent mismatch between the optical classification and X-ray properties of these objects is a challenge for the standard orientation-based AGN unification model. We found that the fraction of objects with detected soft excess is 36%. Using a thermal model, we constrained the soft excess mean rest-frame temperature and intrinsic dispersion to kT 100 eV and σ kT 34 eV. The origin of the soft excess as thermal emission from the accretion disk or Compton scattered disk emission is ruled out on the basis of the temperatures detected and the lack of correlation of the soft excess temperature with the hard X-ray luminosity over more than 2 orders of magnitude in luminosity. Furthermore, the high luminosities of the soft excess rule out an origin in the host galaxy.


Finding AGN at high(er) redshifts

This figure (from Caccianiga et al. 2004) shows the spectrum of a narrow-line AGN. Labeled are many emission lines used to make BPT and other diagnostic diagrams.

This figure shows galaxies (blue), composites (grey), and AGN (red) separated on the traditional BPT diagram, with line emission ratios on each axis.

So if the BPT diagram works so well, why change it? The answer lies in the fact that as one looks to galaxies that are farther away (at higher redshift, z), the emission lines needed to make a BPT diagram quickly shift out of the optical window and into the infrared, where getting spectra is much, much harder. In fact, beyond z > 0.4, the BPT diagram can no longer be used! In 1981, when it was created, a redshift of 0.4 was still relatively high, but now it seems practically nearby. Therefore, the authors of this paper have created a new diagnostic, similar to the BPT diagram, but which relies on the emission lines [NeIII] and [OII] that are at shorter wavelengths (see the spectra above), and uses a completely different feature for the y-axis – restframe (g-z) color. The authors call this the TBT diagram (after their last names – Trouille, Barger, and Tremonti), and in their paper, they show that it works just as well as the BPT diagram to separate star-forming galaxies from AGN, is actually BETTER at finding AGN that are X-ray obscured, and best of all, can be used out to z = 1.4.

The "TBT" diagram, which uses the NeIII to OII ratio and (g-z) color to classify objects as AGN or star-forming galaxies.

Shown at right is an example of the TBT diagram, made with the same SDSS galaxies that were used to create the BPT diagram shown above. The dashed line is the authors’ empirical cut between star-forming galaxies and AGN. As the figure shows, the TBT diagram cleanly separates star-forming galaxies (shown in blue) from AGN (shown in red). The intermediate galaxies (shown in grey), are traditionally called “composites”, because their emission lines are probably excited by a combination of ionizing photons from stars and from an AGN. Unlike the BPT diagram, the TBT diagram places the majority of composite galaxies on the AGN side of the dividing line. To test whether this is valid, the authors look for X-ray emission in these composite galaxies – a sure sign of AGN activity. They find that the galaxies do in fact have weak X-ray emission, indicating that there is probably a heavily obscured AGN at their centers. In addition, the authors find that the TBT diagram places almost all X-ray selected AGN in the correct region, which the BPT diagram fails to do (see Figure 4 in the paper). They suggest that perhaps the TBT diagram does such a good job distinguishing AGN because the [NeIII] emission line it relies on has a higher ionization potential than [OIII], making it a better probe of the high-energy photons that can only come from AGN.


Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. This paper makes use of Chandra data from observation IDs 13401, 16135, 16545, 19581, 19582, 19583, 20630, 20631, 20634, 20635, 20636 and 20797. All raw Chandra data are available for download from the Chandra X-ray Center (https://cda.harvard.edu/chaser/). The Hubble data used in this work is available at the Mikulski Archive for Space Telescopes (MAST https://archive.stsci.edu) under proposal ID 15315. The full raw and reduced FIRE spectroscopy used in this work is freely available upon request. The reduced spectrum is publicly available for download at the Harvard Dataverse (https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/JCFRLB).


4 DISCUSSION

Overall, the X-ray spectrum of NGC 1566 is not unusual for a Sy 1 galaxy, showing standard spectral components. The ionization and velocity of the outflow are well within the normal range seen in other AGNs (Laha et al. 2014). The column density is lower than generally seen, but this is likely due to detection bias: in more distant sources, a low column density outflow is unlikely to be detected. The rapid appearance of this spectrum after a period of quiescence is very unusual, and the increase in brightness by a factor of 30–40 within a short time period is extreme. We do not have X-ray data covering the rise period, but the ASAS-SN V-band light curve shows that the source flux began to rise around September 2017, peaking around the time of our observation (Dai et al. 2018). This means the outburst took ∼9 months to reach the peak. This is longer than previous outbursts: Alloin et al. ( 1986) report a typical rise time of ∼20 days, and the outburst visible in the 105-month Swift BAT catalogue reaches the peak in 3 months. Given the lack of X-ray coverage of previous outbursts, we cannot explain this difference from an X-ray perspective without further data.

