arXiv subjects

Mustafa Muhibullah

Publications and source records attributed to Mustafa Muhibullah.

3 recordsLinked to original sources

arXiv2026

Hypersoft X-ray Sources as a Low-Energy Class of Luminous Cosmic Emitters

X-ray binaries, powered by accretion onto black holes, neutron stars, or white dwarfs from companion stars, are among the brightest non-explosive beacons in galaxies, outshining the Sun by millions. Typical X-ray binaries emit primarily above 0.3 keV, whereas cooler sources, peaking in the extreme ultraviolet (EUV), remain challenging to detect due to a long-standing observational blind spot. Here, we report the discovery of a remarkable class of luminous, point-like, non-nuclear X-ray objects--hypersoft X-ray sources--that have largely been missed by previous surveys. Detected primarily or exclusively below 0.3 keV, the most luminous examples radiate near 1E38 erg/s in the narrow X-ray band, with spectral models suggesting even higher bolometric luminosities, most of which emerge in the EUV. They rank among the most energetic sources in galaxies, yet their EUV-peaking spectra evaded earlier detections. We propose that hypersoft sources represent X-ray binaries spanning multiple physical classes, including accreting white dwarfs or post-nova systems--potential progenitors of Type Ia supernovae--and systems hosting accreting black holes. Beyond their elusive nature, they may play a crucial role in ionizing gas within galaxies.

astro-ph.HE↗

arXiv2025

The Massive and Distant Clusters of $WISE$ Survey. XII. Exploring X-ray AGN in Dynamically Active Massive Galaxy Clusters at z~1

We present an analysis of the cluster X-ray morphology and active galactic nucleus (AGN) activity in nine $z\sim1$ galaxy clusters from the Massive and Distant Clusters of $WISE$ Survey (MaDCoWS) observed with $Chandra$. Using photon asymmetry ($A_{\text{phot}}$) to quantify X-ray morphologies, we find evidence that the four most dynamically disturbed clusters are likely to be mergers. Employing a luminosity cut of $7.6\times10^{42}$ erg/s to identify AGN in the 0.7-7.0 keV, we show that the majority of these clusters host excess AGN compared to the local field. We use the cumulative number-count ($\log N-\log S$) model to predict AGN incidence in cluster isophotes under this luminosity cut. Our analysis finds evidence (at $> 2\sigma$) of a positive correlation between AGN surface densities and photon asymmetry, suggesting that a disturbed cluster environment plays a pivotal role in regulating AGN triggering. Studying AGN incidence in cluster X-ray isophotes equivalent in area to $1.0r_{500}$, we find that the AGN space density inversely scales with cluster mass as $\sim M^{-0.5^{+0.18}_{-0.18}}$ at the 3.18$\sigma$ level. Finally, when we separately explore the cluster mass dependence of excess AGN surface density in disturbed and relaxed clusters, we see tentative evidence that the two morphologically distinct sub-populations exhibit diverging trends, especially near the outskirts, likely due to cluster merger-driven AGN triggering/suppression.

astro-ph.GA↗

arXiv2021

The Massive and Distant Clusters of WISE Survey XI: Stellar Mass Fractions and Luminosity Functions of MaDCoWS Clusters at $z \sim 1$

We present stellar mass fractions and composite luminosity functions (LFs) for a sample of \Ncl\ clusters from the Massive and Distant Clusters of WISE Survey (MaDCoWS) at a redshift range of $0.951 \leq z \leq 1.43$. Using SED fitting of optical and deep mid-infrared photometry, we establish the membership of objects along the lines-of-sight to these clusters and calculate the stellar masses of member galaxies. We find stellar mass fractions for these clusters largely consistent with previous works, including appearing to display a negative correlation with total cluster mass. We measure a composite $3.6~\mathrm{\mu m}$ LF down to $m^*+2.5$ for all 12 clusters. Fitting a Schechter function to the LF, we find a characteristic $3.6~\mathrm{\mu m}$ magnitude of $m^*=19.83\pm0.12$ and faint-end slope of $\alpha=-0.81\pm0.10$ for the full sample at a mean redshift of $\bar{z} = 1.18$. We also divide the clusters into high- and low-redshift bins at $\bar{z}=1.29$ and $\bar{z}=1.06$ respectively and measure a composite LF for each bin. We see a small, but statistically significant evolution in $m^*$ and $\alpha$ -- consistent with passive evolution -- when we study the joint fit to the two parameters, which is probing the evolution of faint cluster galaxies at $z\sim1$. This highlights the importance of deep IR data in studying the evolution of cluster galaxy populations at high-redshift.

astro-ph.GA↗