ROSAT Observations of Recent Supernovae

H.-U. Zimmermann

Max-Planck-Institut ftir Extraterrestrische Physik

85740 Garching, Germany

Abstract

Results from ROSAT observations of recent supernova events are discussed

in the light of a model, where the soft X-rays originate in the shock interac-

tion of the expanding supernova ejecta with the circumstellar material, left

from heavy mass loss of the progenitor object. Special attention is given to

the observations of SN19933 in M81, where lightcurves and spectral meas-

urements allow a more detailed analysis and comparison to numerical model

calculations.

Introduct ion

Every year a few tens of supernovae are detected in the whole sky by exten-

ded optical search programmes. In the Milky Way within the last 1000 years

visual reports are known only from the 4 supernovae in the years 1006, 1054.

1576 and 1604. Most of the detected outbursts are located in rather dis-

tant galaxies where detailed information is difficult to collect. Furthermore

Astrophysics and Space Science 228: 331-347, 1995. © 1995 KluwerAcademic Publishers. Printed in Belgium.

332

supernovae are often detected only weeks, months or even years after the

explosion when the peak luminosity has already passed. Especially the early

phase after the explosion has been observed in detail only in a few cases.

For the soft X-ray emission from supernovae a number of models have been

proposed. They include scenarios for Compton scattered ~/-rays from the

radioactive decay of :Vi 5s, non-thermal emission from a central pulsar and

thermal emission from a hot neutron star. All the observations discussed in

the following are best explained by a model where X-rays originate from the

interaction of the expanding supernova ejecta with the circumstellar mat-

ter (Chevalier 1982). The progenitor stars of type II supernovae, usually

assumed to be red supergiants, are known to posses slow and dense stellar

winds. If the wind remains constant on longer timescales, a density profile

in the circumstellar material declining with rad ius -2 is established in the

neighbourhood of the progenitor. The supernova ejecta expelled into the

circumstellar matter produce a forward moving shock front and a reverse

directed shock wave that runs into the expanding ejecta material. Depend-

ing on the density profiles in the circumsteltar matter and the ejecta, the

emission from the forward or from the reverse shock region can dominate

the observed soft X-ray flux. Temperatures and densities in these 2 regions

333

are quite different. Thus X-ray lightcurves and spectra allow to trace the

density profiles of the circumstellar matter and to investigate in some detail

the interaction processes.

Before ROSAT, the only supernova outburst from which low energy X-r~ ,s

had been seen was SN1980K in NGC 6946. The emission was detected with

the EINSTEIN observatory at a 5 sigma level of confidence 35 days after

maximum optical light (Canizares et al. 1982). 32 days later, the source

could no longer be seen. Assuming a thermal spectrum with kT equal to 5

keV, an Nu column density equal to 1021cra -2 and a distance of 10 Mpc, the

authors calculated a luminosity of 2 x 1039 erg/sec (0.2-4 keV) during the

first observation.

Supernovae observed by ROSAT

Supernova research received a new impulse on February 23, 1987 when in

the Large Magellanic Cloud the progenitor star of SN1987A exploded. At a

distance of 52 kpc it is the nearest supernova observed in the past 400 years.

Half a year after the outburst hard X rays, originating from the radioactive

decay of Ni 56, were detected and could be observed for more than 2 years

(Sunyaev et al., 1987; Dotani et al. 1987; Inoue et al. 1991). A rocket

experiment was launched in August 1987 to search for soft X-ray flux from

334

the supernova. Disappointingly only an upper limit of 1.5 x 1036 erg/sec could

be established for the luminosity in the 0.2 to 2.1 keV band (Aschenbach et

al. 1987).

The excitement about the supernova had still sufficient drive to have the first

pointing of the ROSAT X-ray telescope after the launch on June 1, 1990, at

that object. Again no emission from the supernova was detected, resulting

in a new upper limit of 2.5 x 1034 erg/sec (Trfimper et al. 1991).

Eventually Beuermann et al. (1994) and Gorenstein et al. (1994) detected

weak signals from the direction to the supernova in deeper images that were

taken in the years 1991 and 1992. The measured flux corresponds to a source

luminosity of about 1034 erg/sec. There is an indication of a somewhat higher

flux value in the 1992 data. Future measurements will be necessary to fully

exclude the possibility that the observed emission originates from another

object in the crowded star field around SN1987A.

In terms of the model discussed before, the extremely weak soft X-ray flux

from SN1987A is explained by the fact that the progenitor of SN1987A was a

blue supergiant star instead of the expected red supergiant. Contrary to red

supergiants the blue type is known to possess thin and fast winds, which blow

out a low-density bubble around the star. The density of the circumstellar

335

medium therefore was initially too small for an efficient production of X-rays.