Changing look events like this one involve flux changes in multiple wavebands on time-scales far faster than a standard thin disk can evolve. There are several different mechanisms that are invoked to explain this phenomenon, such as obscuration, disk instabilities, and tidal disruption events (TDEs). Variable obscuration, where clumps of cold gas from the torus block the AGN emission (Matt et al. 2003), can be discounted in this case as the optical broad lines have responded directly to the increased optical and UV flux.

Large changes in accretion rate can produce large changes in the flux at all wavelengths, but for a ‘standard’ disk the time-scales involved are far too long. Dexter & Begelman ( 2018) show that disks supported by magnetic pressure have much faster infall times, and can produce changes of a factor of 2–10 in optical to X-ray flux within 1–10 years. While promising for many changing-look events, this is likely not extreme enough to reproduce the rapid increase in flux seen in NGC 1566, which brightened by a factor of ∼40–70 and has had previous outbursts with rise times of less than a month.

In principle, TDEs can produce repeated events over many years (for example, by repeated tidal stripping of a star), and this has been suggested as a possible explanation of repeat X-ray flares in IC 3599 (Campana et al. 2015). However, we consider this unlikely in this case. The theoretical rate of TDEs is low (∼10 −4 per Galaxy, per year), and to find one in such a nearby galaxy that already hosts an AGN would be very unusual. The similarity of this outburst to other changing look events, which are common in nearby galaxies (e.g. Runco et al. 2016) at a rate far in excess of the predicted TDE rate, suggests a common non-TDE origin. Finally, the X-ray spectrum of NGC 1566 is a classic hard AGN spectrum, whereas X-ray spectra of TDEs are typically extremely soft (Komossa 2017, and references therein).

In our view, the most likely interpretation of this behaviour is an instability in the accretion disk. Grupe, Komossa & Saxton ( 2015) discuss this for IC 3599, a Sy 1.9 AGN that has undergone at least two large outbursts. Saxton et al. ( 2015) and Grupe et al. ( 2015) explore the Lightman & Eardley ( 1974) instability, where the inner disk is quiescent until radiation pressure exceeds gas pressure, at which point the disk rapidly switches on. This mechanism produces variability on around the right time-scales but requires that the rise time be longer than the decay time, a condition which has not been met by previous outbursts in NGC 1566. Additionally, the repeat time is set by the viscous time at the truncation radius which is typically decades, much longer than observed in NGC 1566 (Alloin et al. 1986). Ross et al. ( 2018) explain the changing look of a |$z$| ∼ 0.4, M ∼ 10 8.8 M quasar with a cooling front that propagates away from the ISCO, followed by a returning heating front over 20 years. This is similar in time-scale after scaling for the mass (∼ a few months), but predominantly affects the flux at short wavelengths, so it does not explain the uniform flux increase and decline in NGC 1566. Noda & Done ( 2018) propose that the drop in flux by a factor of 10 in Mrk 1018 and associated change from Sy 1.9 to Sy 1 is caused by a combination of the H instability, which produces the overall drop in luminosity, and evaporation of the inner disk, which causes the associated spectral hardening. These processes work in reverse, so a heating front caused by the H instability could propagate through the disk, causing an outburst. Interestingly, Noda & Done suggest that sources crossing a few per cent of Eddington should go through a changing look along with strong soft excess variability, and no soft excess was observed in earlier quiescent observations of NGC 1566 (Kawamuro et al. 2013). The time-scales of the changing look events in these two sources are similar when the higher mass of Mrk 1048 (M ∼ 10 7.8 M, see Noda & Done 2018) is taken into account (9 months × 10 ∼ 8 years), although the variability amplitude is greater in NGC 1566. Given these similarities between the outbursts, we consider it likely that they are due to the same mechanism.

The inclination measured from the relativistic Fe line is very low (<11°), which is consistent with the accretion disk being aligned with the face-on galaxy. While this is expected, we note that Middleton et al. ( 2016) found that there is not a strong correlation between host and disk inclinations. We find only an upper limit on the spin, of <0.25. A poor constraint is to be expected, given the weak feature and low inclination, which gives a narrow, hard to measure line. The low spin is interesting, and may be indicating some truncation of the accretion disk (although there are other explanations, e.g. Parker, Miller & Fabian 2018). While these results are intriguing, it is not possible to come to robust conclusions because of the limited signal available.


