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142 Chapter 8 8 .3 L U M I N O S I T I E S

8 .3 .1 S o ft X - ra y lu m in o s itie s .

T he Ultra-Soft Survey was originally designed to select hot, isolated neutron stars by searching for a steep slope in the lowest energy channels of the Einstein IPC PHA distribution, as evidence for the high energy tail of a ~ 1 0 eV blackbody. The

discovery of so m any AGN in the USS sample was an interesting and unexpected surprise, but assembling a sample of AGN on the basis of their o b s e rv e d soft X -ray spectra has introduced a strong redshift selection effect. The AGN soft and h ard com ponent luminosities are compared over an energy range which is defined in the rest-fram e. The observed portion of the steep soft component is rapidly shifted through this energy range as the redshift of the AGN increases. This is illustrated in Figure 8.2a where a typical observed USS-type spectrum is

represented by a soft blackbody component with a T eff of 7.85 eV and a hard power-law com ponent w ith an index of 0.7 (see Section 4.3).

Figure 8.2a illustrates how a ‘typical’ USS AGN spectrum registered in the observer frame appears in the rest frame at different redshifts, w ith the spectrum shifting to higher energies as the redshift increases. P lotted as a solid line in Fig­ ure 8.2b is a sim ulated soft component luminosity distribution (integrated over the 0 . 2 to 4.5 keV range in the AGN rest frame), calculated for this ‘typical’ observed

spectrum at different assumed redshifts. The dashed line illustrates the distri­ bution expected for a perfectly flat spectrum so th a t there is no dependence on redshift in the lum inosity other th an the cosmological distance dependence. The actual soft luminosities for the secure USS sources are also plotted for compari­ son w ith the models. This emphasizes th a t the steepness of the soft component spectrum exacerbates the usual selection in favour of higher lum inosity sources at higher redshifts.

It is thus clear from Figure 8 . 2 th a t by selecting a sample of objects at different

redshifts on the basis of their observed spectral shape, a strong redshift dependence is introduced into the soft component lum inosity because it is m easured over a restricted band. I am unable to quantify the luminosity of the soft component as a whole, w ithout d a ta which reach further into the big blue bump from the UV and the soft X -ray regions. The blackbody param eters which I have derived from modelling the X-ray d a ta are applicable only within the 0.16 to 3.5 keV range; because it is so steep, the model soon becomes unreliable if I extrapolate longwards

T he M ultiwavelength Analysis 143 -10 Key IPC range extrapolation -11 -12 -13 0.2 0.1 z=0 -14 -15 0.40 0.35 0.30 0.15 0.20 0.25

R est-fram e Energy (keV )

48 46 44 42 40 U 0.00 0.10 0.20 0.30 0.40 0.50 R edshift

Figure 8 .2 : (a) T he spectra of 6 hypothetical USS AG N, which have identical observed

spectra but different redshifts (from 0.0 to 0.5), plotted as they would appear in the

rest-frame of the AG N. The redshift of each AGN is indicated on the diagram. T he spectra shift towards higher energies as the redshift increases. The dotted line shows the lower lim it o f the range over which I have calculated the X-ray lum inosities (i.e. 0.2 keV ). The solid lines cover the observed range of the IPC and the dashed lines indicate the extrapolated portion of the fit. (b) The redshift dependence of the soft X-ray com ponent luminosity. T he solid line represents the sim ulated soft com ponent lum inosity distribution for a typical U SS-type spectrum observed at redshifts from 0 to 0.5. Measured values of Lsoft for the secure sources are plotted as asterisks. The dashed line represents a sim ulated distribution for a perfectly flat spectrum so that there is no redshift dependence in the lum inosity, other than the cosm ological distance dependence.

into the EUV. So when looking for the possible dependence of other param eters on the soft lum inosity param eters (eg. L80f t , Lo.2keV and a oa) this very strong

redshift dependence m ust be borne in mind.