The subsequent discovery of soft X-rays from the supernova in 1991.4 years

after the explosion, dan then be interpreted as the instant when the shock

wave expanded into denser circumstellar matter. Chevalier (1992) suggested

that the supernova shock wave had reached the region where the fast,-thin

wind from the blue supergiant progenitor phase hit the slow and denser wind

from a previous red supergiant phase of the progenitor.

In 1992 Bregman and Pildis detected with ROSAT soft X-rays from SN1986J

in NGC 891. This supernova exploded probably already in late 1982 and was

first detected in radio. Due to its large radio luminosity measureable soft X-

ray emission had been predicted by Chevalier in 1987. It appears plausible

that in this case a red supergiant progenitor produced the high circumstellar

matter density required to explain the X-ray luminosity of a few times 104o

erg/sec (for a distance of 10 Mpc), Interestingly the absorbing column dens-

ity as deduced from the ROSAT spectrum is appreciably higher than that

derived from the optical data. The authors suggest therefore that part of

the material behind the reverse shock had cooled sufficiently to produce the

required additional absorption.

Supernova 1978K in the nearby galaxy NGC 1313 (at ~4.5 Mpc) appears in

336

many aspects similar to SN1986J. 13 years after the explosion a soft X-ray

flux of 9.5 x 1039 erg/sec was observed with the ROSAT PSPC. Also here

appreciable intrinsic absorbing material is required to explain the observed

low energy cut-off in the X-ray spectrum (Ryder et a1.1993). This supernova

had already been observed with the EINSTEIN satellite ~18 months after

the optical outburst, but had not been detected. Ryder et al. calculated

for the EINSTEIN observation an upper limit for the luminosity of 9 x 103s

erg/sec in the 0.2-2.4 keV band.

It was a unique chance when, at the end of March 1993, a supernova exploded

in the nearby galaxy M81, only 3.6 Mpc away from us. On April 3. 6 days

after the outburst, the ROSAT telescope was pointed towards that position.

From former observations of M81 we knew the X-ray image of that region

and therefore detected immediately a strong new X-ray source at the optical

position of the supernova (Zimmermann et al. 1993a). Figure 1 shows 2

perspective views of the center region of M81, before and after the explosion.

In the image to the right, taken in early April 1993, the supernova shows

up as the righthand source in front of the bright nucleus and only 1 arcmin

off from another strong X-ray source, that was already present in the image

to the left, taken half a year before the explosion. Two days after ROSAT,

337

Figure 1: Perspective plots from weakly smoothed images of the central region of M81. The plot to the left originates from a 20 ksec exposure taken with the PSPC in September 1992. The plot to the right shows the same region after the supernova explosion (27.3 ksec PSPC data). The distance between the nucleus of MS1 in the background and the bright X-ray source at the lower rim is ,~3 arcminutes. The intensity scale is linear. The supernova and the bright neighbouring source are fully resolved.

338

SN1993J was also detected by the Japanese X-ray satellite ASCA (Tanaka et

al. 1993) in the energy range 1 to 10 keV. confirming the high temperatures

of the X-ray emission as deduced from the ROSAT spectrum (Zimmermann

et al. 1993b). SN1993J has probably also been detected at energies > 50

keV by the OSSE instrument (Leising et al. 1994).

During the next 40 days the initial luminosity of 2.9 × 1039 erg/sec decreased

by about 40 % (Zimmermann et a]. 1993c). The source was again observed

during the next visibility window in November 1993 and thereafter in April

1994 (Zimmermann et al. 1994b). Figure 2 shows the lightcurve of the

countrate from the different observations in the ROSAT PSPC and HRI

(scaled) detectors. The lightcurve is characterized by a faster decrease of the

rate during the first 40 d~vs. Half a year later the countrate was higher than

expected on a simple extrapolation of the initial behaviour. The last point,

taken in April 1994, continues the declining tendency.

From the ROSAT spectrum on day 6 after the outburst we could determine

that there is very little absorption, if any, close to the SN. This requires that

the circumstellar wind material of the red supergiant progenitor must have

been efficiently ionized, possibly in an initial ultraviolet or X-ray flash (Klein

and Chevalier, !978). The circumstellar hydrogen column density outside the

339

o f J

v

t l I I,.,

I

rL

0. I

0.08

0.06

0.04

0.02

]

i ~ i , J i ' ' ' I ' '

SN 1993J in ROSAT Observatory

T PSPC observations

HRI obs. (scaled) Z i m m e r m a n n eL al.

it

M81

0 i i t I I I I I 1 i I ~ , ~ i i i i

0 100 200 300 400 days (after explosion)

F i g u r e 2: Light curve of SN 1993J as measured with ROSAT in the 0.1 to 2.4 keV band. The 2 data points with circles represent the HRI rates that have been scaled by a factor of 2.76 (using the measured spectral parameters) to comparable PSPC rates.