An X-ray study of luminous infrared galaxies observed with ASCA

The discovery of ultra-luminous infrared galaxies (ULIRGs) has provided a clue to an evolutionary connection between starburst and active galactic nuclei. The IRAS color is suggested to be a possible trace of the evolution. We present the results of ASCA observations of two ULIRGs, IRAS20551-4250 and IRAS23128-5919, which are southern 100 μm bright galaxies with LIR ∼ 10 12 L. Both are mergers and have a “warm” IRAS color ( 25μm 100μm ≥ 0.15 ). The ASCA spectrum of IRAS20551-4250 can be characterized by two components, one of which is a soft thermal component (kT ∼ 0.3keV) and the other is a hard power-law component absorbed by a column density of 10 22 cm −2 . The observed X-ray luminosity is ∼ 2.5 × 10 42 ergs/s in the rest frame 2–10keV band (assuming H0 = 50 km/s/Mpc). IRAS23128-5919 also shows a hard spectrum (LX ∼ 3 × 10 42 ergs/s), but thermal emission is not as clear as that in IRAS20551-4250. Since these targets are similar in infrared luminosity as well as in hard X-rays but not in soft X-rays, LIR would be associated with hard X-rays. In addition to these results, we here compare X-ray properties of ULIRGs with IR properties.


Iron K-alpha lines

6 keV responds to these variations on timescales of less than

3x10^4 s. The response becomes slower and slower toward the line peak near 6.4 keV. An explanation of the less variable narrow core of the iron line is that it originates in distant, cold matter such as a molecular torus.

Figure 1: The mean line profile from ASCA of Seyfert galaxies (Nandra et al. 1997 ApJ 477, 602 Turner et al. 1997 ApJS 113, 23). The Seyfert 1s (top left panel) show a line peak at the rest energy of 6.4 keV, but most of the flux is shifted to the red. The width of the line implies relativistic velocities. The peculiar profiles, with FWHM of order 50,000 km/s, are only easily accounted for by invoking kinematic and gravitational effects close to a central black hole. NELGs and Seyfert 2s observed in transmitted X-rays (bottom left, top right) show remarkably similar profiles. Seyfert 2s observed in scattered X-rays (bottom right) show stronger emission lines and evidence for emission from highly ionized species. The major component is at 6.4 keV, however, and the emission still extends red-wards. This implies that all these classes have iron line emission from the central regions, close to the black hole.

An outstanding question is that of whether or not the black hole is rotating. In the Kerr Metric, the disk can extend closer to the central hole, approaching the gravitational radius. If this is indeed the case, we expect broader, more redshifted profiles in the Kerr Metric compared to the non-rotating (Schwarzschild) case. Iwasawa et al. (1996 MNRAS 282, 1038) found MCG-6-30-15 to show an extremely broad Fe K-alpha profile during a low-state, with significant line variability over the 4-day ASCA observation. Those authors concluded that a Kerr metric was required given the strength and shape of the line emission. Both Reynolds & Begelman (1997 ApJ 488, 109) and Weaver & Yaqoob (1998 ApJ, in press) have proposed alternative explanations. It is striking that this dispute is over what is occurring within a few gravitational radii of a black hole.

Based on Ginga data, Iwasawa & Taniguchi (1993 ApJ 413, L15) suggested an X-ray Baldwin effect in AGN, whereby the equivalent width of the Fe K-alpha line decreased with increasing luminosity. This result was disputed until the advent of ASCA data. Recent analysis of a large sample of QSOs confirms the X-ray Baldwin effect (Nandra et al. 1997 ApJ 488, 91). Using a large sample, those authors were also able to demonstrate that the observed relationship was not solely due to radio power, as the effect persists when only radio-quiet sources are considered. However, the effect can be due solely to luminosity of the central source. In addition to the change in strength, the line profile also appears to be different in higher luminosity sources (Fig. 2). Nandra et al. (1997 ApJ 488, 91) suggest that the accretion disk ionization state is a function of the accretion rate.

Figure 2. Data/Model ratios, constructed as in Fig.1, split into luminosity bins. The vertical dotted lines are at an energy of 6.4 keV. The profile for all sources (upper left) is dominated by the high signal-to-noise objects which are mostly low luminosity. Below L_X = 10^44 erg/s the line profiles are very similar, but above this luminosity there are clear changes. The line strength reduces with increasing luminosity, in both the core and red wing, but the blue flux is enhanced relative to the total line emission. Above L_X = 10^46 erg/s no evidence for line emission is observed at all. This confirms the ``X-ray Baldwin'' effect suggested by Iwasawa & Tanaguchi. The mean redshift is also shown, and demonstrates the strong correlation between redshift and luminosity in our sample.

