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Number 1, 2013 June 10 (1-10)
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These are the latest articles published in The Astrophysical Journal.
Daniel Anglés-Alcázar et al. 2013 ApJ 770 5 Tag this article
Recent models of black hole growth in a cosmological context have forwarded a paradigm in which the growth is self-regulated by feedback from the black hole itself. Here we use cosmological zoom simulations of galaxy formation down to z = 2 to show that such strong self-regulation is required in the popular spherical Bondi accretion model, but that a plausible alternative model in which black hole growth is limited by galaxy-scale torques does not require self-regulation. Instead, this torque-limited accretion model yields black holes and galaxies evolving on average along the observed scaling relations by relying only on a fixed, 5% mass retention rate onto the black hole from the radius at which the accretion flow is fed. Feedback from the black hole may (and likely does) occur, but does not need to couple to galaxy-scale gas in order to regulate black hole growth. We show that this result is insensitive to variations in the initial black hole mass, stellar feedback, or other implementation details. The torque-limited model allows for high accretion rates at very early epochs (unlike the Bondi case), which if viable can help explain the rapid early growth of black holes, while by z ~ 2 it yields Eddington factors of ~1%-10%. This model also yields a less direct correspondence between major merger events and rapid phases of black hole growth. Instead, growth is more closely tied to cosmological disk feeding, which may help explain observational studies showing that, at least at z 1, active galaxies do not preferentially show merger signatures.
Y. Y. Chun et al. 2013 ApJ 770 10 Tag this article
Galactic black hole transients show many interesting phenomena during outburst decays. We present simultaneous X-ray ( RXTE, Swift, and INTEGRAL), and optical/near-infrared (O/NIR) observations (SMARTS) of the X-ray transient XTE J1752–223 during its outburst decay in 2010. The multiwavelength observations over 150 days in 2010 cover the transition from soft to hard spectral state. We discuss the evolution of radio emission with respect to the O/NIR light curve which shows several flares. One of those flares is bright and long, starting about 60 days after the transition in X-ray timing properties. During this flare, the radio spectral index becomes harder. Other smaller flares occur along with the X-ray timing transition, and also right after the detection of the radio core. We discuss the significances of these flares. Furthermore, using the simultaneous broadband X-ray spectra including INTEGRAL, we find that a high energy cut-off with a folding energy near 250 keV is necessary around the time that the compact jet is forming. The broadband spectrum can be fitted equally well with a Comptonization model. In addition, using photoelectric absorption edges in the XMM-Newton Reflection Grating Spectrometer X-ray spectra and the extinction of red clump giants in the direction of the source, we find a lower limit on the distance of >5 kpc.
Liping Yang et al. 2013 ApJ 770 6 Tag this article
To understand the origin of the solar wind is one of the key research topics in modern solar and heliospheric physics. Previous solar wind models assumed that plasma flows outward along a steady magnetic flux tube that reaches continuously from the photosphere through the chromosphere into the corona. Inspired by more recent comprehensive observations, Tu et al. suggested a new scenario for the origin of the solar wind, in which it flows out in a magnetically open coronal funnel and mass is provided to the funnel by small-scale side loops. Thus mass is supplied by means of magnetic reconnection that is driven by supergranular convection. To validate this scenario and simulate the processes involved, a 2.5 dimensional (2.5D) numerical MHD model is established in the present paper. In our simulation a closed loop moves toward an open funnel, which has opposite polarity and is located at the edge of a supergranulation cell, and magnetic reconnection is triggered and continues while gradually opening up one half of the closed loop. Its other half connects with the root of the open funnel and forms a new closed loop which is submerged by a reconnection plasma stream flowing downward. Thus we find that the outflowing plasma in the newly reconnected funnel originates not only from the upward reconnection flow but also from the high-pressure leg of the originally closed loop. This implies an efficient supply of mass from the dense loop to the dilute funnel. The mass flux of the outflow released from the funnel considered in our study is calculated to be appropriate for providing the mass flux at the coronal base of the solar wind, though additional heating and acceleration mechanisms are necessary to keep the velocity at the higher location. Our numerical model demonstrates that in the funnel the mass for the solar wind may be supplied from adjacent closed loops via magnetic reconnection as well as directly from the footpoints of open funnels.
Didier Barret 2013 ApJ 770 9 Tag this article
High frequency soft reverberation lags have now been detected from stellar mass and supermassive black holes. Their interpretation involves reflection of a hard source of photons onto an accretion disk, producing a delayed reflected emission, with a time lag consistent with the light travel time between the irradiating source and the disk. Independently of the location of the clock, the kHz quasi-periodic oscillation (QPO) emission is thought to arise from the neutron star boundary layer. Here, we search for the signature of reverberation of the kHz QPO emission, by measuring the soft lags and the lag energy spectrum of the lower kHz QPOs from 4U1608-522. Soft lags, ranging from ~15 to ~40 μs, between the 3-8 keV and 8-30 keV modulated emissions are detected between 565 and 890 Hz. The soft lags are not constant with frequency and show a smooth decrease between 680 Hz and 890 Hz. The broad band X-ray spectrum is modeled as the sum of a disk and a thermal Comptonized component, plus a broad iron line, expected from reflection. The spectral parameters follow a smooth relationship with the QPO frequency, in particular the fitted inner disk radius decreases steadily with frequency. Both the bump around the iron line in the lag energy spectrum and the consistency between the lag changes and the inferred changes of the inner disk radius, from either spectral fitting or the QPO frequency, suggest that the soft lags may indeed involve reverberation of the hard pulsating QPO source on the disk.
Joseph R. Masiero et al. 2013 ApJ 770 7 Tag this article
Using albedos from WISE/NEOWISE to separate distinct albedo groups within the Main Belt asteroids, we apply the Hierarchical Clustering Method to these subpopulations and identify dynamically associated clusters of asteroids. While this survey is limited to the ~35% of known Main Belt asteroids that were detected by NEOWISE, we present the families linked from these objects as higher confidence associations than can be obtained from dynamical linking alone. We find that over one-third of the observed population of the Main Belt is represented in the high-confidence cores of dynamical families. The albedo distribution of family members differs significantly from the albedo distribution of background objects in the same region of the Main Belt; however, interpretation of this effect is complicated by the incomplete identification of lower-confidence family members. In total we link 38,298 asteroids into 76 distinct families. This work represents a critical step necessary to debias the albedo and size distributions of asteroids in the Main Belt and understand the formation and history of small bodies in our solar system.