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Accretion onto compact objects



My main research interests are accretion inflows and outflows near compact astrophysical sources such as black holes (BH) and neutron stars (NS). Understanding the physical processes that lead to complex instabilities in these accretion flows is one of the most challenging and exciting fields in high energy astrophysics. These instabilities give rise to dramatic transitions in accretion flow pattern, sometimes occurring within an hour in X-ray binaries (XRB; a binary system where a stellar mass BH or NS is accreting matter from its companion via an accretion disk). Similar instabilities are also suspected to occur in active galactic nuclei (AGN), although on a much longer timescale. An extremely intriguing recent discovery is the association of relativistic outflows or "jets" with accretion states. It appears that marked changes in the X-ray state are almost always accompanied by turning on/off of radio emission from a collimated relativistic jet. Even though it is not clear whether the outflow is the cause or the effect of changes in the accretion disk inflow, the association with accretion states suggests a strong disk-jet coupling. It is not known why jets are seen only during low mass accretion states and are either turned off or are quenched by a factor of al least 50-100 at higher mass accretion rates. Since from the accretion physics point of view AGNs are scaled up versions of stellar XRBs, understanding the physical nature of the accretion flow and disk-jet coupling is not only crucial for understanding gravity in the strong-field limit, but will also have significant impact for large-scale phenomena such as AGN activity cycles, AGN feedback mechanism, and the cooling flows in galaxy clusters.

Recent results:

  • The totally weird X-ray Binary MAXI J0556-332

    The Monitor of All-sky X-ray Image (MAXI) discovered a new X-ray source on 2011 January 11, which was subsequently named MAXI J0556-332 (hereafter we will refer to the source as J0556). The discovery was immediately followed up by pointed observations in other wavelengths. X-ray observations made with Swift indicated that the interstellar extinction toward J0556 is small (NH ~ 1021 cm-2) compared to most Galactic X-ray binaries (XRBs), which typically have NH ~ 1022 cm-2. The low NH, largely owing to the high Galactic latitude of the source (b = –25.183), implies that minimal line-of-sight absorption will facilitate efforts to study its low-energy X-ray spectrum. The optical counterpart for this source was found to be a stellar object with USNO B1.0 magnitudes of mR = 19.91 and mB = 19.52, and an optical spectrum obtained by Jules Halpern showed emission lines from H-alpha, He I, and He II, indicating the presence of an accretion disk near the compact accretor.

    XMM-Newton observed J0556 on MJD 55608 and 55653, each observation being of about 40 ks duration. We analyzed the data obtained by the reflection grating spectrometer and found some interesting variability in the X-ray lightcurve (shown below)

    RGS light curve of the first XMM-Newton observation showing its complex morphology, especially epochs of dipping and non-dipping behavior. The variously colored regions represent the intervals from which spectra were analyzed in the paper.

    However this wasn't the most interesting thing that we noted in this source. Far more weird was the RGS spectra. The spectra of both observations showed a (comparatively) very strong emission line at 0.5 keV (24.8 angstrom), and almost no other emission/absorption line! The RGS spectrum during a portion of the first observation (the cyan region in the previous figure) is shown below (click on the figure to see the high-res version).

    XMM-Newton/RGS spectrum of J0556, in 7-36 angstrom range, obtained on MJD 55608 during the steady-high period (i.e., the cyan region in the above figure). Top panel: RGS1 data in red and the RGS2 data in blue (binned for visual clarity only). Data from both RGS units were fitted jointly as described in the paper. The joint best-fit models for RGS1 and RGS2 are shown by red and blue histograms, respectively. Rest-frame energies of few different lines/edges typically prominent in this range are labeled. The energies of the resonance (r), intercombination (i), and forbidden (f) lines are also shown. The broad feature in the 10-14 angstrom range was modeled with a Gaussian. Middle panel: residuals in units of standard deviation, when the normalization of the narrow line at 24.8 angstrom (0.5 keV) was set to zero. Bottom panel: residuals in units of standard deviation, to the best-fit continuum + line model.

    So what is going on in this source? The emission line is exactly where one would expect the N VII line to be. But typically, for an XRB with a solar-type donor, the oxygen abundance is ~8 times greater than that of nitrogen, and the X-ray spectra show lines from H- and He-like ions of oxygen. Nitrogen lines, if present, are usually much weaker. We ran the photoionization code XSTAR to get an idea about the relative abundances that might produce such a strong nitrogen line and little or no oxygen lines. It turns out that one'd need the donor star to have an extremely high N/O abundance (greater than 57) relative to solar, based on the weakness of oxygen lines of similar charge states!!!

    Given the lack of source distance and any signature of orbital periodicity so far, it is only possible to speculate on the nature of the donor star in this system based on its strong N/O overabundance and B-R color-temperature, and exotic stars such as hot, core-helium-burning subdwarfs (sdB, sdO), or degenerate white dwarfs (WD) appear to be strong candidates. It would indeed be very cool if such an exotic star is the donor because it'd be the first XRB system to have a such a donor.

