Spectral transition of ultraluminous X-ray source, NGC 2403 SRC3 N. Isobe (RIKEN, Suzaku Help Desk) and Suzaku SWG team of NGC 2403 K. Makishima (RIKEN/U. Tokyo), M. Kawaharada, A. Senda (RIKEN), H. Takahashi, T. Mizuno (Hirishima U.), R. Miyawaki (U. Tokyo), T. Yoshida (Tokyo Univ of Schience), R. Mushotzky (GSFC/NASA) Good morning. I am Naoki Isobe, from RIKEN Japan. Today, I’d like to show you the Suzaku observation of the Ultraluminous X-ray source in NGC 2403, in comparison with the Chandra and XMM-Newton observations. 2007/10/24 8 Years of Science with Chandra

Ultraluminous X-ray Sources (ULXs) X-ray point sources with a luminosity of L >> 1039 ergs s-1, frequently found in nearby normal galaxies (Fabbiano et al. 1989) Promising candidate of intermediate mass black holes with a mass of M >> 10 M◎, (M◎ is the solar mass). (e.g. Makishima et al. 2000) Two spectral states (e.g., Kubota et al. 2001) Multi-color disk (MCD) and Power Law (PL) Important sources to study black holes at high accretion rate. Ultraluminous X-ray sources (ULXs), are point like X-ray sources with a X-ray luminosity significantly higher than ten to the thirty-ninth erg/s. They are frequently found in nearby normal galaxies since the Einstein era. Although the true nature of ULXs is still one of the important unresolved issue, they are regarded as a promising candidate of intermediate mass black holes with a mass larger than 10 times the solar masses, due to their high luminosity. Similar to galactic black holes, ULXs show two kinds of spectral states, multi-color disk state and Power-law state. Recent studies indicated that ULXs are important to study black holes at a high accretion rate near the Eddington limit. 2007/10/24 8 Years of Science with Chandra

8 Years of Science with Chandra Suzaku 5th Japanese X-ray observatory, launched in June 2005 (Mitsuda et al. 2007) Two instruments X-ray Imaging Spectrometer (XIS) 3 FI and 1 BI CCDs 0.2 – 12 keV Hard X-ray Detector (HXD) PIN : 10 – 70 keV GSO : 40 – 600 keV Advantages Low and stable background Good energy resolution High sensitivity Wide energy band Results on ULXs NGC 1313 (Mizuno et al, 2007) Suzaku J1305-4931 (Isobe et al. 2008) Suzaku XIS HXD Suzaku is the 5th Japanese X-ray observatory, which was launched in the summer of 2005. Currently two instruments are active on Suzaku, XIS and HXD. For ULX study, Suzaku has several advantages; such as low and stable background, high sensitivity, good energy resolution and wide energy coverage. We have already reported the Suzaku results on the several ULXs during the scientific working group phase, NGC 1313 by Mizuno et al. in PASJ Suzaku special issue and Suzaku J1305-4931, newly discovered in the famous Seyfert II galaxy NGC 4945, by myself. I strongly recommend you to read these papers. @ Terada (Saitama U.) 2007/10/24 8 Years of Science with Chandra

8 Years of Science with Chandra NGC 2403 Spiral galaxy (SABcd) at a distance of 3.2 ± 0.4 Mpc (Freedman & Madore 1988) Several bright X-ray sources, including ULX. (Fabianno & Trinchieri 1987) Suzaku observation on March 16th, 2006 during SWG phase ASCA observation in 1997 (Kotoku et al.,1999) Discovery of a supernova SN2004dj motivated a series of Chandra, XMM-Newton observations SRC5 SRC3 Today I present you another target NGC 2403, observed with Suzaku. This image shows the optical image of NGC 2403. This is a normal spiral galaxy at a distance of 3.2 Mpc. Since the Einstein era, the galaxy is known to contain several X-ray sources, including ULX. We observed NGC 2403 with Suzaku in March 2006, during the scientific working group phase. I overlaid the Suzaku XIS image on the optical one. We notice two bright X-ray sources. Following the nomenclature of Fabbiano et al. the brightest source is called SRC 3 and the this one is called SRC 5. Here, I concentrate on SRC 3. Prior to the Suzaku observation, NGC 2403 is observed with ASCA in 1997. It is also observed with Chandra and XMM-Newton, several times. Especially the discovery of a supernovae in the galaxy motivated a series of Chandra and Newton observations. With the best use of these data, we can extensively study the spectral variation of NGC 2403 SRC 3. 4 arcmin = 3.7 kpc Optical image (DSS) Suzaku XIS image (0.5 – 10 keV) 2007/10/24 8 Years of Science with Chandra

