CAL- 1683 

A* 

EXOSAT DATA ANALYSIS PROGRAM 

FINAL TECHNICAL REPORT. 

NASA Grant NAG 8-570 

Submitted to: 

National Aeronautics and Space Administration 

George C. Marshall Space Flight Center 

Marshall Space Flight Center, AL 35812 

Title of Research: 

VARIABLE SPECTRA OF ACTIVE GALAXIES 

Prepared by: 

Jules P. Halpern 

Columbia Astrophysics Laboratory 

Departments of Astronomy and Physics 

Columbia University 

New York, New York 10027 

Submitted by: 

Columbia University 

Box 20, Low Memorial Library 

New York, New York 10027 

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May 1988 
EXOSAT DATA ANALYSIS PROGRAM 

FINAL TECHNICAL REPORT' 

NASA Grant NAG 8-570 

Submitted to: 

National Aeronautics and Space Administration 

George C. Marshall Space Flight Center 

Marshall Space Flight Center, AL 35812 

Title of Research: 

VARIABLE SPECTRA OF ACTIVE GALAXIES 

Prepared by: 

Jules P. Halpern 

Columbia Astrophysics Laboratory 

Departments of Astronomy and Physics 

Columbia University 

New York, New York 10027 

Submitted by: 

Columbia University 

Box 20, Low Memorial Library 

New York, New York 10027 

May 1988 
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There are no new technology items reportable under this contract. 
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SUMMARY 

This grant supported the participation 0: J. Halpern as a coinvestigator 

on the analysis of EXOSAT spectra of active galaxies in collaboration with the 

University of Leicester. 

were MR 2251-178 and 3C 120. 

investigations, as well as additional results on X-ray spectral variability 

related to EXOSAT observations of active galaxies. In addition, the 

participation of J. Halpern as a coinvestigator on the EXOSAT observations of 

the dipping X-ray source 4U1624-49 was also supported by this grant. 

The objects examined for X-ray spectral variability 

This report describes the results of these 
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- FIR 2251-178 

MR 2251-178 was the first X-ray selected QSO and the second after 3C 273 

to be detected as an X-ray source. The spectrum was measured by the Einstein 

Observatory on two occasions approximately 1 year apart. 

the MPC 2-10 keV data yielded energy indices a of 0.52 f 0.17 and 0.72 

(+0.35,-0.30) (Halpern 1984). There was evidence for an increase in absorbing 

column density by a large factor over the 1 year interval. Simultaneous 

measurements with the HRI showed that the highly absorbed spectrum (NH 

Power-law fits to 

2x1OZ2 cm-2) must have excess emission below 1 keV. 

variable X-ray absorption in active galactic nuclei were discussed. The 

favored model was one in which photoionzation by the X-ray continuum ionizes a 

cloud or shell of material, so that the emergent spectrum is transparent below 

the K-edge of 0 VI1 at 739 eV, but highly absorbed above this energy. The 

required change in the ionization parameter by a factor of 3 could be due to a 

number of causes, such as varying luminosity, radial motions, and density 

changes. The alternative model, in which the variable absorption is due to 

the passage of cold clouds across the line of sight to the X-ray source, was 

thought to be less likely, primarily because the size of the broad line clouds 

Two possible causes of 

cm) is probably much smaller than that of the X-ray source cm). 

The purpose of the EXOSAT observations of MR 2251-178 was to investigate 

in more detail the time history of the changes in column density in order to 

address the proposed models for the location and physical conditions of the 

absorbing material. 

using the ME instrument and the CMA with thin Lexan filter (Stewart et al. 

1985). Figure 1 shows the count rates in each instrument. While variability 

Nine observations over a period of 80 days were obtained 
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by at least a factor of 2 was seen, the lack of a correlation between the LE 

and ME fluxes implies that the spectral shape of the source must vary. 

Spectral fits to the ME data show column densities varying between 0 and 

3x1022 

large changes were not seen in the LE count rates, which confirms the Einstein 

HRI results. 

(Figure 21, similar to the Einstein results (Halpern 1984). Such 

The EXOSAT results do not unambiguously distinguish between the two 

hypotheses which were suggested to explain the variable absorption, although 

they do show that the absorption can change on a time scale of 1 month. 

Sensitive X-ray grating spectroscopy would help to distinguish between cold 

and warm absorbers, as the energies of the oxygen edges could be directly 

observed. Any future observation of column density variations on time scales 

of 10,000 s or less would be strong evidence for X-ray heating effects, as 

opposed to varying covering by cold clouds. 