8.3.1.1 The frequency o f ultra-soft X -ray excesses in A G N

T he USS and th e EMSS are draw n from the same parent sample (ie. the IPC d atabase), therefore the fraction of EMSS AGN which are USS sources gives an indication of th e ubiquity of ultra-soft X-ray excesses in AGN.f There are 132 EMSS AGN w ith a z<0.5 (the Wilkes et al. and USS redshift distribution suggests th a t the soft com ponent m ay be shifted out of the IPC range at larger redshifts; see Section 8.2) and an N ^ < 3 x 1 0 20 cm- 2 (above this value, most of the soft

X-rays below 0.56 keV are absorbed; see Figure 3.4). Out of these 132 EMSS AGN, 13 are USS sources, representing 1 0% of X-ray selected AGN.

T ab le 8.1 : S ecu re U S S A G N X -ra y and o p tic a l co n tin u u m co rrela tio n s Linear correlation coefRcent array

hs o f t T ^_{s o f t} Lhard L 0 p t h o . 2 k e V L2k e V L2 5 0 0A z & 0 8 T ^ 8 0 f t 1.0(25) f^hard 0.7(17) 0.7(17) L 0 p t 0.6(25) 0.6(25) 0.8(18) Lo . 2 k e V 1.0(25) 1.0(25) 0.7(17) 0.6(25) l > 2ke V 0.7(17) 0.7(17) 1.0(18) 0.9(19) 0.7(17) L 2 5 0 0 A 0.6(25) 0.6(25) 0.8(18) 0.9(29) 0.6(25) 0.8(19) Z 0.8(25) 0.8(25) 0.7(18) 0.6(29) 0.8(25) 0.7(19) 0.5(29) & 0 8 -0 .6 (2 5 ) -0 .6 (2 5 ) -0.3(17) -0 .1 (2 5 ) -0 .6 (2 5 ) -0 .3 (1 7 ) -0 .1 (2 5 ) -0 .6 (2 5 ) &OX 0.0(17) 0.0(17) -0 .5 (1 8 ) 0.1(19) 0.0(17) -0 .4 (1 9 ) 0.2(19) -0 .6 (1 9 ) 0.3(17) &SX 0.9(17) 0.9(17) 0.3(17) 0.3(17) 0.9(17) 0.3(17) 0.3(17) 0.7(17) -0 .9 (1 7 )

Linear correlation coefficient (number o f data pairs)

f T h e EMSS sources have a S /N > 4 in the 0.3-3.5keV band (Stocke et al. 1991) cf.> 3 for the USS, thus there are m any sources in the USS which do not appear in th e EMSS.

I he iviuitiwaveiength A nalysis 14 &

8 .3 .2 H ard X -ra y lu m in o sitie s

The hard component X-ray lum inosity for the secure USS AGN from 0.5 to 4.5 keV, L/tarrf? (see C hapter 4) is plotted in Figure 8.3 as a function of redshift, w ith the X- ray luminosities of the EMSS and HGLS samples included for comparison. These are plotted over the redshift range of 0.0 to 0.5, where most AGN in all 3 surveys lie (see Figure 8.1 and Section 8.2). I have converted HGLS counts to fluxes using the conversion graph in Giommi et al. (1991), assuming a power-law energy index of 1.5 (which was the best fit inferred for the E X O S A T sample; Giommi et al. 1991) and the Nh listed in the HGLS. The EMSS fluxes used have been taken from Stocke et al. (1991) and were converted from count rates assuming an energy index of 1.0.

The hard component luminosities of the USS AGN are on average lower than the X-ray luminosities of the EMSS AGN. The HGLS X-ray luminosities are gen­ erally consistent with those of the EMSS.

M in >o o 0 <>: o. o 0.0 0.1 0.2 0 .3 0.4 0 .5 Redshift

Figure 8.3 : T he hard com ponent lum inosity from 0.5 to 4.5 keV, plotted as a function o f redshift, for the low-redshift (z < 0 .5 ) secure USS A G N (asterisks within boxes). The ‘total’ X-ray lum inosities o f the EMSS (dots) and the HGLS (diamonds) over the same range, are plotted for comparison.