Analysis of some bright ``intermediate'' Seyferts has allowed detailed analysis of the complex shape of the Fe K-alpha line. Weaver et al. (1997 ApJ 474, 675) demonstrate that the Fe K-alpha line in MCG-5-23-16 cannot be modeled with the profile predicted by standard disk theories. An additional component is required, most likely a narrow line produced in the putative torus or the broad-line-region. This result is the strongest direct evidence to date for two distinct reprocessors within a single Seyfert galaxy.

Recent analysis of ASCA observations of type-2 Seyfert galaxies showed that both Seyfert-2 galaxies and Narrow Emission Line Galaxies (NELGs) have a significant red-wing on the Fe K-alpha profile, at the same level as that observed in the Seyfert-1 sample (Fig 1 Turner et al. 1997 ApJ S 113, 23). A relatively strong core is observed in these lines, suggesting several line components contribute to the observed profile. In the case where we observe a spectrum dominated by scattered X-rays, then the scattering region may see a face-on disk, whose spectrum is scattered towards the observer. This can explain the broad base and red wing in the Fe K-alpha line observed in these sources. Iwasawa, Fabian & Matt (1997 MNRAS 289, 443) suggest another mechanism for producing the red wing in scattering-dominated sources, and the existence of a Compton-shoulder has been suggested in some cases.

In other Seyfert 2s the nuclear X-rays are observed, transmitted through a large column of gas (10^22-10^24 cm^-2). If these sources have their inner regions inclined edge-on, as predicted by AGN Unification Models, then the profiles of the Fe K-alpha lines should be systematically different from those observed for Seyfert

1 galaxies. In fact the mean profiles from several sub-groups of Seyfert 2 galaxies look remarkably similar, leading Turner et al. (1998 ApJ 493, 91) to suggest the orientation of the inner regions may be the same as for Seyfert

1 galaxies. If this were true, factors other than orientation would have to explain the observed differences between the classes. Unfortunately it is difficult to separate the components contributing to the Fe K-alpha line profiles. A narrow line component is likely to arise in circumnuclear material well outside of the accretion disk. Weaver & Reynolds (1998 ApJ, in press) challenge the conclusions of Turner et al. (1998 ApJ 493, 91), asserting that consistency with Unification Models can be achieved if the narrow core of the line is stronger than indicated purely from the column provided by the line-of-sight absorber.

Other X-ray Line Emission in AGN

1 galaxies alone. In particular, it is possible to detect many soft X-ray emission lines in Seyfert

2 galaxies, which are rarely detected in Seyfert

1s. Hydrogen-like and helium-like lines are detected from Fe, Ne, Si, S, Ar and Mg and these cannot be explained solely by the presence of starburst emission (Turner et al. 1997 ApJ S. 113, 23). Netzer et al. (1998 ApJ, in press) find many lines to be consistent with an origin in narrow-line-region clouds. Arguably the most spectacular X-ray result for a Seyfert 2 galaxy to date is provided by the ASCA spectrum of Circinus (Matt et al. 1996 MNRAS 280, 823). The observed spectrum appears to be purely reprocessed radiation, allowing precise measurement of many emission lines.

Warm Absorbers

The essential features of this model were supported by the observed variability of the absorption edges in MCG-6-30-15 on time scales of hours (Otani et al. 1996 PASJ 48, 211). However, the variability patterns of the edges in MCG-6-30-15 and of warm absorbers in general suggest a complex absorption structure. In some objects, rapid variability of the warm absorber coupled with a lack of correlated variability of the edges suggests there are at least two absorption ``zones'', the first being highly ionized and located close to the central source (perhaps situated within the broad-line region), and the second being a more diffuse ionized medium, possibly a wind from the molecular torus or the warm scattering medium seen in Seyfert 2s. The highly-ionized zone may have been observed previously in the deep Fe K-shell edges observed with Ginga (Nandra & Pounds 1994 MNRAS 268, 405). In other objects, such as NGC 1365, photoionization modeling of simultaneous far-UV and X-ray spectroscopic observations imply an absorbing region with a broad range of ionization parameters and column densities (Kriss et al. 1996 ApJ 467, 629). Still other objects have significant optical reddening and display deep OVII edges, which suggests the existence of a dusty warm absorber (Reynolds 1997 MNRAS 286, 513 George et al. 1998 ApJ S 114, 73).

Narrow-Line Seyfert

1 galaxies of comparable luminosity. The distinctions between Seyfert 1 and NLSy1 galaxies may be driven by a fundamental difference between the two, such as accretion rate.