8 Years of Science with Chandra Suzaku Spectrum of SRC3 Suzaku XIS spectrum of NGC 2403 SRC 3 is successfully described with multi-temperature black body emission from accretion disk (Multi-color disk model; MCD). Tin = 1.09 ± 0.03 keV Rin = 116.9 + 5.9 – 5.5 km Lbol = 1.75 x 1039 ergs s-1 is not fitted with a PL model. MCD XIS 0+2+3 (FI) XIS 1 (BI) This figure shows the Suzaku XIS spectrum of SRC 3 in 0.5 – 10 keV. The data of XIS 1 (BI CCD) is shown with red points and those of FI CCDs with black ones. The spectrum of SRC 3 is successfully described with the multi-color disk (MCD) model. The innermost disk temperature and radius are determined to be 1.09 keV and 117 km, respectively. The bolometric luminosity of SRC 3, 1.75 time 10 to the 39th erg/s is at the lowest end of typical ULXs. On the other hand, the spectrum is not fitted with a PL model. MCD　(c2 = 296.5/334 ) PL (c2 = 585.3 / 334) 2007/10/24 8 Years of Science with Chandra

Relation between Lbol and Tin SRC3 SRC5 Except for one Chandra observation, NGC 2430 SRC 3 shows the MCD-like spectrum. This figure plotted the bolometric luminosity against the innermost disk temperature Tin. The data for well-studied black holes and ULXs are shown with black and blue points respectively. The dotted lines indicate the constant mass and the dashed lines indicated the constant Eddington ratio; the Eddington luminosity, 10 % of the Eddington luminosity. The back holes follows the constant mass relation, although ULXs dose not. The red points shows the ASCA, Suzaku, Chandra and Newton results for SRC 3. The temperature and luminosity of SRC 3 are both rather stable during this 10 years. The figure indicates that the source is apparently a black hole with 10 – 15 times the solar masses shining almost at the Eddington limit. Although we did not discuss in detail, the magenta points shows the results for SRC 5. (Kotoku et al. 2000, Makishima et al. 2000, Isobe et al. 2007 and reference therein) 2007/10/24 8 Years of Science with Chandra

Spectral transition of SRC3 Only the Chandra data obtained in December, 2004 showed the PL-like spectrum. G = 2.37 ± 0.07 The X-ray flux of the observation is slightly smaller than the Suzaku observation by 15 %. Spectral transition between the MCD-like and the PL-like states is Frequently observed in Galactic black holes. Observed in some well-studied ULXs. (e.g. Kubota et al. 2001) Suzaku (2006 May) MCD Chandra (2004 Aug.) MCD Chandra (2004 Dec.) PL Newton (2004 Sep.) MCD Chandra (2004 Aug.) The upper panel shows the unfolded spectra of SRC 3, obtained in the Suzaku, Chandra and Newton observations. As already mentioned, in allmost all the observations, SRC 3 showed the MCD-like spectrum. However only in the Chandra observation conducted in December, 2004, the sources was in the PL-like spectral state with the photon index of G = 2.37. We can notice the transition more clearly in the bottom panel which shows the ratio to the best-fit MCD model to the Suzaku spectrum. The X-ray flux of the PL-like state is about 15 % smaller than the Suzaku observation. Such a kind of spectral transition is frequently observed in Galactic black holes and is regarded as an important characteristics of black holes. Therefore, we summarize the spectral state of Glactic black holes. Chandra (2004 Dec.) Newton (2004 Sep.) Ratio to the best-fit MCD model to Suzaku data 2007/10/24 8 Years of Science with Chandra