Figure 8.3 shows th a t about one th ird of the USS AGN have a value for Lhard th a t is significantly lower th an the band defined by the X-ray luminosity

146 Chapter 8 distribution of the EMSS (note th a t there are 11 secure USS sources where there are only upper lim its on the hard component flux; only eight have z<0.5), These are th e ‘hard X -ray qu iet’ AGN. O ther USS AGN have an Lhard th a t is comparable to th e EMSS X-ray luminosity distribution (although generally lower th an the average for the EMSS), indicating th a t for these objects, a strong soft component is superposed on a ‘norm al’ underlying hard X-ray power-law.

47 46 45 o CO 44 o 43 42 0.0 0.1 0.2 0.3 0.4 0.5 Redshift

Figure 8.4 : O ptical lum inosities o f the secure USS AGN (asterisks within boxes) plotted as a function o f redshift for the low-redshift ( z < 0 .5 ) AG N. Corresponding lum inosities Me plotted for the EMSS (dots) and the HGLS (diam onds). A ll lum inosities are calculated over the 3000 A to 6000 A range from V m agnitudes.

N ote th a t the ‘h a rd ’ component luminosities of the EMSS and HGLS sources will include any photons from a soft component, if present, since m ultiple spectral com ponents were not differentiated in those studies. Thus if any of the EMSS or HGLS sources have significant soft component emission, their hard component lum inosities will be overestim ated in Figure 8.3. As an illustration of this, if I calculate luminosities for the USS sources on the basis of a single hard power-law fit to the Einstein spectra, I obtain values th a t are consistent w ith those in the EMSS (as expected since bo th are derived from the same count-lim ited sample). However, I presume th a t any contribution from the soft com ponent is relatively small in th e m ajority of the EMSS sources since only a small fraction of Einstein

The M ultiw avelength A nalysis 147 AGN appear in the USS sample. By inference, the same is true of the HGLS since the relative num bers of EMSS and USS AGN suggest th at the incidence of observable strong soft X-ray em itting components among X-ray em itting AGN is relatively low when they are selected without spectral discrimination.

8 .3 .3 O p tica l lu m in o sitie s

The optical luminosities have been calculated in the 3000A to 6000A range for the EMSS and HGLS AGN from the V magnitudes, assuming an optical power-law index of 1.0. I have recalculated the USS optical luminosities in the same way in order to make a comparison. The results are shown in Figure 8.4 for z<0.5 and dem onstrate th a t most of the USS optical luminosities are typical of other X-ray selected AGN (the only notable exception being E1704+608 which is known to be variable).

8.3.3.1 Relationship between X -ray and optical lum inosity

3 2 1 0 0 0 1 2L_ 0.0 0.1 0.2 0.3 0.4 0.5 Redshift

Figure 8.5 : The ratio o f broadband optical to hard X-ray lum inosity (shown separately in Figures 8a and 8b) for the secure low-redshift (z < 0 .5 ) USS AGN (asterisks within boxes), plotted as a function o f redshift. The ratio of broad­ band optical to ‘to ta l’ X-ray lum inosity is plotted for the EMSS (dots) and HGLS (diam onds) for comparison.

Strong correlations between X-ray and optical lum inosity have been reported by previous authors in both X-ray selected (eg. Kriss and Canizares 1982) and

148 Chapter 8 optically selected samples (eg. Zamorani et al. 1981, Kriss and Canizares, 1985). Figure 8.5 shows th a t this correlation also exists in the EMSS and HGLS AGN (although the la tte r is not as tight).

2.5 2.0 o o -o ;9o o o h.o.. rssir 1.0 0.5 0.5 0.0 0.1 0.2 0.3 0.4 Redshift

Figure 8 .6 : T he ratio o f monochromatic optical to hard X-ray lum inosities,

Otoxi plotted as a function of redshift for the low redshift (z < 0 .5 ) secure USS AG N (asterisks within boxes: see Section 5.3.2 for the definition of olox).

A lso plotted is the distribution of o l o x for the EMSS (dots) and the HGLS

(diam onds).

