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Register a new user{it NuSTAR} Reveals an Intrinsically X-ray Weak Broad Absorption Line Quasar in the Ultraluminous Infrared Galaxy Markarian 231
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Stacy H. Teng
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We present high-energy (3--30 keV) {it NuSTAR} observations of the nearest quasar, the ultraluminous infrared galaxy (ULIRG) Markarian 231 (Mrk 231), supplemented with new and simultaneous low-energy (0.5--8 keV) data from {it Chandra}. The source was detected, though at much fainter levels than previously reported, likely due to contamination in the large apertures of previous non-focusing hard X-ray telescopes. The full band (0.5--30 keV) X-ray spectrum suggests the active galactic nucleus (AGN) in Mrk 231 is absorbed by a patchy and Compton-thin (N$_{rm H} sim1.2^{+0.3}_{-0.3}times10^{23}$ cm$^{-2}$) column. The intrinsic X-ray luminosity (L$_{rm 0.5-30 keV}sim1.0times10^{43}$ erg s$^{1}$) is extremely weak relative to the bolometric luminosity where the 2--10 keV to bolometric luminosity ratio is $sim$0.03% compared to the typical values of 2--15%. Additionally, Mrk 231 has a low X-ray-to-optical power law slope ($alpha_{rm OX}sim-1.7$). It is a local example of a low-ionization broad absorption line (LoBAL) quasar that is intrinsically X-ray weak. The weak ionizing continuum may explain the lack of mid-infrared [O IV], [Ne V], and [Ne VI] fine-structure emission lines which are present in sources with otherwise similar AGN properties. We argue that the intrinsic X-ray weakness may be a result of the super-Eddington accretion occurring in the nucleus of this ULIRG, and may also be naturally related to the powerful wind event seen in Mrk 231, a merger remnant escaping from its dusty cocoon.
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With 40 ks of Chandra ACIS-S3 exposure, new information on both the starburst and QSO components of the X-ray emission of Markarian 231, an ultraluminous infrared galaxy and Broad Absorption Line QSO, has been obtained. The bulk of the X-ray luminosity is emitted from an unresolved nuclear point source, and the spectrum is remarkably hard with the majority of the flux emitted above 2 keV. Most notably, significant nuclear variability (a decrease of ~45% in approximately 6 hours) at energies above 2 keV indicates that Chandra has probed within light hours of the central black hole. Though we concur with Maloney & Reynolds that the direct continuum is not observed, this variability coupled with the 188 eV upper limit on the equivalent width of the FeKalpha emission line argues against the reflection-dominated model put forth by these authors based on their ASCA data. Instead, we favor a model in which a small, Compton-thick absorber blocks the direct X-rays, and only indirect, scattered X-rays from multiple lines of sight can reach the observer. Extended soft, thermal emission encompasses the optical extent of the galaxy and exhibits resolved structure. An off-nuclear X-ray source with a 0.35-8.0 keV luminosity of L_X=7x10^{39} erg s^{-1}, consistent with the ultraluminous X-ray sources in other nearby starbursts, is detected. We also present an unpublished FOS spectrum from the HST archive showing the broad C IV absorption.
We report the results from a 2011 Suzaku observation of the nearby low-ionization BAL quasar/ULIRG Markarian 231. These data reveal that the X-ray spectrum has undergone a large variation from the 2001 XMM-Newton and BeppoSAX observations. We interpret this finding according to a scenario whereby the X-ray continuum source is obscured by a two-component partial-covering absorber with NH ~10^22 and ~10^24 cm^-2, respectively. The observed spectral change is mostly explained by a progressive appearance of the primary continuum at <10 keV due to the decrease of the covering fraction of the denser absorption component. The properties of the X-ray obscuration in Mrk 231 match well with those of the X-ray shielding gas predicted by the theoretical models for an efficient radiatively-driven acceleration of the BAL wind. In particular, the X-ray absorber might be located at the extreme base of the outflow. We measure a 2-10 keV luminosity of L(2-10) = 3.3 x 10^43 erg s^-1 for the 2011 data set, i.e. an increase of 30% with respect to the 2001 value.
