arXiv subjects

T. Surti

Publications and source records attributed to T. Surti.

4 recordsLinked to original sources

arXiv2026

Early Multiwavelength Observations of AT 2026fgk: The Luminous Afterglow to Sub-luminous GRB 260310A, Identified Independently of a Gamma-ray Trigger

The origins of sub-luminous ($L_\mathrm{\gamma,\mathrm{iso}} < 10^{49.5}$\,erg\,s$^{-1}$) gamma-ray bursts (GRBs) associated with broad-lined Type~Ic supernovae (Ic-BL SNe) are poorly understood, in part due to the low discovery rate and faint afterglows. Here we present the identification of the optical afterglow of Fermi-GBM-detected GRB\,260310A (AT\,2026fgk) as a rapidly rising ($>1\,$mag\,d$^{-1}$), red ($g-r=0.4$\,mag) transient using the Gravitational-wave Optical Transient Observatory, Large Array Survey Telescope, and Zwicky Transient Facility (ZTF) data streams. We present multiwavelength follow-up observations from the first 50\,days, which reveal that GRB 260310A/AT\,2026fgk was sub-luminous ($L_\mathrm{\gamma,iso}=10^{48.8}\,$erg\,s$^{-1}$); it was the most nearby ($z=0.153$) afterglow identified blindly by an optical survey; and that it is one of the brightest afterglows ever observed at X-ray, optical, and radio (cm to mm) wavelengths. We spectroscopically confirm an underlying Ic-BL SN with properties typical of GRB-SNe ($M_\mathrm{ej}\approx3\,M_\odot$, $E_{\rm K}\approx 10^{52}\,$erg). With basic modeling of the afterglow, including the long optical rise ($\approx10^{3}\,$s), we infer either a low initial Lorentz factor ($\Gamma_0\approx40$) or a slightly off-axis viewing angle ($\lesssim3^\circ$). The host galaxy's mass and star formation rate are similar to the hosts of other sub-luminous GRBs. ZTF's flux-limited survey gives a volumetric rate of AT\,2026fgk-like events of $0.30^{+1.37}_{-0.29}\,$Gpc\,$^{-3}$\,yr$^{-1}$, which is consistent with the on-axis, high luminosity ($L_{\rm \gamma,iso}>10^{49.5}$\,erg\,s$^{-1}$) long-GRB rate. The similarity in the rates strongly constrains the prevalence of low-$\Gamma_0$ bursts and the beaming of the initial relativistic material in GRBs.

astro-ph.HE↗

arXiv2025

Compelling Evidence for a Harmonic in the Light Curve of the Supermassive Black Hole Binary Candidate PKS J1309+1154

We recently discovered a supermassive black hole binary (SMBHB) candidate, PKS J1309+1154, in the combined 46-yr University of Michigan Radio Astronomy Observatory (UMRAO) plus Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz. The light curve of PKS 1309+1154 exhibits a 17.9 year periodicity. We also reported a hint of a first harmonic with a 9 year periodicity in this object. Further analysis of the PKS J1309+1154 light curve provides compelling evidence that both the fundamental and the harmonic are real, confirming the existence of real periodicities in blazar light curves. This is the first case, to our knowledge, of watertight evidence for a fundamental and a harmonic periodicity in a blazar light curve. It makes PKS J1309+1154 a \textit{strong\/} supermassive black hole binary (SMBHB) candidate, and thus the third such candidate to be revealed through long-term radio monitoring, the other two being PKS J0805--0111 and PKS 2131--021, both discovered through the OVRO 40 m Telescope monitoring program. It is argued that hundreds of SMBHB candidates will be discovered by the Vera Rubin and Simons Observatories. Coherent searches for gravitational waves from a network of SMBHB candidates, starting immediately, are strongly motivated.

astro-ph.HE↗

arXiv2025

A Search for Supermassive Black Hole Binary Candidates in 46-Year Radio Light Curves of 83 Blazars

The combined University of Michigan Radio Astronomy Observatory (UMRAO) and Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz provide uninterrupted light curves of $\sim~46-50$ yr duration for 83 blazars, selected from amongst the brightest and most rapidly flaring blazars north of declination $-20^\circ$. In a search for supermassive black hole binary (SMBHB) candidates, we carried out tests for periodic variability using generalized Lomb-Scargle (GLS), weighted wavelet-Z (WWZ), and sine-wave fitting (SWF) analyses of this sample. We used simulations to test the effects of the power law spectrum of the power spectral density (PSD) on our findings, and show that the irregular sampling in the observed light curves has very little effect on the GLS spectra. Apparent periodicities and putative harmonics appear in all 83 of the GLS spectra of the blazars in our sample. We tested the reality of these apparent periodicities and harmonics with simulations, and found that in the overwhelming majority of cases they are due to the steep slope of the PSD, and should therefore be treated with great caution. We find one new SMBHB candidate: PKS 1309+1154, which exhibits a 17.9 year periodicity. The fraction of SMBHB candidates in our sample is $2.4_{-0.8}^{+3.2}\%$.

astro-ph.HE↗

arXiv2024

The Radio Spectra of High Luminosity Compact Symmetric Objects (CSO-2s): Implications for Studies of Compact Jetted Active Galactic Nuclei

This paper addresses, for the first time, a key aspect of the phenomenology of Compact Symmetric Objects (CSOs) -- the characteristics of their radio spectra. We present a radio-spectrum description of a complete sample of high luminosity CSOs (CSO-2s), which shows that they exhibit the \textit{complete} range of spectral types, including flat-spectrum sources ($\alpha \ge -0.5$), steep-spectrum sources ($\alpha < -0.5$), and peaked-spectrum sources. We show that there is no clear correlation between spectral type and size, but there is a correlation between the high-frequency spectral index and both object type and size. We also show that, to avoid biasing the data and to understand the various classes of jetted-AGN involved, the complete range of spectral types should be included in studying the general phenomenology of compact jetted-AGN, and that complete samples must be used, selected over a wide range of frequencies. We discuss examples that demonstrate these points. We find that the high-frequency spectral indices of CSO-2s span $-1.3 <\alpha_{\rm hi} < -0.3$, and hence that radio spectral signatures cannot be used to discriminate definitively between CSO-2s, binary galactic nuclei, and millilensed objects, unless they have $\alpha_{\rm hi} >-0.3$.

astro-ph.HE↗