340

shock front along the line of sight, assuming a 1/r 2 density distribution in

the wind material, can be expressed as ~'\-u.22 = 13 x l~_5/(v~,~oVs,4) -1. NH,22

is in units of 1022 cm -2. With the time after the explosion t~ = 6 in days, the

wind velocity v,o,20 = 1 in units of 20 km/sec, the shock velocity v8,4 = 2 in

units of 104 km/sec and mass loss rates ),~-5 of 0.46 to 5.5 solar masses per

year (required to achieve the observed luminosity), we obtain values for Nu,22

between 0.5 and 6. Neutral matter with such column density would strongly

absorb the X-rays, in contradiction to the observation. The 90% confidence

upper limit of 0.05 for NH,22, as derived from the ROSAT spectra, means

that the wind is optically thin against photo-electric absorption for photon

energies > 0.3 keV. Furthermore, our observations are consistent with no

change of the absorption column density until the end of the PSPC observa-

tion period, i.e. day 39 after the outburst. This indicates that the degree of

ionization, effective in the soft X-ray range, was essentially maintained over

that timescale (Zimmermann et al. 1994a).

The ROSAT measurements in November 1993 showed that within the first

half year the temperature, as derived from the X-ray spectrum, had cooled

drastically down to values in the order of 0.5 keV (Zimmermann et al. 1993d).

] 'he strong cooling was later on confirmed by ASCA measurements (Kohmura

341

et al. 1994). The obvious complexity of the X-ray spectrum in the November

1993 observation and the limited spectral resolution of the ROSAI' PSPC

do not allow a good fit to the data with simple spectral models. But the

spectrum suggests that the intrinsic absorption had appreciably increased (by

about a factor of 5) within the half year period, possibly by recombination

in the circumstellar material. It may be the same kind of absorbing material

that was already required in the observations of SN1986J and SN1978K at

much older supernova ages.

Based on the observed parameters detailed numerical models of the evolution

of SN1993J have been published by Nomoto et al. (1993), Suzuki et al.

(1993), Shigeyama et al. (1994) and recently by Fransson et al. (1994). In

the latter work it is shown that in the first few months after the outburst

the bulk of the soft X-rays originate from the circumstellar shock region,

while in the November 1993 measurements X-rays from the reverse shock

region dominate the observed emission. The presented models predict also

increasing luminosity values in the ROSAT band after about 100 days. While

the November 1993 value may be still within the uncertainties of the model

calculations, the next data point, taken in April 1994, certainly follows a

different tendency. It is plausible that there are large uncertainties in the

342

assumed boundary conditions for such calculations. Especially the density

profiles may be strongly affected by the scenario before the explosion. In a

number of papers evidence has been claimed that the progenitor of SN1993J

was a member of a binary system and lost most of its hydrogen envelope

to the companion star (Podsiadlowski et al. 1993; Nomoto et al. 1993i

Shigeyama et al. 1994).

The success with the X-ray observations of SN1993J has raised the hope

for further observations of similar events. In April 1994 a supernova in the

grand spiral M51 (at ~7.7 Mpc) showed up. Unfortunately ROSAT could

observe that sky region only 53 days after the explosion. No X-ray source

was detected at the optical position of the supernova. Compared to SN1993J,

which had a luminosity of about 1039 erg/sec at about the same age, SN1994I

was at least an order of magnitude weaker in soft X-rays (Lewin et al. 1994;

Lewin 1994). Comparing the radio measurements of SN1994I with those of

the supernova in M81, Rupen et al. (t994) concluded that the amount of

circumstellar material around SN1994I was probably appreciably lower.

C o n c l u s i o n s

Present observations of the soft X-ray emission can be reasonably explained

343

bv models where the X-rays are produced bv the interaction of the supernova

ejecta with surrounding material, left from mass loss in the progenitor phase.

Up to now only 2 detections of soft X-ray emission from the very early stages

of a supernova are known (SN1980K and SN1993J) and only in one case, with

SN1993J, a lightcurve has been measured that covers the first few weeks of

the evolution. More examples are known from events where soft X-ray emis-

sion was only detected many years after the outburst (SN1978K, SN1986J,

SN1987A), although in some cases (SN1978K, SN1987A) previous observa-

tions were not successful. The delayed appearance of X-rays could be due

to sufficiently dense circumstellar matter that absorbs for some time the soft

X-ray emission from the expanding shock region. Another explanation is sug-

gested by the case of SN1987A, where obviously the expanding shock wave

hit denser circumstellar material after 4 years. Such material mav originate

from earlier wind phases of the progenitor or, in other scenarios, from the

mass exchange history in a binary system.

