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Register a new userChandra Observation of the Big Dipper X 1624-490
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We present the results of a 73 ks long Chandra observation of the dipping source X 1624-490. During the observation a complex dip lasting 4 hours is observed. We analyse the persistent emission detecting, for the first time in the 1st-order spectra of X 1624-490, an absorption line associated to ion{Ca}{xx}. We confirm the presence of the ion{Fe}{xxv} K$_alpha$ and ion{Fe}{xxvi} K$_alpha$ absorption lines with a larger accuracy with respect to a previous XMM observation. Assuming that the line widths are due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines have been produced in a photoionized absorber between the coronal radius and the outer edge of the accretion disk.
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We present a detailed spectral study (photoionization modelling and variability) of the Big Dipper 4U 1624-490 based on a chandra-High Energy Transmission Gratings Spectrometer (HETGS) observation over the $sim76$ ks binary orbit of 4U 1624-490. While the continuum spectrum can be modeled using a blackbody plus power-law, a slightly better fit is obtained using a single $Gamma=2.25$ power-law partially (71%) covered by a local absorber of column density $N_{rm H, Local}=8.1_{-0.6}^{+0.7}times 10^{22} rm cm^{-2}$. The data show a possible quasi-sinusoidal modulation with period $43_{-9}^{+13}$ ks that might be due to changes in local obscuration. Photoionization modeling with the {sc xstar} code and variability studies of the observed strong ion{Fe}{25} and ion{Fe}{26} absorption lines point to a two-temperature plasma for their origin: a highly ionized component of ionization parameter $xi_{rm hot} approx 10^{4.3} {rm ergs cm s^{-1}}$ ($Tsim 3.0times 10^{6}$ K) associated with an extended accretion disk corona of radius $R sim3times10^{10}$ cm, and a less ionized more variable component of $xi approx 10^{3.4} {rm ergs cm s^{-1}}$ ($Tsim 1.0times 10^{6}$ K) and $rm xi approx 10^{3.1} ergs cm s^{-1}$ ($Tsim 0.9times 10^{6}$ K) coincident with the accretion disk rim. We use this, with the observed ion{Fe}{25} and ion{Fe}{26} absorption line variations (in wavelength, strength, and width) to construct a viewing geometry that is mapped to changes in plasma conditions over the 4U 1624-490 orbital period.
We present X-ray dust scattering halo results based on our 76 ks {it Chandra} ACIS-S/HETGS observation of the LMXB dipping source 4U 1624-490. Through analysis of the halo light curves with 2-6 keV spectra over the persistent and dipping periods, we estimate a geometric distance of $sim$15 kpc to 4U 1624-490. We also fit halo radial profiles with different ISM dust grain models to assess the location, uniformity, and density of the halo. Our analysis shows that the dust spatial distribution is not uniform along the line-of-sight; rather, it is consistent with the spiral arm structure mapped in {sc Hii}. The large difference between the absorption Hydrogen column ($N_{rm H}^{abs} sim 8 times10^{22} rm cm^{-2}$; probes all gas along the line-of-sight) derived from broadband spectral fitting, and the scattering Hydrogen column ($N_{rm H}^{sca} sim 4 times10^{22} rm cm^{-2}$; probes only Galactic gas) derived from our studies of the 4U 1624-490 X-ray halo suggests that a large fraction of the column is local to the X-ray binary. We also present (and apply) a new method for assessing the {it Chandra} point spread function at large ($> 50$) angles, through use of the time delays from the observed dips.
We present Chandra X-ray observations of the nearby Seyfert 1.5 galaxy NGC 4151. The images show the extended soft X-ray emission on the several hundreds of pc scale with better sensitivity than previously obtained. The spectrum of the unresolved nuclear source may be described by a heavily absorbed (N_{H} simeq 3 times 10^{22} cm^-2), hard power-law (Gamma simeq 0.3) plus soft emission from either a power-law (Gamma simeq 2.6) or a thermal (kT simeq 0.6 keV) component. The flux of the high energy component has decreased from that observed by ASCA in 1993 and the spectrum is much harder.The large difference between the soft and hard spectral shapes does not favor the partial covering or scattering model of the ``soft excess. Instead, it is likely that the hard and soft nuclear components represent intrinsically different X-ray sources. Spectra of the extended emission to almost 1 kpc NE and SW of the nucleus have also been obtained. The spectra of these regions may be described by either thermal bremsstrahlung (kT simeq 0.4-0.7 keV) or power-law (Gamma simeq 2.5-3.2) continua plus 3 emission lines. There is an excellent correlation between the extended X-ray and [O III]lambda 5007 line emissions. We discuss the nature of the extended X-ray emission. Upper limit to the electron scattering column was obtained. This upper limit is much too low for the soft X-rays to be electron scattered nuclear radiation.
We report on the precise localization of the low mass X-ray binaries X1624-490 and X1702-429 with the Chandra HRC-I. We determine the best positions to be 16:28:02.825 -49:11:54.61 (J2000) and 17:06:15.314 -43:02:08.69 (J2000) for X1624-490 and X1702-429, respectively, with the nominal Chandra positional uncertainty of 0.6. We also obtained deep IR observations of the fields of these sources in an effort to identify the IR counterparts. A single, faint (Ks=18.3 +/- 0.1) source is visible inside the Chandra error circle of X1624-490, and we propose this source as its IR counterpart. For X1702-429, a Ks=16.5 +/- 0.07 source is visible at the edge of the Chandra error circle. The brightness of both counterpart candidates is comparable to that of other low mass X-ray binary IR counterparts when corrected for extinction and distance.
SDSS J2232-0806 (the Big Dipper) has been identified as a slow-blue nuclear hypervariable: a galaxy with no previously known active nucleus, blue colours and large-amplitude brightness evolution occurring on a timescale of years. Subsequent observations have shown that this source does indeed contain an active galactic nucleus (AGN). Our optical photometric and spectroscopic monitoring campaign has recorded one major dimming event (and subsequent rise) over a period of around four years; there is also evidence of previous events consistent with this in archival data recorded over the last twenty years. Here we report an analysis of the eleven optical spectra obtained to date and we assemble a multiwavelength data set including infrared, ultraviolet and X-ray observations. We find that an intrinsic change in the luminosity is the most favoured explanation of the observations, based on a comparison of continuum and line variability and the apparent lagged response of the hot dust. This source, along with several other recently-discovered changing-look objects, demonstrate that AGN can exhibit large-amplitude luminosity changes on timescales much shorter than those predicted by standard thin accretion disc models.
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