Broad-line radio galaxies (BLRGs) are active galactic nuclei that produce powerful, large-scale radio jets, but appear as Seyfert 1 galaxies in their optical spectra. In the X-ray band, BLRGs also appear like Seyfert galaxies, but with flatter spectra and weaker reflection features. One explanation for these properties is that the X-ray continuum is diluted by emission from the jet. Here, we present two NuSTAR observations of the BLRG 3C 382 that show clear evidence that the continuum of this source is dominated by thermal Comptonization, as in Seyfert 1 galaxies. The two observations were separated by over a year and found 3C 382 in different states separated by a factor of 1.7 in flux. The lower flux spectrum has a photon-index of $Gamma=1.68^{+0.03}_{-0.02}$, while the photon-index of the higher flux spectrum is $Gamma=1.78^{+0.02}_{-0.03}$. Thermal and anisotropic Comptonization models provide an excellent fit to both spectra and show that the coronal plasma cooled from $kT_e=330pm 30$ keV in the low flux data to $231^{+50}_{-88}$ keV in the high flux observation. This cooling behavior is typical of Comptonizing corona in Seyfert galaxies and is distinct from the variations observed in jet-dominated sources. In the high flux observation, simultaneous Swift data are leveraged to obtain a broadband spectral energy distribution and indicates that the corona intercepts $sim 10$% of the optical and ultraviolet emitting accretion disk. 3C 382 exhibits very weak reflection features, with no detectable relativistic Fe K$alpha$ line, that may be best explained by an outflowing corona combined with an ionized inner accretion disk.
We report NuSTAR observations of a sample of six X-ray weak broad absorption line (BAL) quasars. These targets, at z=0.148-1.223, are among the optically brightest and most luminous BAL quasars known at z<1.3. However, their rest-frame 2 keV luminosities are 14 to >330 times weaker than expected for typical quasars. Our results from a pilot NuSTAR study of two low-redshift BAL quasars, a Chandra stacking analysis of a sample of high-redshift BAL quasars, and a NuSTAR spectral analysis of the local BAL quasar Mrk 231 have already suggested the existence of intrinsically X-ray weak BAL quasars, i.e., quasars not emitting X-rays at the level expected from their optical/UV emission. The aim of the current program is to extend the search for such extraordinary objects. Three of the six new targets are weakly detected by NuSTAR with <45 counts in the 3-24 keV band, and the other three are not detected. The hard X-ray (8-24 keV) weakness observed by NuSTAR requires Compton-thick absorption if these objects have nominal underlying X-ray emission. However, a soft stacked effective photon index ({Gamma}~1.8) for this sample disfavors Compton-thick absorption in general. The uniform hard X-ray weakness observed by NuSTAR for this and the pilot samples selected with <10 keV weakness also suggests that the X-ray weakness is intrinsic in at least some of the targets. We conclude that the NuSTAR observations have likely discovered a significant population (>33%) of intrinsically X-ray weak objects among the BAL quasars with significantly weak <10 keV emission. We suggest that intrinsically X-ray weak quasars might be preferentially observed as BAL quasars.
We investigate the X-ray properties of PG 1004+130, a low-redshift radio-loud broad absorption line (BAL) quasar with a hybrid FR I/FR II radio morphology. The 22.2 ks XMM-Newton and 41.6 ks Chandra observations presented here are the first X-ray detections of PG 1004+130 and constitute the highest spectral quality X-ray observations of a radio-loud BAL quasar available to date. The Chandra ACIS-S spectrum shows evidence for complex soft X-ray absorption not detected in the data obtained 1.7 yr previously with XMM-Newton, with a best-fit intrinsic column density of N_H=1.2e22 cm-2 for the preferred partial-covering model. There is no significant difference in the hard-band power-law photon index of ~1.5 between the two observations. The Chandra image also reveals extended X-ray emission ~8 (30 kpc) south-east of the nucleus, aligned with the FR I jet but upstream of the 1.4 GHz radio-brightness peak. The jet is not detected by HST, and the optical upper limit rules out a simple single-component synchrotron interpretation of the radio-to-X-ray emission. The multiwavelength characteristics of the PG 1004+130 jet, including its relatively flat X-ray power law and concave spectral energy distribution, are similar to those of powerful FR II jets. The lack of strong beaming in PG 1004+130 limits the efficiency of inverse Compton upscattering, and we consider the X-ray emission to most likely arise from a second synchrotron component generated by highly energetic electrons.
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