-- 3c 120 

In contrast to the previous source, 3C 120 is a object in which the 

spectral index was observed by Einstein to vary, while the column density 

remained constant (Halpern 1985). 

of 2.5 on time scales of days to months. The spectral slope of the 2-10 keV 

X-rays was steeper when intensity was higher, and varied from 0.51 t 0.13 at 

minimum to 0.83 2 0.03 at maximum, with a continuous variation in between. 

The results were interpreted (Halpern 1985) in terms of a composite spectrum 

consisting of variable beamed synchrotron emission from a relativistic jet, 

and a stationary isotropic component. This interpretation is rendered 

plausible by the fact that 3C 120 is a rapidly varying superluminal radio 

The spectral luminosity varied by a factor 
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source. 

The objectives of the EXOSAT monitoring were to confirm the Einstein 

results on X-ray variability and to search for the multicomponent spectra 

suggested by the Einstein data. Five observations were done in October 1984 

and one in February 1985. Unfortunately, the flux and spectral index remained 

constant throughout these observations, with a = 1.850.2, and NH consistent 

with the galactic value. 

-- Mrk 335 

Einstein HRI observations of Mrk 335 showed a soft X-ray flare with an 

amplitude of a factor of 2.5 and a rise time of only 10,000 s (Lee & a. 

1988). The MPC flux varied by a much smaller amplitude. The HRI flux implies 

that there is a soft excess above the extrapolation of the best fit a = 1.25 f: 

0.19 to the MPC data. This is the first observation of rapid, large amplitude 

variability in a soft excess component, which restricts the size of the 

emitting region to less than 3x1Ol4 cm. 

obtained in EXOSAT observations (Pounds a. 1987; Turner and Pounds 1988). 

Thus, the theory that soft X-ray excesses arise near the inner edge of an 

accretion disk around a black hole is strongly supported. In this case, the 

Similar results on Mrk 335 were 

0 be lo' - 10 No. Origin in a hot medium confining the 

emission-line clouds is ruled out, at least for Mrk 335. 
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- 4U 1624-49 

J. Halpern was a coinvestigator on two EXOSAT observations of 4U 1624-49, 

a low mass X-ray binary for which a "dip" was seen.in a short Einstein 

observation. The first EXOSAT observation, of duration 6 hours, resulted in 

the detection of a series of dips in which the flux fell to - 25% of the 

steady level on a timescale of seconds (Watson et al. 1985). The data are 

shown in Figure 3. The dips were accompanied by strong variations in spectral 

hardness consistent with large changes in the absorbing column density. A 

second observation, of approximately 56 hours duration, showed that the dips 

recur with a period of 21 5 2 hours, presumably the binary period of the 

system, which gives 4U 1624-29 the distinction of having the longest orbital 

period of the 8 dip sources (Watson et al. 1985). Subsequently, the 21 hour 

period was confirmed in HEAO A-1 data (P. Hertz, private communication). 
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REFERENCES 

Halpern, J.P., "Variable X-ray Absorption in the QSO MR 2251-178", Ap. J., 

281, 90 (1984). 

Halpern, J.P. 1985, "X-ray Spectrum and Variability of 3C 12O", Ap. J., 290, 

130 (1985). 

Lee, M.G., Balick, B., Halpern, J.P., and Heckman, T.M., "A Soft X-ray Flare 

in the Seyfert 1 Galaxy Mrk 335", Ap. J., in press (1988). 

Pounds, K.S., Stanger, V.J., Turner, T.J., King, A.R., and Czerny, B., 

M.N.R.A.S., 224, 443 (1987). 

Stewart, G.C., Pounds, K.A., and Stanger, V.A., Space Science Reviews, 40 

(1985). 

Turner, T.J., and Pounds, K.A., M.N.R.A.S., in press (1988). 

Watson, M., Willingale, R., King, A., Grindlay, J., and Halpern, J., "The Big 

Dipper: 4U1624-49", Space Science Reviews, 40, 195 (1985). 

Watson, M., Willingale, R., King, A.R., Grindlay, J.E., and Halpern, J., "4U 

1624-49", IAU Circular No. 4051 (1985). 
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Figure 1 - EXOSAT ME 2-6 keV Flux of MR 2251-178 

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Day of 1983 

- EXOSAT LE Count Rate for MR 2251-178 

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Day of 1983 
Figure 2 - MSOAT ME Column Density for MR 2251 -1 78 

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Day of 1983 

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Figure 3. The complete EXOSAT ME X-ray light curve of 4U1624-49 (-2- 

10 keV) shown with 20s time resolution. 

plot is 1984 March 5, 12:23 UT. 

Start time on this