    Another intriguing possibility, assuming solar-abundance plasma, is that the observed line is a gravitationally redshifted O VIII Lyman-alpha line (rest frame wavelength = 18.967 angstrom) originating from the surface of the accreting neutron star. Based on the most precise mass estimates from pulsar timing experiments, observed NS masses range between 1.25 and 2 solar (see, e.g., Kramer and Wex 2009; Demorest et al. 2010). Assuming the observed 24.8 angstrom line in J0556 is gravitationally redshifted O VIII, the above mass range would imply a radius of 8.9-14.2 km. However, a redshifted oxygen line scenario would also require other redshifted lines of oxygen to be present in the spectra, which are not seen. Also, there is a hint of N VI resonance and intercombination lines in the spectra which if indeed present, would favor the N/O overabundance scenario.

    For more details please read our article entitled "A strong emission line near 24.8 angstrom in the X-ray binary system MAXI J0556-332: gravitational redshift or unusual donor" which was published in the ApJ Letters, Volume 743, Issue 1, L11 (2011).

    We really need high-resolution optical/UV spectroscopy to understand the nature of this unique source ...

  • Modeling the Jet in the Seyfert AGN NGC 4051

    Recent radio VLBI observations of the ~parsec-scale nuclear region of the narrow line Seyfert 1 galaxy NGC 4051 hint toward the presence of outflowing plasma. From available literature we have collected high-quality, high-resolution broadband spectral energy distribution data of the nuclear region of NGC 4051 spanning from radio through X-rays, to test whether the broadband SED can be explained within the framework of a relativistically outflowing jet model. We show that once the contribution from the host galaxy is taken into account, the broadband emission from the active galactic nucleus of NGC 4051 can be well described by the jet model. Contributions from dust and ongoing star-formation in the nuclear region tend to dominate the IR emission even at the highest resolutions. In the framework of the jet model, the correlated high variability of the extreme ultraviolet and X-rays compared to other wavelengths suggests that the emission at these wavelengths is optically thin synchrotron originating in the particle acceleration site(s) in the jet very close (few rg=GMBH/c2) to the central supermassive black hole of mass MBH}. Our conclusions support the hypothesis that narrow line Seyfert 1 galaxies (which NGC 4051 is a member of) harbor a "jetted" outflow with properties similar to what has already been seen in low-luminosity AGNs and stellar mass black holes in hard X-ray state.

    The figure above shows the broadband spectral energy distribution of NGC 4051. Chandra/ACIS (solid black lines) and VLA/EVLA 8.4 GHz (red points) data taken from King et al. (2011). Suzaku/PIN data (magenta points connected by lines) from Miller et al. (2010). Swift/BAT data (brown points connected by line) from publicly available NASA archives. EUVE data (violet) from Uttley et al. (2000), FUSE data (blue points) are from Kaspi et al. (2004), HST/STIS fluxes (magenta line) from Collinge et al. (2001), and 5100 angstrom continuum data point (orange) from Denney et al. (2009). The Gemini/NIFS data (Riffel et al. 2008) and the Spitzer/IRS data Sani et al. (2010) are shown in cyan and green respectively. The SED was corrected for Galactic absorption assuming a column density of E(B–V)=0.013 and NH=1.3x1020 cm-2 in the direction of NGC 4051 (Elvis et al. 1989), and assuming the extinction law of Cardelli et al. (1989). The fits to high-resolution Chandra spectra discussed in King et al. (2011) suggest that intrinsic absorption is negligible. The thick solid lines show the various jet model components as well as the total model predicted spectral energy distribution.

    For more details please read our article entitled "A Jet Model for the Broadband Spectrum of the Seyfert-1 Galaxy NGC 4051" which was published in the Astrophysical Journal, Volume 735, Issue 2, pp. 107 (2011).

  • What might have caused the quasi-periodic oscillations in a nearby AGN?

    A recent observation of the nearby (z = 0.042) narrow-line Seyfert 1 galaxy RE J1034+396 on 2007 May 31 showed strong quasi-periodic oscillations (QPOs) in the 0.3-10 keV X-ray flux. We present phase-resolved spectroscopy of this observation, using data obtained by the EPIC PN detector on board XMM-Newton. The "low" phase spectrum, associated with the troughs in the light curve, shows (at >4sigma confidence level) an absorption edge at 0.86 ± 0.05 keV with an absorption depth of 0.3 ± 0.1. Ionized oxygen edges are hallmarks of X-ray warm absorbers in Seyfert active galactic nuclei; the observed edge is consistent with H-like O VIII and implies a column density of N O VIII ~ 3 x 1018 cm-2. The edge is not seen in the "high" phase spectrum associated with the crests in the light curve, suggesting the presence of a warm absorber in the immediate vicinity of the supermassive black hole that periodically obscures the continuum emission. If the QPO arises due to Keplerian orbital motion around the central black hole, the periodic appearance of the O VIII edge would imply a radius of ~ 9.4(M/[4x 106MSun])-2/3(P/[1 hr])2/3 rg for the size of the warm absorber.