In an exciting new result, Leighly et al. (1997 ApJ 489, L25) report the observation of features near 1 keV in the ASCA spectra from three NLSy1s. Those authors interpret these as oxygen absorption in a highly relativistic outflow. If interpreted as absorption edges, the implied velocities are 0.2--0.3 c, near the limit predicted by "line-locking" radiative acceleration. If instead interpreted as broad absorption lines, the implied velocities are

0.57 c, interestingly near the velocity of particles in the last stable orbit around a Kerr black hole, although a physical interpretation of this is not obvious. The features are reminiscent of the UV absorption lines seen in broad absorption line quasars, but with larger velocities.

Observation of the bright NLSy1 Ton S

180 shows an Fe K-alpha line which is broad and asymmetric, indicating an origin in the accretion disk. However, that line has an energy close to 7 keV, this, and the detection of spectral features consistent with soft X-ray emission lines suggest the existence of an ionized accretion disk in Ton S

180, consistent with a higher accretion rate in this source, compared to Seyfert 1 galaxies.

Halpern & Moran (1998 ApJ 494, 194) use ASCA data to confirm the NLSy1 nature of IRAS 20181-2244. This source also shows rapid X-ray variability, ruling out the previous supposition that it is a hidden/scattered QSO, or type 2 QSO. The distinction is very important because high-luminosity NLSy1 objects are not uncommon, but type-2 QSO are very rare. This ASCA result leaves no known cases of X-ray emitting QSOs of type-2. The absence of such a population must be accounted for by Unification Models.

Starburst and Ultra-luminous infrared galaxies

M82 and NGC 253 are the most well-studied Starburst galaxies in X-rays. ASCA observations (Tsuru et al. 1997 PASJ 49, 619 Moran & Lehnert 1997 ApJ 478, 172 Ptak et al. 1997. AJ 113, 1286 Dahlem, Weaver & Heckman 1998 ApJS, in press) have proven the intricate nature of their spectra and have allowed the physics of these systems to be examined in great detail for the first time. Between 0.1 and a few keV, the spectra are dominated by thermal emission with lines from highly ionized Mg, Si, and S. Above a few keV, there is a hard component extending to >10 keV. The soft X-rays are extended, absorbed by the galaxy disk, and clearly associated with the outflow, while the hard X-rays are pointlike and are most likely due to XRBs (or a possible weak AGN). The data indicate the presence of at least two gas phases, one at

0.2-0.4 keV and the other at

0.65-0.9 keV. There is some evidence that the abundances of O, Ne, Mg, Si, S, and Fe are significantly lower than the cosmic value (Tsuru et al. 1997 Ptak et al. 1997), in which case neither type-Ia nor type-II supernova explosions can reproduce the observed abundance ratio. However, if the data are modeled to allow for the absorption of the X-ray emitting gas by the edge-on plane of the host galaxy (seen in the Rosat images), the abundances are not required to be significantly sub-solar (Dahlem et al. 1998).

Ultra-luminous infrared galaxies (ULIRGs) may provide an evolutionaly link between starburst nuclei and active galactic nuclei. If ULIRGs evolve from being starburst-dominated to being powered by an embedded AGN, then we should find examples of both stages of evolution, with the IR emission (indicating the amount of dust) and X-ray emission (indicating a buried AGN) being the best tracers of the change from the starburst-dominated phase to gas-dust enshrouded AGN phase. ASCA, with its broad bandpass coverage of soft energies (where the starburst contribution dominates) to hard energies (where the emission from the buried AGN can leak out) is thus perfectly poised to address the question of whether ULIRGs are powered by star formation and/or a deeply embeddd AGN.

To date, both types of objects have been found, but how they fit into an evolutionary sequence is not clear. IRAS20551-4250 (Misaki et al. 1997, IAU Symp. 186, in press), IRAS 20460+1925 (Ogasaka et al. 1997 PASJ 49, 179) and NGC 6240 (Turner etal 1997 Iwasawa & Comastri 1998 MNRAS, in press) all possess a hard X-ray, absorbed power-law component and an Fe K-alpha emission line that is consistent with fluorescence from neutral Fe. Their X-ray characteristics, which are consistent with Seyfert 2 galaxies, imply that these merging systems contain and are powered by an embedded AGN. On the other hand, NGC3690+IC694(Arp299), NGC1614 (Watarai et al. 1998, IAU Symp. 186, in press), and Arp 220 (Nakagawa et al. 1998, IAU Symp. 186, in press) have spectra dominated by thermal plasma emission with kT

1 keV, similar to starbursts. A lack of any sign of hidden AGN activities implies that these objects are powered by star formation. It is interesting to note that Arp220 and NGC 6240 are at almost the same evolutionary stage. This suggests that the contribution of AGN activity to total luminosity varies from galaxy to galaxy and there is no obvious correlation with evolutionary sequence.