The observations of X-ray lightcurves and spectra have provided valuable in-

put for tuning the density profiles assumed in detailed numerical calculations.

The appearance of intrinsic absorption in the X-r~v spectrum of SN1993J in

M81, a few months after the outburst, might be a general evolution fea-

344

ture. The intrinsic absorption in the much older spectra of SN1980K and

SN1986J are consistent with that suggestion. According to the numerical

models of Franson et al. (1994), the absorption should increase as soon as

the emission from the reverse shock region starts to overcome the flux from

the circumstellar shock.

The ROSAT observations are a promising first step towards a better un-

derstanding of the structure and the conditions of the circumstellar matter

around young supernovae.

R e f e r e n c e s :

1. Aschenbach, B., Briel, U.G., Pfeffermann, E., Br£uninger, H., Hippmann,

H., and Trfimper, J. 1987, Nature 330. 232

2. Bregman, J.N., and Pildis, R.A., 1992. ApJ. 398, L107

3. Beuermann, K., Brandt, S., and Pietsch, W., 1994, ApJ. 281. L45

4. Canizares, C.R., Kriss. G.A. and Feigelson, E.D., 1982, ApJ. 253, L17

5. Chevalier, R.A.

- 1982, ApJ. 259. 302

1987. Nature 329. 611

- 1992, Nature 355.617

345

6. Dotani. T. et al.. 1987, Nature 330, 230

7. Franson, C., Lundqvist, P., Chevalier, R.A., 1994. ApJ. submitted

8. Gorenstein, P., Hughes, J.P., and Tucker, W.H., 1994, preprint

9. Inoue, H., Haushida, K., Itoh, M., Kondo, K., Takeshima, T.. Yoshida.

K., and Tanaka, Y., 1991, Publ. Astron. Soc., Japan 43, 213

10. Klein, R.I. and Chevalier, R.A., 1978, ApJ. 223, L109

11. Kohmura, Y., Ishisaki, Y., Makishima, K., Matsushita, K., Inoue, H.,

Aoki, T., Ishida, M., Itoh, M., Kotani, T., Tanaka, Y.,Gotthelf. E.,

Ricker, G., and Tsusaka, Y., 1994, preprint

12. Leising et al., 1994, ApJ. in press

13. Lewin, W.H.G., 1994, preprint

14. Lewin, W.H.G., Zimmermann, H.-U., Pietsch, W., and Beck, R., 1994,

IAU Circ. No. 6019

15. Nomoto, K., Suzuki, T., Shigeyama, T., Kumagai, S., Yamaoka, H., and

Saio, H., 1993, Nature 364, 509

16. Podsiadlowski, P., Hsu, J.J.L., Joss, P.C., and Ross, R.R., 1993. Nature

364, 507

17. Rupen, M.P., Sramek, R.A., Van Dyk, S.D., Weiler, K.W.. and Panagia,

N., 1994, IAU Circ. No. 5963

346

18. Ryder, S., Staveley-Smith, L., Dopita, M., Petre, R., Colbert, E., Malin,

D., and Schlegel, E., 1993, ApJ. 416. 167

19. Shigeyama, T.,Suzuki, T., Kumagai, S., Nomoto, K., Saio, H., Yamaoka,

H., 1994, ApJ. 420, 341

20. Sunyaev, R., et at., 1987, Nature 330, 227

21. Suzuki. T., Kumagai, S., Shigeyama, T., Nomoto, K., Yamaoka, H. and

Saio, H., preprint 1993

22. Tanaka, Y., 1993, IAU Circ. No. 5753

23. Tr/imper, J., Hasinger, G., Aschenbach, B., Brguninger, H., Briel, U.G.,

Burkert, W., Fink, H., Pfeffermann, E., Pietsch, W., Predehl, P., Schmitt,

J.H.M.M., Voges, W., Zimmermann, H.-U., Beuermann, K., 1991, Nature

349, 579

24. Zimmermann, H.-U., Lewin, W.H.G., Magnier, E., Predehl, P., Aschen-

bach. B., Pietsch, W., Hasinger, G., Triimper, J., Fabbiano, G., van

Paradijs, J., Petre, R., and Lubin, L.

- 1993a. IAU Circular No. 5748

- 1993b. IAU Circular No. 5750

- 1993c. IAU Circular No. 5766

- 1993d. IAU Circular No. 5899

1994a. Nature 367, 621

- 1994b, IAU Circular No. 6014

347