Spectral states of black holes Lbol / LEdd Spectral transition of NGC 2403 SRC 3 Slim disk state ⇔ Very high state 1 10 100 Energy [keV] Slim disk state (MCD-like) 1 Very high state (PL-like) High/Soft state (MCD) 2 < G < 3 0.1 G < 2 These figures schematically show the spectral states of Galactic black holes, according to the luminosity in the unit of the Eddington limit. The luminosity of the source is below a few % of the Emission limit, the source is in the low/hard state with a PL-like spectrum in the X-ray band. The luminosity is higher than the low/hard state, but lower than the Eddington limit, the source is in the high/soft state with MCD spectrum. Moreover, recent observational and theoretical studies find out two other spectral states, around the Eddington limit. One is the Very high state. In the states, the X-ray spectrum becomes PL-like with a photon index of about 2 – 3 , due to significant Comptonization in the hot coronae, around the accretion disk. The other is slim disk states, in which advection becomes important. In this state, the source shows the MCD-like spectrum with higher temperature. Considering the high luminosity and photon index of the PL-like state, we regard that the transition of SRC 3 is between Slim disk state and very high state. Suzaku XIS Chandra/Newton Low/Hard state (PL) 0.01 2007/10/24 8 Years of Science with Chandra (e.g. Kubota et al. 2004)

Slim disk scenario for SRC 3 Standard disk (in MCD) Radial temperature profile T ∝ r -0.75 Slim disk Advection and/or photon trapping become important. ⇒ Flat profile (Watarai et al. 2000) T ∝ r –p with 0.5 < p < 0.75 (p-free disk model; Mineshige et al. 1994, Vierdayanti et al. 2006) Summary of p-free disk fitting MCD : p = 0.75 (Standard disk) We would like to examine the slim disk for SRC 3. In the standard accretion disk adopted in the MCD model, it is assumed that the radial temperature profile is r to the -0.75. On the other hand, in the slim accretion disk, advection and/or photon trapping effect makes the profile more flat. In order to quantitatively estimate this effect, so-called p-free disk (or extended MCD) model is used, with the index of temperature profile being a free parameter. We re-analyzed all the spectra of SRC 3 in the MCD-like spectra, with the p-free disk model. We plotted the index p against the flux of the SRC 3 in the figure. The p in the MCD model is here and the limit in the slim disk is here. In SRC 3, the value of p becomes smaller when the source gets brighter. This seems very natural, because the slim disk properties are thought to be significant at high accretion rate. Limit in Slim disk state p = 0.5 2007/10/24 8 Years of Science with Chandra

8 Years of Science with Chandra Summary We examined the spectral variation of the ULX, NGC 2403, in nearly 10 years, using ASCA, Suzaku, Chandra and XMM-Newton. NGC 2403 SRC3 showed the MCD-like spectra, except for one Chandra observation. Tin = 1.09 ± 0.03 keV Rin = 116.9 + 5.9 – 5.5 km 　(in the Suzaku Observation) Lbol = 1.75 x 1039 ergs s-1 We found the transition to the PL-like spectrum in the Chandra observation, conducted on December 2004, with the flux about 10 % lower than the other observations. G = 2.37 ± 0.07 The spectral transition of NGC 2403 SRC 3 can be naturally explained to be a transition between Very High state (PL-like) and slim disk state (MCD-like). 2007/10/24 8 Years of Science with Chandra

8 Years of Science with Chandra 2007/10/24 8 Years of Science with Chandra

8 Years of Science with Chandra Chandra image 2007/10/24 8 Years of Science with Chandra

Spectral transition of SRC 3 PL Chandra ACIS S5 MCD (c2 = 194.8/123) PL (c2 = 110.7/123) 2007/10/24 8 Years of Science with Chandra