Low-luminosity AGN and LINERs

1/3 of nearby galaxies have low-level activity classifying them as LINERs (low ionization nuclear emission-line regions). The ionization mechanisms of LINERs, which includes shock excitation and photoionization by low luminosity AGN are still under debate. ASCA observations of LINERs and low-luminosity AGN (Ptak 1988, Ph.D. thesis, UMD Terashima et al. 1997, IAU Symp. 184, Kyoto Japan) show that in most cases the spectra are well described by a canonical model consisting of a power-law with a photon index, gamma

1.7-2.0, plus soft, optically-thin thermal emission with kT

0.6-0.8 keV. LINERs typically show a hard point-like nuclear source of X-ray luminosity of 10^40 - 10^41 erg/s, which provides strong support for the presence of a low luminosity AGN. The soft component is usually extended and the nuclear, point-like emission is sometimes absorbed by column densities in the range NH = 10^20-10^23 cm^-2. The hard component is similar to the X-ray spectra of quasars and classical Seyfert galaxies however, no significant short time scale ( 5. Since production of TeV photons requires Comptonization by TeV electrons, both TeV and keV photons are produced by electrons with gamma_e

10^6. The key evidence that this version of the SSC model is correct comes from the spectral variability observed in X-rays. The time-resolved spectral variability observed by ASCA manifests itself as a ``soft lag'' (or ``hard lead'') (Takahashi et al. 1996, ApJ 470, L89), and implies a lifetime of radiating electrons to synchrotron losses t_s (E/1 keV) of 6000

s. This, with delta = 5, implies B

0.2 Gauss, and yields gamma_e

5x10^5 (E/1 keV)^1/2, in excellent agreement with the value obtained by the Compton energy transfer argument above.

With the recent detection of two more TeV-emitting BL Lac objects, and several multi-wavelength campaigns planned for the current cycle, ASCA observations are crucial to our understanding of the structure of these enigmatic sources of the highest energy photons observed in the Universe. A good example is an observation of another TeV blazar, Mkn 501 (Kataoka et al. 1998, ApJ, submitted) however, this observation was clearly undersampled. A continuous ``long look'' is needed to learn more about radiation processes in this object and to allow for a comparison with Mkn 421 to find out whether the behavior described above is a general property of BL Lac - type blazars, or is unique to Mkn 421.

Broad-line Radio Galaxies

2x10^21 cm^-2), a steep soft excess, and a broad (sigma

300 eV) plus a possible hard component, corresponding either to reflection from an accretion disk or to a flatter power law from a jet (Grandi et al. 1997, ApJ, 487, 636). The broad-line radio galaxy 3C 445 also shows an absorbed, relatively flat spectrum (nH

10^22-23 cm^-2) and a broad Fe K-alpha line (sigma

0.2 keV indicating gas with velocities of

270 eV (Sambruna et al. 1998 ApJ 495, 749). The significant intrinsic absorption and broad Fe K line indicate that the X-ray emission in the 0.5--10 keV energy band is dominated by a Seyfert-like component. The inverse-Compton X-ray emission from the radio jets appears to become important at > 10 keV. These results support unified schemes for active galaxies, and demonstrate a remarkable similarity between the X-ray properties of powerful radio sources and those of lower luminosity, Seyfert 1 galaxies. The inverse-Compton X-ray emission from the radio jets appears to become important at > 10 keV.


We present new observational determinations of the evolution of the 2–10 keV X-ray luminosity function (XLF) of active galactic nuclei (AGN). We utilize data from a number of surveys including both the 2 Ms Chandra Deep Fields and the AEGIS-X 200 ks survey, enabling accurate measurements of the evolution of the faint end of the XLF. We combine direct, hard X-ray selection and spectroscopic follow-up or photometric redshift estimates at z < 1.2 with a rest-frame UV colour pre-selection approach at higher redshifts to avoid biases associated with catastrophic failure of the photometric redshifts. Only robust optical counterparts to X-ray sources are considered using a likelihood ratio matching technique. A Bayesian methodology is developed that considers redshift probability distributions, incorporates selection functions for our high-redshift samples and allows robust comparison of different evolutionary models. We statistically account for X-ray sources without optical counterparts to correct for incompleteness in our samples. We also account for Poissonian effects on the X-ray flux estimates and sensitivities and thus correct for the Eddington bias. We find that the XLF retains the same shape at all redshifts, but undergoes strong luminosity evolution out to z

1 , and an overall negative density evolution with increasing redshift, which thus dominates the evolution at earlier times. We do not find evidence that a luminosity-dependent density evolution, and the associated flattening of the faint-end slope, is required to describe the evolution of the XLF. We find significantly higher space densities of low-luminosity, high-redshift AGN than in prior studies, and a smaller shift in the peak of the number density to lower redshifts with decreasing luminosity. The total luminosity density of AGN peaks at z= 1.2 ± 0.1 , but there is a mild decline to higher redshifts. We find that >50 per cent of black hole growth takes place at z > 1 , with around half in L_X < 10^(44) erg s^(−1) AGN.