I investigate the relationship between X-ray and optical lum inosity directly using their ratio param eterized by a ox (see Section 5.3.1). Using the m ethod de­ tailed in Avni et al. (1980) which takes into account the upper limits on L2keV-,

I calculate an effective a ox of 1.36 ± 0.05 for the USS secure sources (the aver­ age detected a oz is 1.37). For the EMSS sample, I calculate an average a ox of 1.33 ± 0.01 (this excludes BL Lac objects, ‘norm al’ galaxies and AGN w ith an uncertain redshift) and an average of 1.35 ± 0.05 for the HGLS. Due to the red­ shift effect in the soft com ponent luminosities, values for a os and o l8 X are strongly

dependent on redshift (see Section 7.1.1) and it is not appropriate to calculate the corresponding averages.

Values of otox for the secure USS, EMSS and HGLS sources for z<0.5 are plotted against redshift in Figure 8.6. For the EMSS, the a oxs are those listed in Stocke et al. (1991). I have calculated values of a ox for the HGLS AGN, where

The Multiwavelength Analysis 149 the flux at 2500

A

was derived from the V m agnitudes using the equations in Schm idt (1968) and the flux at 2 keV was derived using the same m ethod as for the broadband X-ray flux (see Section 8.3.2). Note th a t values of a ox for the USS have been calculated after th e subtraction of the soft component: EMSS and HGLS o 0xS include any soft com ponent flux.

A bout two thirds of the USS sources have an olox which lies w ithin the EMSS range; for these objects, the soft com ponent may be superposed on a ‘norm al’ un­ derlying hard X-ray to optical continuum . The rem aining aoxs are high, suggesting th a t the hard component is depressed relative to the optical.

T he ratios of the broad band luminosities (L opt/Lhard) for the secure USS, EMSS and HGLS, are plotted as a function of redshift in Figure 8.5, and bear out the results of the a ox distributions.

8 .3 .4 I n f r a - r e d lu m in o s itie s

Rest-fram e IR luminosities have been calculated at 1 //m, (Li^m) and 1.65/zm (Li ,6 5/zm) for the twelve USS AGN for which IR images have been obtained (see

C hapter 6). These include eight secure and four non-secure sources. Linear regres­ sion correlations between IR param eters and other properties of the USS AGN, ie. X -ray and optical continuum param eters and optical line continuum param e­ ters, are given in Table 8.2.

IR luminosities are com pared w ith those of two other samples, W orrall (1987) and Kriss (1988) in Figure 8.7 and Figure 8.8. T he W orrall sample is made up of 69 radio-quiet QSOs drawn from the optically selected sample of Avni and T ananbaum (1986), while the 88 Kriss AGN are X-ray observed, radio-quiet QSOs from Kriss and Canizares (1985) and Tananbaum et al. (1986), w ith li2keV <

6 x 1024 ergs s 1 Hz 1 and a starlight contribution <50% at lp m . Almost all of the W orrall AGN (62 out of 69) are also in the Kriss sample, but both are shown because Kriss has m easured the IR flux at l p m, whereas W orrall made her m easurem ents at 1.65pm. Also, the IR luminosities given by W orrall include any flux from the host galaxy so they are com pared w ith the to tal USS Li.65/am whereas Kriss has quoted fractional contributions to the to tal IR flux from the host galaxy; I have subtracted these to obtain nuclear IR luminosities and these are com pared with the nuclear IR luminosities of the USS. Only one USS AGN is also found in these samples, E1352+183, which appears in both. The IR flux of this object m easured from UKIRT images is approxim ately 30% lower th an th at

£ b O 32 31 30 29 00 5 32 31 30 29 28 0.00 0.10 0.20 0.30 0.40 0.50 Redshift 0.00 0.10 0.20 0.30 0.40 0.50 Redshift bo 5 34 33 32 31 30 29 28 29 30 31 32 33 34 bo 3 34 33 32 31 30 29 24 25 Log (L2500a) 26 27 Log ( L ^ ) 28 29

Figure 8 .7 : The integrated IR properties of the secure and non-secure USS AG N for which IR images were taken (plotted as asterisks within boxes) compared with the sample of Worrall (1987; triangles), (a) Integrated IR luminosities at 1.65/im, plotted as a function of redshift. (b) Optical luminosities at 2500 A, L2 5 0 0A1 plotted as a

function of redshift. (c) Integrated plotted against L2500A- (d) Integrated

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