    The top panel in the picture above shows the X-ray light curve where the crests and troughs have been colorued differently. The lower-left panel shows the combined spectrum from the crests only, modeled using two blackbodies and a power law. The lower-right panel shows the combined spectrum from the troughs only, and modeled using the same model for the crests. Note the edge at 0.87 keV (due to ionized oxygen) during the troughs.

    For more details please read our article entitled "Evidence of a Warm Absorber That Varies with Quasi-periodic Oscillation Phase in the Active Galactic Nucleus RE J1034+396" which was published in the Astrophysical Journal, Volume 718, Issue 1, pp. 551-557 (2010).

  • A weak jet from the supermassive black hole in the Galactic Center

    The source of emission from Sgr A*, the supermassive black hole at the Galactic Center, is still unknown. Flares and data from multiwavelength campaigns provide important clues about the nature of Sgr A* itself. Here we attempt to constrain the physical origin of the broadband emission and the radio flares from Sgr A*. We developed a time-dependent jet model, which for the first time allows one to compare the model predictions with flare data from Sgr A*. Taking into account relevant cooling mechanisms, we calculate the frequency-dependent time lags and photosphere size expected in the jet model. The predicted lags and sizes are then compared with recent observations. Both the observed time lags and size-frequency relationships are reproduced well by the model. The combined timing and structural information strongly constrain the speed of the outflow to be mildly relativistic, and the radio flares are likely to be caused by a transient increase in the matter channelled into the jets. The model also predicts light curves and structural information at other wavelengths which could be tested by observations in the near future. We show that a time-dependent relativistic jet model can successfully reproduce:
    (1) the quiescent broadband spectral energy distribution of Sgr A*;
    (2) the observed 22 and 43 GHz light curve morphologies and time lags; and
    (3) the frequency-size relationship.

    The results suggest that the observed emission at radio frequencies from Sgr A* is most easily explained by a stratified, optically thick, mildly relativistic jet outflow. Frequency-dependent measurements of time-lags and intrinsic source size provide strong constraints on the bulk motion of the jet plasma.

    The above-left figure shows the comparison of model with data for the flare from Sgr A* on 2006 July 17 at 43 and 22 GHz. The 43 GHz data with errorbars (from Yusef-Zadeh et al. 2008) are shown in red, and 22 GHz data+model in blue. As noted in the paper, we model only the flare that created a peak in the 43 GHz light curve close to 6.5 h UT.
    The above-right figure shows a comparison of model-predicted frequency-size relationship (solid line) with observations. The data are from Bower et al. (2004), Shen et al. (2005), and Doeleman et al. (2008), which were compiled and corrected for scattering by Falcke et al. (2009).

    For more details please read our article entitled "A time-dependent jet model for the emission from Sagittarius A*" which was published in Astronomy and Astrophysics, Volume 508, Issue 1, 2009, pp.L13-L16.

  • Constraining jet/disc geometry and radiative processes in stellar black holes XTE J1118+480 and GX 339-4

    We present results from modelling of quasi-simultaneous broad-band (radio through X-ray) observations of the Galactic stellar black hole (BH) transient X-ray binary (XRB) systems XTE J1118+480 and GX 339-4 using an irradiated disc + compact jet model. In addition to quantifying the physical properties of the jet, we have developed a new irradiated disc model which also constrains the geometry and temperature of the outer accretion disc by assuming a disc heated by viscous energy release and X-ray irradiation from the inner regions. For the source XTE J1118+480, which has better spectral coverage of the two in optical and near-infrared (OIR) wavelengths, we show that the entire broad-band continuum can be well described by an outflow-dominated model + an irradiated disc. The best-fitting radius of the outer edge of the disc is consistent with the Roche lobe geometry of the system, and the temperature of the outer edge of the accretion disc is similar to those found for other XRBs. Irradiation of the disc by the jet is found to be negligible for this source. For GX 339-4, the entire continuum is well described by the jet-dominated model only, with no disc component required. For the two XRBs, which have very different physical and orbital parameters and were in different accretion states during the observations, the sizes of the jet base are similar and both seem to prefer a high fraction of non-thermal electrons in the acceleration/shock region and a magnetically dominated plasma in the jet. These results, along with recent similar results from modelling other galactic XRBs and AGNs, may suggest an inherent unity in diversity in the geometric and radiative properties of compact jets from accreting black holes.

    The figures above (click on any of the figures to see a larger/clearer image) shows how jet model fits (and residuals) to the broadband (radio through X-ray) spectral energy distributions (SED) of galactic X-ray binaries. The left panel is for the SED of the source XTE J1118+480, obtained on MJD 51652 during its outburst of the year 2000. The middle panel is also for the same source, but for its outburst in 2005. The right panel is for the source GX 339-4, obtained on MJD 52367 during its outburst of 2002. Data from different observatories are shown in different colors and different symbols. Similarly the different model components are also shown in different colored curves. For more details please read our article entitled "Constraining jet/disc geometry and radiative processes in stellar black holes XTE J1118+480 and GX 339-4" which was published in the Monthly Notices of the Royal Astronomical Society, Volume 398, Issue 4, pp. 1638-1650.

  • For more details please take a look at my publications here.

    This page is maintained by Dipankar Maitra.   Last updated: May 3, 2011