SOLID THIN FILMS AND LAYERS

2.3.2.1 Structure Analysis

To study the structural properties of the Ta2O5 films we performed X-ray diffraction measurements on the 2θθ mode on a series of samples obtained under nonheated and heated substrates. All of the samples corresponding to certain Ts showed identical X-ray spectra. For the base silicon substrate we obtained a sharp and intense X-ray reflection peak at 2θ = 30.005°, d = 2.976 Å, and three week peaks at 2θ = 23.635°, 39.970°, 48.900° and d = 3.716 Å, 2.254 Å, 1.865 Å, respectively. These peaks correspond to reflections from the bulk silicon crystal. Figure 15 shows the XRD pattern (20) plot of the as-deposited film, and in Table VI the diffraction peaks extracted are listed. The nature of the X-ray spectra indicates an amorphous structure of Ta2O5—no diffraction peaks (except for those from the Si substrate and the SiO2 layer, probably presented at the interface with Si) were observed in the spectra of the as-deposited films. The interplanar spacing showed good agreement with that of a SiO2-α cristobalite difftactogram. As is seen, two intensive peaks at 2θ = 69.41° and 69.60° for the layers obtained at room temperature are registered. A slight shift of the peaks positioned with respect to the position of SiO2 [214] is observed. The spectrum corresponding to the layers obtained at Ts = 493 K showed the existence of a sharp reflex at 20 = 56.93° and a smaller one at 58.93°. A detailed study of these two most intense peaks in the diffractogram reveal the same slight difference in the peak position. This can also be seen in Table VI , where the position of the observed peaks is compared with the corresponding reflexes [301] and [222] of SiO2 [ 71 , card no. 11−695]. Generally it may be concluded that the as-deposited Ta2O5 layers have an amorphous structure for the unheated and the heated substrates. At the same time there is strong evidence for the presence of crystalline SiO2, most likely at the interface with Si. Our electrical and XPS measurements undoubtedly indicated the formation of an ultrathin SiO2 film at the Si-Ta2O5 interface. The formation of an interfacial SiO2 is obviously an unavoidable process during RF sputtering and is due to the simultaneous action of two factors favorable for its formation, namely, a discharge containing active oxidizing particles and a silicon surface that is easily oxidized. Now the present XRD data are in accordance with the electrical and XPS results. The Ta2O5 films annealed at 873 K reveal the same tendency in the diffraction pattern as the above results for the as-deposited samples. The Ta2O5 layers are X-ray amorphous no diffraction peaks, except for those from the silicon substrate and from the intermediate SiO2 at the interface with Si, were observed in the spectra of the annealed films. It is also in accordance with the results of others [ 14 , 55 , 56 ]. After the annealing of the layers corresponding to the unheated substrate, small-intensity [111], [202], and [220] reflections appear in the spectrum ( Fig. 16a , Table VII ). A little change in crystal modification of SiO2 after annealing of the layers with Ts = 493 K is observed ( Fig. 16b , Table VII ). The annealed films have an amorphous structure of Ta2O5, and the interplanar spacings for SiO2 are very similar to that of crystalline SiO2 (α-cristobalite). The XRD pattern of the films annealed at 1123 K for 30 min shows diffraction peak characteristics of Ta2O5 crystals. The spectra exhibit well-defined peaks of orthorombic phase. It is seen from Figure 17 that Ta2O5 has a crystalline structure after annealing at 1123 K—sharp diffraction lines appear in the XRD pattern. The temperature of the phase transition is therefore between 873 and 1123 K. In Table VIII the observed diffraction peaks obtained from a random powder sample [ 71 ] are also given for comparison. From the Table one can identify the polycrystalline Ta2O5 of the samples as the low-temperature β-Ta2O5 modification. As is seen, most of the observed peaks agree well with the orthorombic crystal structure of tantalum oxide films [ 71 , card no. 25−0922]. From the comparison of the measured and the reference spectra it can be seen that they are identical, with the exception of certain peaks in crystal direction, such as [1 12 1] [3 3 0]. Inasmuch as the latter missing two peaks have a low theoretical intensity, their absence from the spectrum of the samples investigated is not significant. This is why we can conclude that the experimental spectrum for layers obtained at 293 K and annealed at 1123 K are very well fitted with the reference spectrum of orthorombic phase. Other authors also observed a β crystalline phase in Ta2O5 after high-temperature annealing, at 973 K [ 65 ], 1173 K [ 65 ], 1173–1273 K [ 63 ], and temperatures higher than 1273 K [ 72 ]. The [2 1 0], [0 0 1], and [0 8 1] peaks are strong and the rest are weaker ( Fig. 17 ). The peaks with small intensities appearing at 2θ = 39.375°, 47.600°, 48.400°, and 52.860° can be associated with the crystal phase of SiO2 [ 71 , card no. 11−695]. The highly intensive peak at 68.610° and the peak at 74.490° are due to the signal from the silicon substrate. The spectrum of the layer deposited at Ts = 493 K also corresponds to the presence of an orthorombic phase of Ta2O5, and again the peaks [0 0 1] and [0 8 1] are the strongest ones. A well-pronounced peak appears at [2 1 1], instead of the [2 0 1] peak corresponding to the case where Ts = 293 K ( Fig. 17 ). A number of small peaks present in the spectrum for Ts = 293 K ( Fig. 17 ) disappear in the spectrum corresponding to Ts = 493 K, with the exception of the small peak at 57.215°, which can be associated with a small shift of the peak corresponding to the phase [3 12 1]. So, in general, the spectrum for Ts = 493 K exhibits more pronounced peaks, but their number is smaller in comparison with the spectrum of the sample obtained at 293 K. The peaks at 69.455° and 69.650° also correspond to the silicon substrate. The only peak that could be related to the crystal phase of SiO2 at 47.180° is not very pronounced, with a significant (three to four times) intensity reduction. This why we interpret the spectrum as not showing crystal SiO2. Then it can be concluded that the higher substrate temperature obviously stimulates the formation of amorphous SiO2 (and consequently of better quality) rather than the crystalline one. Then one can expect that the layers obtained at 493 K will have better interface properties. In addition, the substrate temperature during deposition has a negligible effect on the structure of the layers (crystal phases corresponding to heated and unheated substrate in general are the same the differences are small details).

Fig. 15 . XRD spectra of as-deposited Ta2O5 layers (d = 27–28 nm). (a) Ts = 293 K. (b) Ts = 493 K.


Bright X-Ray Galactic Nuclei

A Chandra X-Ray Observatory image of a field of galaxies in the costellation Bootes. A new study of 703 galaxies with supermassive black holes in this field finds that although infrared from dust and X-ray emission from the nucleus tend to be correlated, the infrared emitted by the supermassive black holes is not well correlated with the dust, suggesting the role of our viewing angle of a torus around the black hole nuclei.

All massive galaxies are believed to host supermassive black holes (SMBH) at their centers that grow by accreting mass from their environment. The current picture also imagines that the black holes grow in size as their host galaxy evolves, perhaps because galaxy evolution includes accretion triggered, for example, by galaxy mergers. This general picture has been substantiated by two lines of data. The peak epoch of black hole accretion can be measured by observations of nuclear activity, and coincides with the peak epoch of star formation in the universe about ten billion years after the big bang. Star formation is associated with disruptions that stir up the gas and induce accretion. Moreover, the local universe shows a tight correlation between SMBH mass, host galaxy bulge mass, and the spread of stellar velocities. These methods (but with weaker confirmation) can similarly estimate the sizes of SMBH in galaxies in the earlier universe, and find that SMBH growth and galaxy growth are co-evolutionary processes. Indeed, it seems the processes may regulate each other over time to produce the galaxy and SMBH sizes we observe today.

Both central black hole growth and star formation are fed by the abundance of molecular gas and dust that can be traced by the infrared emitted by the dust. Dust grains, heated by the radiation from young stars and AGN accretion, emit strongly in the infrared. Since AGN activity also produces X-rays, the expectation is that AGN should track strong dust emission and that X-ray and infrared emission should be correlated. CfA astronomer Mojegan Azadi was a member of a team that examined 703 galaxies with active SMBH nuclei using both X-ray data from Chandra and infrared from Spitzer and Herschel, the largest sample to date making this comparison. Although the team did find a trend consistent with the infrared correlating with AGN X-ray activity over a wide range of cases, they did not find one when compared with the AGN's infrared (not- X-ray) contributions. Since the AGN infrared comes largely from a dusty emitting torus around the SMBH, the difference could point to the role of the angle with which we view the torus. These results help to refine the current models of AGN activity, but the authors note that more sensitive, deeper observations should be able to sort out more clearly the physical processes associated with the AGN.

"Infrared Contributions of X-Ray Selected Active Galactic Nuclei in Dusty Star-forming Galaxies," Arianna Brown, Hooshang Nayyeri, Asantha Cooray, Jingzhe Ma, Ryan C. Hickox, and Mojegan Azadi, The Astrophysical Journal 871, 87, 2019.